20 Amp T Slot Receptacle
Most important, while 15-amp outlets can be installed on 20-amp circuits, 20-amp outlets cannot be installed on 15-amp circuits. In older homes where there may be no circuit ground wire, the original receptacles may be only two-slot models, without the grounding slot.
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A 20-amp outlet is only compatible with a 20-amp circuit. The circuit's ampere rating can be found by checking the breaker box. A 20-amp outlet is identifiable by horizontal slots that cross the more typical vertical slots of a normal outlet. This kind of outlet is served by either 12-gauge or 10-gauge wiring.
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
The 20 denotes the device is rated for 20 amps. The 'R' at the end denotes that this is a receptacle. The plug side would be a 'P'. The purpose for this outlet is to allow a variety of devices to be plugged in as long as the device does not exceed 125 volts and 20 amps.
Types of electrical receptacles: how to select the right type of electrical receptacle (outlet):
It's important to use 20-A rated receptacles if the electrical circuit is a 20-amp circuit. Don't install a grounded electrical receptacle plug on a circuit that has no electrical ground. Remember to install AFCI or GFCI devices where they are required.
This article explains how to match the receptacle type to the circuit type and use. We describe both 120V receptacles and 240V receptacles.
This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or 'plugs' or 'sockets') are simple devices that are easy to install, but there are details to get right if you want to be safe.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need. Choose the Proper Electrical Receptacle Type
The proper type of electrical receptacle must be selected : some receptacles are rated only for 15-Amp circuits and must not be installed on a 20-Amp circuit.
20-Amp electrical receptacles may be designed to only accept plugs for 20-Amp appliances (which may have a different plug-spade configuration in which one of the plug terminals is twisted to be at 90 degrees to the other).
[Click to enlarge any image]
Some 20-Amp electrical receptacles are designed to accept either conventional plugs used by a 15-A appliance as well as 20-A plugs used by a 20-A appliance.
It's generally ok to plug a 15-A appliance into a 20-A circuit since that appliance is not going to overload the circuit in normal use. But the opposite is not true
. If you plug a 20-Amp appliance into a 15-Amp circuit you are risking overloading the circuit and tripping the circuit breaker, blowing the fuse, or worse, overheating the circuit and risking a fire.
Below our photographs illustrate a 15-Amp grounded electrical (below left) and a 20-Amp grounded electrical receptacle (below right).
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
Details about how to wire up an electrical receptacle are at ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires . Two-Wire Electrical Receptacle Circuits
Older two-wire electrical circuits , such as the two circuits depicted at the right of our sketch above may provide only the hot and neutral wires and no ground wire. 20 Amp T Slot Receptacle 30 Amp
If no ground wire or ground path is provided, it is improper and unsafe to install a grounding (3-prong) electrical receptacle on that circuit.
Our photo (above left) shows a conventional grounded three-prong electrical receptacle - the round hole is the ground connection - at the left end of the picture closest to my thumb.
At right in the photo is an ungrounded electrical receptacle. This is the right device to install if no ground is present on the electrical circuit.
You don't want to 'fool' a building occupant into thinking that a ground is present when there is not one, so you don't install a receptacle that has that third ground opening in its face.
Some older two-wire circuits which are covered with a flexible metal jacket ('BX' or 'armored cable' wire) may provide a ground path by means of the cable jacket itself.
We don't rely on it, and in event of certain short circuits it's unsafe: the exposed metal sheathing of the wire becomes live, risking a shock.
Details about how to wire up an un-grounded receptacle are at 2-WIRE RECEPTACLE CONNECTIONS - no ground
The illustration at above-left shows the typical wiring of an electrical outlet or 'receptacle', courtesy of Carson Dunlop Associates.
Click any image to see an enlarged, detailed view of electrical wiring details for 'plugs' or electrical receptacles. Choosing GFCI and AFCI Receptacles
Ground fault protection - GFCI's: The NEC also requires that only special ground fault circuit interrupter (GFCI) protected outlets can be installed in certain hazardous locations like kitchens, baths, garages, outdoors.
A GFCI-protected electrical receptacle includes circuitry that turns the electric power off at the outlet quickly should a ground-fault (electricity flowing to earth, such as through your hand and down a water pipe) be detected. [4]
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. [4]
AFCI's are similar to GFCI's discussed above, but they include an additional level of protection against fire by detecting small electrical arcing at a connection - a condition that can lead to overheating and fire.
As you can see from this US CPSC photo, you can add Arc fault protection to a home circuit by installing a special circuit breaker in the electrical panel.
By this means you can provide arc fault protection and thus improved fire safety for all electrical outlets on the circuit - for example in the building's bedrooms
In the FAQs (below) we discuss the importance of wiring the Line and Load terminals of GFCIs and AFCIs correctly.
See AFCI GFCI WIRING, TESTING SAFETY
and AFCIs ARC FAULT CIRCUIT INTERRUPTERS for details about these devices.
Readers of this article should also see SAFETY for ELECTRICAL INSPECTORS.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Question: Is it safe to plug a 10-Amp A/C into an outlet that is back wired?
is it safe to plug in an ac unit that runs 10 amps , into outlet that is backed wired, i had read that you don't like this method, the outlet is on third floor and is on a 15 amp breaker - Johnny B 5/2/12 Reply: Comparing Three Types of Backwired Receptacles: 20-A Clamp Type 15-ASpring Type Clamp Type Backwiring Devices
Johnny, that's an interesting question and one I'm scared to answer - by online posting one cannot assure the electrical safety of your building.
Details about back-wired electrical devices (receptacles switches) are at BACK-WIRED ELECTRICAL DEVICES.
Here we will illustrate three different types of electrical receptacles that can be wired from their back-side.
Our photo (left) illustrates a spec-grade 20-Amp, 125V rated electrical receptacle that looks as if it is 'back-wired' - in fact while a wire can be wrapped around the terminal screws on this device.
The screw is intended to be used to tighten a rectangular brass plate against a square metal nut (silver in color) that makes a very strong and positive connection over a good area of wire surface.
This receptacle is marked on its back surface as CU Wire Only - copper only . [Click images to see enlarged details.]
That said, I agree that older, spring-type back-wired electrical connections (shown at below left) are not as reliable as connections made under a screw or clamp, as the total contact area between the back-wire spring edge and the wire surface is minimal.
Nevertheless, on a 15-A circuit using 15-A devices such as receptacles, the circuit and its devices are rated and intended to be able to support the 10-amp load you describe, so long as the sum of all of the items plugged into that electrical circuit don't overload it.
Contractor-grade 15-A spring-type -connector back-wired electrical receptacles (below left) provide a single opening at each of the four terminals (two neutral wires, two hot wires) on the back of the receptacle (red arrow).
The yellow arrow points to a release spring that will allow removal of the wire, but we prefer not to re-use this type of back-wired receptacle. Tightening the screw at the main wire terminal (blue arrow) has nothing to do with the spring-clamp that is securing the back-wired terminal wire.
Some newer heavy-duty 15-A back-wired electrical receptacles (above right) o not rely on a simple spring-edge to contact the electrical wire, as we illustrate in our second photo (above right).
Rather, when the wire is inserted into a receiving hole on the back of the receptacle (either of the two red arrows).
When the terminal screw is tightened (blue arrow) that actually snugs up a clamp that contacts a much larger surface area of the back-wired wire.
That's a more secure connection mechanically. On this receptacle, instead on a single back terminal accepting a single wire, there are a pair of back terminal openings at each of the four terminal screws.
Follow-up:
Thank you for responding, my town home was built in 1999, not sure if that is considered newer or older, lights do dim though when i use 10 amp vacuum . - Johnny B.
Reply:
Backwiring electrical receptacles is a permitted installation and might be found in a 1999 home - but as we show above, there are two different approaches, the second of which is a better quality installation and is in our opinion more reliable. Older Antique Electrical Receptacles or 'Outlets'
At above and below are photos of an un-grounded electrical receptacle found in many older homes where the electrical circuit wiring did not include a grounding conductor.
These receptacles can still be purchased and are the only receptacle type that should be installed on an un-grounded electrical receptacle circuit as the absence of a third ground-prong opening makes clear to the user that no ground is present.
See FALSE GROUND at RECEPTACLES for a discussion of how this particular receptacle enjoyed giving the author a shock.
Above at right is an antique surface-mount plastic or bakelite wall receptacle or 'wall plug' or 'wall outlet' in common parlance. This installation is not just obsolete but improper and unsafe as you can see exposed wires poking through the wall trim below the receptacle. Property inspector Steve Smallman, Raleigh NC, who contributed this receptacle points out that
We expect the receptacle to have been designed for a 20A circuit based on the Tee-shaped slot
The receptacle has been heavily painted over, interfering with its use
The wiring exposed below the receptacle is stripped-back armored cable ('BX') without proper strain relief or protection.
If the actual circuit is powered by a 15A circuit breaker or fuse, the fact that it pretends to offer 20A is mitigated by proper fusing but could also cause trouble for the homeowner as it invites overloading.
Thanks to frequent InspectApedia contributor also cited at ABOUT Inspect A pedia.com
Steve Smallman, Raleigh, NC, Email: steve@stevesmallman.com, Website: http://stevesmallman.com/ Tel: 919-669-3639 - Quoting: Steve Smallman Property Inspections (SSPI) inspectors have performed or supervised over 25,000 inspections since we introduced home inspections to the Triangle area in 1980. Mr. Smallman, located in Raleigh N.C., is a contributor to InspectApedia.com and has commented on or provided information on plumbing traps, electrical wiring, commercial FPE electrical panels and DIY Tests of FPE equipment, roofing underlayment, and building exteriors.
Below are two versions of another antiquated and complex electrical receptacle.
I think that these electrical receptacles provide either 120V or 240V connections depending on the receptacle and slots used and on the actual wiring present. Comments are invited.
Below: an older Harvey Hubbell (1857-1927) design electrical receptacle designed to accept 120VAC wall plugs sporting either the 'modern' 15A standard parallel blade design OR the older tandem design.
Hubbell, a prolific inventor of electrical receptacles, plugs, and also bulb sockets and controls, designed and patented a number of key electrical receptacles that are cited below.
Bossert, William F. 'Outlet-box for interior conduits.' U.S. Patent 754,414, issued March 15, 1904.
Hubbell Inc. HUBBELL CORPORATE HISTORY [PDF] Hubbell Inc., 584 Derby Milford Road Orange, CT 06477-4024 USA, Tel: (203) 799-4100 Website:
Hubbell Inc. HUBBELL ELECTRIC CO CATALOG 1906 [PDF] Western Electric Company, Eleventh York Sts, Philadelphia PA USA, retrieved 2018/12/20, original source https://ia600607.us.archive.org/14/items/ElectricalSpecialitiesCatalogueAndPriceListNo.9August1906/Cca49543WesternElectricCo..pdf
Below: illustration from Harvey Hubbell's Separable Attachment Plug patent from 1904.
[Click to enlarge any image]
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Excerpt:
I have devised the simple and novel separable attachment-plug which I will now describe, referring to the accompanying drawings forming part of this specification
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,251, issued November 8, 1904.
Excerpt:
Be it known that I, HARVEY HUBBELL, acitizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Separable Attachment-Plug, of which the following is a specification.
My invention has for its general object to produce a practical, durable, and inexpensive direct-acting separable attachment-plug that will avoid the objections raised by the underwriters to attachment-plugs of this type now upon the market and shall have for its special objects to produce a direct-acting attachment-plug in which ample current-carrying capacity shall be provided independently of locking-springs, which may or may not be used, and in which the possibility of arcing or sparkingin making the contact shall be wholly done away with.
With these and other objects in view I have devised a novel attachment-plug in which a base is provided with insulating-chambers to receive contact-plates and locking-springs, if used, and with contracted insulating-passages leading to said chambers, in which the end block is made independent of the base and is secured thereto, covering the chambers, and in which an insulating-cap is provided with knife-blade contacts which engage the contact plates after they have been passed through the contracted insulating-passages.
It should be noted that the object in making the base and end block independent is to permit the formation of the insulating-chambers in the base, which receive the contact-plates and the locking-springs, if usechsaid chambers being covered by the end block, and, furthermore, that by providing contact-plates of ample current-carrying capacity in the chambers independent of the locking-springs it becomes immaterial, so far as the currentcarrying is concerned, whether or not locking-springs are used.
My present construction therefore enables me to avoid certain of the most serious objections raised by the underwriters to attachment-plugs heretofore placed upon the market.
Hubbell, Harvey. 'Cartridge-fuse and fuse-block.' U.S. Patent 819,657, issued May 1, 1906.
Photo above: a round pin receptacle, possibly with ground and missing one mounting screw, possibly an early Hubbell-design round two-pin floor receptacle, courtesy of Bay State Home Inspections and discussed originally
This receptacle is also similar to George Thomas' receptacle and plug design from 1910 and cited below.
at OLD ELECTRICAL WIRING TYPES
Below: an excerpt from the patent illustration for Harvey Hubbell's multiple attachment plug patent issued in 1904.
Knife blade (flat) plug and receptacle connectors designed by Hubbell were produced as early as 1904 and became the dominant connector design used in the U.S. and Canada.
Hubbell, Harvey. 'Multiple attachment-plug.' U.S. Patent 776,326, issued November 29, 1904.
Excerpt:
I Harvey Hubbell, a citizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Multiple Attachment-Plug, of which the following is a specification.
My invention has for its object to provide a multiple attachment-plug which shall comprise a body adapted to engage floor, wall, or ceiling receptacles or other fixtures and a plurality of separable caps which shall be adapted to engage the body, so that a plurality of lights, groups of lights, fans, motors, heating, soldering, surgical, or other appliances may be placed in the circuit simultaneously or independently.
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Hubbell, Harry C., 'Portable Electrical Lamp', U.S. Patent 753,138, issued February 23, 1904
Excerpt:
I, HARRY C. HUBBELL, a citizen of the United States, residing at Brooklyn, in the county of Kings and State of New a portable electric lamp for miners use or for other purposes which will be safe, efficient, and durable.
Hubbell, Harvey. 'Attachment-plug.' U.S. Patent 783,275, issued February 21, 1905.
Hubbell, Harvey. 'Flush attachment-plug.' U.S. Patent 857,176, issued June 18, 1907.
Hubbell, Harvey. 'Self-closing flush-receptacle plate.' U.S. Patent 961,720, issued June 14, 1910.
Hubbell, Harvey. 'Pull-socket.' U.S. Patent 1,053,176, issued February 18, 1913.
Krantz, Hubert. 'Electrical outlet-box.' U.S. Patent 740,663, issued October 6, 1903.
Excerpt:
Be it known that I, HUBERT KRANTZ, a citizen of the United States of America, residing in the borough of Brooklyn, county of Kings, State of New York, have invented Improved Electrical Outlet-Boxes, of which the following is a specification.
The main object of my invention is to provide a simple and eificient electrical outletbox the face of which can be readily adjusted toward or from the body of the box as the particular place of use may require, and this without interfering with the wires in the interior of the box.
Thomas, George B. 'Electrical plug-receptacle.' U.S. Patent 952,961, issued March 22, 1910. - illustrated above.
Excerpt:
Be it known that I, GEORGE B. THoMAs, a citizen of the United States of America, and residing at Bridgeport, in the county of Fairfield and State of Connecticut, have invented a certain new and useful Improvement in Electrical Plug-Receptacles, of which the following is a specification.
My invention relates to plug receptacles and particularly to flush receptacles of the type described in Goodridge 7 54,863 the object of my -invention being to improve in several features the construction shown therein. 240VAC Electrical Receptacles
There are several common appliances or tools that require a 240-Volt-AC electrical receptacle and a number of 240VAC (or 220 VAC or 204VAC depending on where you live) electrical receptacle designs.
When wiring a 240V receptacle the circuit wires used will include
A black 'hot' wire (120V)
A red 'hot' wire (120V) normally from separate circuit that is on the opposite side or different elecrical phase in the electrical panel from the black wire
A grounding conductor wire
No neutral wire is typically connected to this device. The 240V power is achieved across the two individual 120V 'hot' circuits.
Examples are listed below.
Our photo of a 240V electrical receptacle shown here is a 20-Amp 240-volt electrical outlet that would be installed on a 20-Amp wall or window air conditioner circuit in a building.
This particular recetpacle, a Leviton product, is UL-listed for 20 Amp, 250 Volt, NEMA 6-20R, 2-pole, 3-wire circutis; it's further described as a narrow-body single receptacle, straight blade, commercial grade, grounding, side-wired, steel strap; available in ivory, white, brown. Similar products are sold in North America by Cooper, Eagle Wiring, and Pass Seymour.
240-Volt window or through-wall room air conditioner
240-Volt equipment such as small welders
Below are two additional configurations and types of 240 VAC electrical receptacles.
The white three-bladed 240-V receptacle below is a 20-Amp Leviton product that is a non-grounding type electrical receptacle. This is a 20A, 125/250 V, NEMA 10-20R, 2P, 3W, narrow-body single receptacle, straight bladeed, non-grounding, side-wired, steel strap device.
The orange 240 V receptacle below is a also a Leviton product, a flush-mounting twistlock single 20-Amp receptacle wired to provide an isolated ground and is described as a 20-A, 250V, NEMA L6-20R, 2P, 3W, flush mount locking receptacle, industrial grade with locking ground.
...
Continue reading at BACK-WIRED ELECTRICAL DEVICES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
HOW TO ADD an ELECTRICAL OUTLET - home
GFCI PROTECTION, GFCI CODES - ground-fault-circuit-interruptor receptacles (and circuit breakers) Suggested citation for this web page
ELECTRICAL RECEPTACLE TYPES at Inspect A pedia.com - online encyclopedia of building environmental inspection, testing, diagnosis, repair, problem prevention advice. INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION TESTING
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[1] ... Books Articles on Building Environmental Inspection, Testing, Diagnosis, Repair
Our recommended books about building mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Timothy Hemm has provided photographs of various electrical defects used at the Inspect A Pedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor [Website: /www.house-whisperer.com ] is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-873-8534 or by Email: johncranor@verizon.net
[3] NFPA - the National Fire Protection Association can be found online at www.nfpa.org
[4] The 2008 NEC National Electrical Code (ISBN 978-0877657903) Online Access LINK (you'll need to sign in as a professional or as a visitor)
[5] Special thanks to our reader Steve who pointed out prior errors in our illustrations.
[6] Simpson Strong-Tie, 'Code Compliant Repair and Protection Guide for the Installation of Utilities in Wood Frame Construction', web search 5/21/12, original source strongtie.com/ftp/fliers/F-REPRPROTECT09.pdf, [copy on file as /Structures/Framing/Simpson_Framing_Protectors.pdf ]. 'The information in this guide is a summary of requirementsfrom the 2003, 2006 and 2009 International Residential Code(IRC), International Building Code (IBC), International PlumbingCode (IPC), International Mechanical Code (IMC), 2006 UniformPlumbing Code (UPC) and the 2005 National Electrical Code.'
'Electrical System Inspection Basics,' Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'How to plan and install electric wiring for homes, farms, garages, shops,' Montgomery Ward Co., 83-850.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'Home Wiring Inspection,' Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
'Evaluating Wiring in Older Minnesota Homes,' Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
'Electrical Systems,' A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
'Basic Housing Inspection,' US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
...
Carson, Dunlop Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. The firm provides professional HOME INSPECTION SERVICES and also extensive HOME INSPECTION EDUCATION and home inspection-related PUBLICATIONS. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book illustrations from The Illustrated Home . Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The ILLUSTRATED HOME illustrates construction details and building components, a reference for owners inspectors.
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The HOME REFERENCE BOOK - the Encyclopedia of Homes, Carson Dunlop Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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Types of electrical receptacles: how to select the right type of electrical receptacle (outlet):
It's important to use 20-A rated receptacles if the electrical circuit is a 20-amp circuit. Don't install a grounded electrical receptacle plug on a circuit that has no electrical ground. Remember to install AFCI or GFCI devices where they are required.
This article explains how to match the receptacle type to the circuit type and use. We describe both 120V receptacles and 240V receptacles.
This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or 'plugs' or 'sockets') are simple devices that are easy to install, but there are details to get right if you want to be safe.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need. Choose the Proper Electrical Receptacle Type
The proper type of electrical receptacle must be selected : some receptacles are rated only for 15-Amp circuits and must not be installed on a 20-Amp circuit.
20-Amp electrical receptacles may be designed to only accept plugs for 20-Amp appliances (which may have a different plug-spade configuration in which one of the plug terminals is twisted to be at 90 degrees to the other).
[Click to enlarge any image]
Some 20-Amp electrical receptacles are designed to accept either conventional plugs used by a 15-A appliance as well as 20-A plugs used by a 20-A appliance.
It's generally ok to plug a 15-A appliance into a 20-A circuit since that appliance is not going to overload the circuit in normal use. But the opposite is not true 20 Amp T Slot Receptacle Wall Plate
. If you plug a 20-Amp appliance into a 15-Amp circuit you are risking overloading the circuit and tripping the circuit breaker, blowing the fuse, or worse, overheating the circuit and risking a fire.
Below our photographs illustrate a 15-Amp grounded electrical (below left) and a 20-Amp grounded electrical receptacle (below right).
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
Details about how to wire up an electrical receptacle are at ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires . Two-Wire Electrical Receptacle Circuits
Older two-wire electrical circuits , such as the two circuits depicted at the right of our sketch above may provide only the hot and neutral wires and no ground wire.
If no ground wire or ground path is provided, it is improper and unsafe to install a grounding (3-prong) electrical receptacle on that circuit.
Our photo (above left) shows a conventional grounded three-prong electrical receptacle - the round hole is the ground connection - at the left end of the picture closest to my thumb.
At right in the photo is an ungrounded electrical receptacle. This is the right device to install if no ground is present on the electrical circuit.
You don't want to 'fool' a building occupant into thinking that a ground is present when there is not one, so you don't install a receptacle that has that third ground opening in its face.
Some older two-wire circuits which are covered with a flexible metal jacket ('BX' or 'armored cable' wire) may provide a ground path by means of the cable jacket itself.
We don't rely on it, and in event of certain short circuits it's unsafe: the exposed metal sheathing of the wire becomes live, risking a shock.
Details about how to wire up an un-grounded receptacle are at 2-WIRE RECEPTACLE CONNECTIONS - no ground
The illustration at above-left shows the typical wiring of an electrical outlet or 'receptacle', courtesy of Carson Dunlop Associates.
Click any image to see an enlarged, detailed view of electrical wiring details for 'plugs' or electrical receptacles. Choosing GFCI and AFCI Receptacles
Ground fault protection - GFCI's: The NEC also requires that only special ground fault circuit interrupter (GFCI) protected outlets can be installed in certain hazardous locations like kitchens, baths, garages, outdoors.
A GFCI-protected electrical receptacle includes circuitry that turns the electric power off at the outlet quickly should a ground-fault (electricity flowing to earth, such as through your hand and down a water pipe) be detected. [4]
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. [4]
AFCI's are similar to GFCI's discussed above, but they include an additional level of protection against fire by detecting small electrical arcing at a connection - a condition that can lead to overheating and fire.
As you can see from this US CPSC photo, you can add Arc fault protection to a home circuit by installing a special circuit breaker in the electrical panel.
By this means you can provide arc fault protection and thus improved fire safety for all electrical outlets on the circuit - for example in the building's bedrooms
In the FAQs (below) we discuss the importance of wiring the Line and Load terminals of GFCIs and AFCIs correctly.
See AFCI GFCI WIRING, TESTING SAFETY
and AFCIs ARC FAULT CIRCUIT INTERRUPTERS for details about these devices.
Readers of this article should also see SAFETY for ELECTRICAL INSPECTORS.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Question: Is it safe to plug a 10-Amp A/C into an outlet that is back wired?
is it safe to plug in an ac unit that runs 10 amps , into outlet that is backed wired, i had read that you don't like this method, the outlet is on third floor and is on a 15 amp breaker - Johnny B 5/2/12 Reply: Comparing Three Types of Backwired Receptacles: 20-A Clamp Type 15-ASpring Type Clamp Type Backwiring Devices
Johnny, that's an interesting question and one I'm scared to answer - by online posting one cannot assure the electrical safety of your building.
Details about back-wired electrical devices (receptacles switches) are at BACK-WIRED ELECTRICAL DEVICES.
Here we will illustrate three different types of electrical receptacles that can be wired from their back-side.
Our photo (left) illustrates a spec-grade 20-Amp, 125V rated electrical receptacle that looks as if it is 'back-wired' - in fact while a wire can be wrapped around the terminal screws on this device.
The screw is intended to be used to tighten a rectangular brass plate against a square metal nut (silver in color) that makes a very strong and positive connection over a good area of wire surface.
This receptacle is marked on its back surface as CU Wire Only - copper only . [Click images to see enlarged details.]
That said, I agree that older, spring-type back-wired electrical connections (shown at below left) are not as reliable as connections made under a screw or clamp, as the total contact area between the back-wire spring edge and the wire surface is minimal.
Nevertheless, on a 15-A circuit using 15-A devices such as receptacles, the circuit and its devices are rated and intended to be able to support the 10-amp load you describe, so long as the sum of all of the items plugged into that electrical circuit don't overload it.
Contractor-grade 15-A spring-type -connector back-wired electrical receptacles (below left) provide a single opening at each of the four terminals (two neutral wires, two hot wires) on the back of the receptacle (red arrow).
The yellow arrow points to a release spring that will allow removal of the wire, but we prefer not to re-use this type of back-wired receptacle. Tightening the screw at the main wire terminal (blue arrow) has nothing to do with the spring-clamp that is securing the back-wired terminal wire.
Some newer heavy-duty 15-A back-wired electrical receptacles (above right) o not rely on a simple spring-edge to contact the electrical wire, as we illustrate in our second photo (above right).
Rather, when the wire is inserted into a receiving hole on the back of the receptacle (either of the two red arrows).
When the terminal screw is tightened (blue arrow) that actually snugs up a clamp that contacts a much larger surface area of the back-wired wire.
That's a more secure connection mechanically. On this receptacle, instead on a single back terminal accepting a single wire, there are a pair of back terminal openings at each of the four terminal screws.
Follow-up:
Thank you for responding, my town home was built in 1999, not sure if that is considered newer or older, lights do dim though when i use 10 amp vacuum . - Johnny B.
Reply:
Backwiring electrical receptacles is a permitted installation and might be found in a 1999 home - but as we show above, there are two different approaches, the second of which is a better quality installation and is in our opinion more reliable. Older Antique Electrical Receptacles or 'Outlets'
At above and below are photos of an un-grounded electrical receptacle found in many older homes where the electrical circuit wiring did not include a grounding conductor.
These receptacles can still be purchased and are the only receptacle type that should be installed on an un-grounded electrical receptacle circuit as the absence of a third ground-prong opening makes clear to the user that no ground is present.
See FALSE GROUND at RECEPTACLES for a discussion of how this particular receptacle enjoyed giving the author a shock.
Above at right is an antique surface-mount plastic or bakelite wall receptacle or 'wall plug' or 'wall outlet' in common parlance. This installation is not just obsolete but improper and unsafe as you can see exposed wires poking through the wall trim below the receptacle. Property inspector Steve Smallman, Raleigh NC, who contributed this receptacle points out that
We expect the receptacle to have been designed for a 20A circuit based on the Tee-shaped slot
The receptacle has been heavily painted over, interfering with its use
The wiring exposed below the receptacle is stripped-back armored cable ('BX') without proper strain relief or protection.
If the actual circuit is powered by a 15A circuit breaker or fuse, the fact that it pretends to offer 20A is mitigated by proper fusing but could also cause trouble for the homeowner as it invites overloading.
Thanks to frequent InspectApedia contributor also cited at ABOUT Inspect A pedia.com
Steve Smallman, Raleigh, NC, Email: steve@stevesmallman.com, Website: http://stevesmallman.com/ Tel: 919-669-3639 - Quoting: Steve Smallman Property Inspections (SSPI) inspectors have performed or supervised over 25,000 inspections since we introduced home inspections to the Triangle area in 1980. Mr. Smallman, located in Raleigh N.C., is a contributor to InspectApedia.com and has commented on or provided information on plumbing traps, electrical wiring, commercial FPE electrical panels and DIY Tests of FPE equipment, roofing underlayment, and building exteriors.
Below are two versions of another antiquated and complex electrical receptacle.
I think that these electrical receptacles provide either 120V or 240V connections depending on the receptacle and slots used and on the actual wiring present. Comments are invited.
Below: an older Harvey Hubbell (1857-1927) design electrical receptacle designed to accept 120VAC wall plugs sporting either the 'modern' 15A standard parallel blade design OR the older tandem design.
Hubbell, a prolific inventor of electrical receptacles, plugs, and also bulb sockets and controls, designed and patented a number of key electrical receptacles that are cited below.
Bossert, William F. 'Outlet-box for interior conduits.' U.S. Patent 754,414, issued March 15, 1904.
Hubbell Inc. HUBBELL CORPORATE HISTORY [PDF] Hubbell Inc., 584 Derby Milford Road Orange, CT 06477-4024 USA, Tel: (203) 799-4100 Website:
Hubbell Inc. HUBBELL ELECTRIC CO CATALOG 1906 [PDF] Western Electric Company, Eleventh York Sts, Philadelphia PA USA, retrieved 2018/12/20, original source https://ia600607.us.archive.org/14/items/ElectricalSpecialitiesCatalogueAndPriceListNo.9August1906/Cca49543WesternElectricCo..pdf 20 Amp T Slot Receptacle Adapter
Below: illustration from Harvey Hubbell's Separable Attachment Plug patent from 1904.
[Click to enlarge any image]
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Excerpt:
I have devised the simple and novel separable attachment-plug which I will now describe, referring to the accompanying drawings forming part of this specification
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,251, issued November 8, 1904.
Excerpt:
Be it known that I, HARVEY HUBBELL, acitizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Separable Attachment-Plug, of which the following is a specification.
My invention has for its general object to produce a practical, durable, and inexpensive direct-acting separable attachment-plug that will avoid the objections raised by the underwriters to attachment-plugs of this type now upon the market and shall have for its special objects to produce a direct-acting attachment-plug in which ample current-carrying capacity shall be provided independently of locking-springs, which may or may not be used, and in which the possibility of arcing or sparkingin making the contact shall be wholly done away with.
With these and other objects in view I have devised a novel attachment-plug in which a base is provided with insulating-chambers to receive contact-plates and locking-springs, if used, and with contracted insulating-passages leading to said chambers, in which the end block is made independent of the base and is secured thereto, covering the chambers, and in which an insulating-cap is provided with knife-blade contacts which engage the contact plates after they have been passed through the contracted insulating-passages.
It should be noted that the object in making the base and end block independent is to permit the formation of the insulating-chambers in the base, which receive the contact-plates and the locking-springs, if usechsaid chambers being covered by the end block, and, furthermore, that by providing contact-plates of ample current-carrying capacity in the chambers independent of the locking-springs it becomes immaterial, so far as the currentcarrying is concerned, whether or not locking-springs are used.
My present construction therefore enables me to avoid certain of the most serious objections raised by the underwriters to attachment-plugs heretofore placed upon the market.
Hubbell, Harvey. 'Cartridge-fuse and fuse-block.' U.S. Patent 819,657, issued May 1, 1906.
Photo above: a round pin receptacle, possibly with ground and missing one mounting screw, possibly an early Hubbell-design round two-pin floor receptacle, courtesy of Bay State Home Inspections and discussed originally
This receptacle is also similar to George Thomas' receptacle and plug design from 1910 and cited below.
at OLD ELECTRICAL WIRING TYPES
Below: an excerpt from the patent illustration for Harvey Hubbell's multiple attachment plug patent issued in 1904.
Knife blade (flat) plug and receptacle connectors designed by Hubbell were produced as early as 1904 and became the dominant connector design used in the U.S. and Canada.
Hubbell, Harvey. 'Multiple attachment-plug.' U.S. Patent 776,326, issued November 29, 1904.
Excerpt:
I Harvey Hubbell, a citizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Multiple Attachment-Plug, of which the following is a specification.
My invention has for its object to provide a multiple attachment-plug which shall comprise a body adapted to engage floor, wall, or ceiling receptacles or other fixtures and a plurality of separable caps which shall be adapted to engage the body, so that a plurality of lights, groups of lights, fans, motors, heating, soldering, surgical, or other appliances may be placed in the circuit simultaneously or independently.
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Hubbell, Harry C., 'Portable Electrical Lamp', U.S. Patent 753,138, issued February 23, 1904
Excerpt:
I, HARRY C. HUBBELL, a citizen of the United States, residing at Brooklyn, in the county of Kings and State of New a portable electric lamp for miners use or for other purposes which will be safe, efficient, and durable.
Hubbell, Harvey. 'Attachment-plug.' U.S. Patent 783,275, issued February 21, 1905.
Hubbell, Harvey. 'Flush attachment-plug.' U.S. Patent 857,176, issued June 18, 1907.
Hubbell, Harvey. 'Self-closing flush-receptacle plate.' U.S. Patent 961,720, issued June 14, 1910.
Hubbell, Harvey. 'Pull-socket.' U.S. Patent 1,053,176, issued February 18, 1913.
Krantz, Hubert. 'Electrical outlet-box.' U.S. Patent 740,663, issued October 6, 1903.
Excerpt:
Be it known that I, HUBERT KRANTZ, a citizen of the United States of America, residing in the borough of Brooklyn, county of Kings, State of New York, have invented Improved Electrical Outlet-Boxes, of which the following is a specification.
The main object of my invention is to provide a simple and eificient electrical outletbox the face of which can be readily adjusted toward or from the body of the box as the particular place of use may require, and this without interfering with the wires in the interior of the box.
Thomas, George B. 'Electrical plug-receptacle.' U.S. Patent 952,961, issued March 22, 1910. - illustrated above.
Excerpt:
Be it known that I, GEORGE B. THoMAs, a citizen of the United States of America, and residing at Bridgeport, in the county of Fairfield and State of Connecticut, have invented a certain new and useful Improvement in Electrical Plug-Receptacles, of which the following is a specification.
My invention relates to plug receptacles and particularly to flush receptacles of the type described in Goodridge 7 54,863 the object of my -invention being to improve in several features the construction shown therein. 240VAC Electrical Receptacles
There are several common appliances or tools that require a 240-Volt-AC electrical receptacle and a number of 240VAC (or 220 VAC or 204VAC depending on where you live) electrical receptacle designs.
When wiring a 240V receptacle the circuit wires used will include
A black 'hot' wire (120V)
A red 'hot' wire (120V) normally from separate circuit that is on the opposite side or different elecrical phase in the electrical panel from the black wire
A grounding conductor wire
No neutral wire is typically connected to this device. The 240V power is achieved across the two individual 120V 'hot' circuits.
Examples are listed below.
Our photo of a 240V electrical receptacle shown here is a 20-Amp 240-volt electrical outlet that would be installed on a 20-Amp wall or window air conditioner circuit in a building.
This particular recetpacle, a Leviton product, is UL-listed for 20 Amp, 250 Volt, NEMA 6-20R, 2-pole, 3-wire circutis; it's further described as a narrow-body single receptacle, straight blade, commercial grade, grounding, side-wired, steel strap; available in ivory, white, brown. Similar products are sold in North America by Cooper, Eagle Wiring, and Pass Seymour.
240-Volt window or through-wall room air conditioner
240-Volt equipment such as small welders
Below are two additional configurations and types of 240 VAC electrical receptacles.
The white three-bladed 240-V receptacle below is a 20-Amp Leviton product that is a non-grounding type electrical receptacle. This is a 20A, 125/250 V, NEMA 10-20R, 2P, 3W, narrow-body single receptacle, straight bladeed, non-grounding, side-wired, steel strap device.
The orange 240 V receptacle below is a also a Leviton product, a flush-mounting twistlock single 20-Amp receptacle wired to provide an isolated ground and is described as a 20-A, 250V, NEMA L6-20R, 2P, 3W, flush mount locking receptacle, industrial grade with locking ground.
...
Continue reading at BACK-WIRED ELECTRICAL DEVICES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
HOW TO ADD an ELECTRICAL OUTLET - home
GFCI PROTECTION, GFCI CODES - ground-fault-circuit-interruptor receptacles (and circuit breakers) Suggested citation for this web page
ELECTRICAL RECEPTACLE TYPES at Inspect A pedia.com - online encyclopedia of building environmental inspection, testing, diagnosis, repair, problem prevention advice. INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION TESTING
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[1] ... Books Articles on Building Environmental Inspection, Testing, Diagnosis, Repair
Our recommended books about building mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Timothy Hemm has provided photographs of various electrical defects used at the Inspect A Pedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor [Website: /www.house-whisperer.com ] is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-873-8534 or by Email: johncranor@verizon.net
[3] NFPA - the National Fire Protection Association can be found online at www.nfpa.org
[4] The 2008 NEC National Electrical Code (ISBN 978-0877657903) Online Access LINK (you'll need to sign in as a professional or as a visitor)
[5] Special thanks to our reader Steve who pointed out prior errors in our illustrations.
[6] Simpson Strong-Tie, 'Code Compliant Repair and Protection Guide for the Installation of Utilities in Wood Frame Construction', web search 5/21/12, original source strongtie.com/ftp/fliers/F-REPRPROTECT09.pdf, [copy on file as /Structures/Framing/Simpson_Framing_Protectors.pdf ]. 'The information in this guide is a summary of requirementsfrom the 2003, 2006 and 2009 International Residential Code(IRC), International Building Code (IBC), International PlumbingCode (IPC), International Mechanical Code (IMC), 2006 UniformPlumbing Code (UPC) and the 2005 National Electrical Code.'
'Electrical System Inspection Basics,' Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'How to plan and install electric wiring for homes, farms, garages, shops,' Montgomery Ward Co., 83-850.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'Home Wiring Inspection,' Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
'Evaluating Wiring in Older Minnesota Homes,' Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
'Electrical Systems,' A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
'Basic Housing Inspection,' US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
...
Carson, Dunlop Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. The firm provides professional HOME INSPECTION SERVICES and also extensive HOME INSPECTION EDUCATION and home inspection-related PUBLICATIONS. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book illustrations from The Illustrated Home . Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The ILLUSTRATED HOME illustrates construction details and building components, a reference for owners inspectors.
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The HOME REFERENCE BOOK - the Encyclopedia of Homes, Carson Dunlop Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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A 20-amp outlet is only compatible with a 20-amp circuit. The circuit's ampere rating can be found by checking the breaker box. A 20-amp outlet is identifiable by horizontal slots that cross the more typical vertical slots of a normal outlet. This kind of outlet is served by either 12-gauge or 10-gauge wiring.
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
The 20 denotes the device is rated for 20 amps. The 'R' at the end denotes that this is a receptacle. The plug side would be a 'P'. The purpose for this outlet is to allow a variety of devices to be plugged in as long as the device does not exceed 125 volts and 20 amps.
Types of electrical receptacles: how to select the right type of electrical receptacle (outlet):
It's important to use 20-A rated receptacles if the electrical circuit is a 20-amp circuit. Don't install a grounded electrical receptacle plug on a circuit that has no electrical ground. Remember to install AFCI or GFCI devices where they are required.
This article explains how to match the receptacle type to the circuit type and use. We describe both 120V receptacles and 240V receptacles.
This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or 'plugs' or 'sockets') are simple devices that are easy to install, but there are details to get right if you want to be safe.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need. Choose the Proper Electrical Receptacle Type
The proper type of electrical receptacle must be selected : some receptacles are rated only for 15-Amp circuits and must not be installed on a 20-Amp circuit.
20-Amp electrical receptacles may be designed to only accept plugs for 20-Amp appliances (which may have a different plug-spade configuration in which one of the plug terminals is twisted to be at 90 degrees to the other).
[Click to enlarge any image]
Some 20-Amp electrical receptacles are designed to accept either conventional plugs used by a 15-A appliance as well as 20-A plugs used by a 20-A appliance.
It's generally ok to plug a 15-A appliance into a 20-A circuit since that appliance is not going to overload the circuit in normal use. But the opposite is not true
. If you plug a 20-Amp appliance into a 15-Amp circuit you are risking overloading the circuit and tripping the circuit breaker, blowing the fuse, or worse, overheating the circuit and risking a fire.
Below our photographs illustrate a 15-Amp grounded electrical (below left) and a 20-Amp grounded electrical receptacle (below right).
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
Details about how to wire up an electrical receptacle are at ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires . Two-Wire Electrical Receptacle Circuits
Older two-wire electrical circuits , such as the two circuits depicted at the right of our sketch above may provide only the hot and neutral wires and no ground wire. 20 Amp T Slot Receptacle 30 Amp
If no ground wire or ground path is provided, it is improper and unsafe to install a grounding (3-prong) electrical receptacle on that circuit.
Our photo (above left) shows a conventional grounded three-prong electrical receptacle - the round hole is the ground connection - at the left end of the picture closest to my thumb.
At right in the photo is an ungrounded electrical receptacle. This is the right device to install if no ground is present on the electrical circuit.
You don't want to 'fool' a building occupant into thinking that a ground is present when there is not one, so you don't install a receptacle that has that third ground opening in its face.
Some older two-wire circuits which are covered with a flexible metal jacket ('BX' or 'armored cable' wire) may provide a ground path by means of the cable jacket itself.
We don't rely on it, and in event of certain short circuits it's unsafe: the exposed metal sheathing of the wire becomes live, risking a shock.
Details about how to wire up an un-grounded receptacle are at 2-WIRE RECEPTACLE CONNECTIONS - no ground
The illustration at above-left shows the typical wiring of an electrical outlet or 'receptacle', courtesy of Carson Dunlop Associates.
Click any image to see an enlarged, detailed view of electrical wiring details for 'plugs' or electrical receptacles. Choosing GFCI and AFCI Receptacles
Ground fault protection - GFCI's: The NEC also requires that only special ground fault circuit interrupter (GFCI) protected outlets can be installed in certain hazardous locations like kitchens, baths, garages, outdoors.
A GFCI-protected electrical receptacle includes circuitry that turns the electric power off at the outlet quickly should a ground-fault (electricity flowing to earth, such as through your hand and down a water pipe) be detected. [4]
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. [4]
AFCI's are similar to GFCI's discussed above, but they include an additional level of protection against fire by detecting small electrical arcing at a connection - a condition that can lead to overheating and fire.
As you can see from this US CPSC photo, you can add Arc fault protection to a home circuit by installing a special circuit breaker in the electrical panel.
By this means you can provide arc fault protection and thus improved fire safety for all electrical outlets on the circuit - for example in the building's bedrooms
In the FAQs (below) we discuss the importance of wiring the Line and Load terminals of GFCIs and AFCIs correctly.
See AFCI GFCI WIRING, TESTING SAFETY
and AFCIs ARC FAULT CIRCUIT INTERRUPTERS for details about these devices.
Readers of this article should also see SAFETY for ELECTRICAL INSPECTORS.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Question: Is it safe to plug a 10-Amp A/C into an outlet that is back wired?
is it safe to plug in an ac unit that runs 10 amps , into outlet that is backed wired, i had read that you don't like this method, the outlet is on third floor and is on a 15 amp breaker - Johnny B 5/2/12 Reply: Comparing Three Types of Backwired Receptacles: 20-A Clamp Type 15-ASpring Type Clamp Type Backwiring Devices
Johnny, that's an interesting question and one I'm scared to answer - by online posting one cannot assure the electrical safety of your building.
Details about back-wired electrical devices (receptacles switches) are at BACK-WIRED ELECTRICAL DEVICES.
Here we will illustrate three different types of electrical receptacles that can be wired from their back-side.
Our photo (left) illustrates a spec-grade 20-Amp, 125V rated electrical receptacle that looks as if it is 'back-wired' - in fact while a wire can be wrapped around the terminal screws on this device.
The screw is intended to be used to tighten a rectangular brass plate against a square metal nut (silver in color) that makes a very strong and positive connection over a good area of wire surface.
This receptacle is marked on its back surface as CU Wire Only - copper only . [Click images to see enlarged details.]
That said, I agree that older, spring-type back-wired electrical connections (shown at below left) are not as reliable as connections made under a screw or clamp, as the total contact area between the back-wire spring edge and the wire surface is minimal.
Nevertheless, on a 15-A circuit using 15-A devices such as receptacles, the circuit and its devices are rated and intended to be able to support the 10-amp load you describe, so long as the sum of all of the items plugged into that electrical circuit don't overload it.
Contractor-grade 15-A spring-type -connector back-wired electrical receptacles (below left) provide a single opening at each of the four terminals (two neutral wires, two hot wires) on the back of the receptacle (red arrow).
The yellow arrow points to a release spring that will allow removal of the wire, but we prefer not to re-use this type of back-wired receptacle. Tightening the screw at the main wire terminal (blue arrow) has nothing to do with the spring-clamp that is securing the back-wired terminal wire.
Some newer heavy-duty 15-A back-wired electrical receptacles (above right) o not rely on a simple spring-edge to contact the electrical wire, as we illustrate in our second photo (above right).
Rather, when the wire is inserted into a receiving hole on the back of the receptacle (either of the two red arrows).
When the terminal screw is tightened (blue arrow) that actually snugs up a clamp that contacts a much larger surface area of the back-wired wire.
That's a more secure connection mechanically. On this receptacle, instead on a single back terminal accepting a single wire, there are a pair of back terminal openings at each of the four terminal screws.
Follow-up:
Thank you for responding, my town home was built in 1999, not sure if that is considered newer or older, lights do dim though when i use 10 amp vacuum . - Johnny B.
Reply:
Backwiring electrical receptacles is a permitted installation and might be found in a 1999 home - but as we show above, there are two different approaches, the second of which is a better quality installation and is in our opinion more reliable. Older Antique Electrical Receptacles or 'Outlets'
At above and below are photos of an un-grounded electrical receptacle found in many older homes where the electrical circuit wiring did not include a grounding conductor.
These receptacles can still be purchased and are the only receptacle type that should be installed on an un-grounded electrical receptacle circuit as the absence of a third ground-prong opening makes clear to the user that no ground is present.
See FALSE GROUND at RECEPTACLES for a discussion of how this particular receptacle enjoyed giving the author a shock.
Above at right is an antique surface-mount plastic or bakelite wall receptacle or 'wall plug' or 'wall outlet' in common parlance. This installation is not just obsolete but improper and unsafe as you can see exposed wires poking through the wall trim below the receptacle. Property inspector Steve Smallman, Raleigh NC, who contributed this receptacle points out that
We expect the receptacle to have been designed for a 20A circuit based on the Tee-shaped slot
The receptacle has been heavily painted over, interfering with its use
The wiring exposed below the receptacle is stripped-back armored cable ('BX') without proper strain relief or protection.
If the actual circuit is powered by a 15A circuit breaker or fuse, the fact that it pretends to offer 20A is mitigated by proper fusing but could also cause trouble for the homeowner as it invites overloading.
Thanks to frequent InspectApedia contributor also cited at ABOUT Inspect A pedia.com
Steve Smallman, Raleigh, NC, Email: steve@stevesmallman.com, Website: http://stevesmallman.com/ Tel: 919-669-3639 - Quoting: Steve Smallman Property Inspections (SSPI) inspectors have performed or supervised over 25,000 inspections since we introduced home inspections to the Triangle area in 1980. Mr. Smallman, located in Raleigh N.C., is a contributor to InspectApedia.com and has commented on or provided information on plumbing traps, electrical wiring, commercial FPE electrical panels and DIY Tests of FPE equipment, roofing underlayment, and building exteriors.
Below are two versions of another antiquated and complex electrical receptacle.
I think that these electrical receptacles provide either 120V or 240V connections depending on the receptacle and slots used and on the actual wiring present. Comments are invited.
Below: an older Harvey Hubbell (1857-1927) design electrical receptacle designed to accept 120VAC wall plugs sporting either the 'modern' 15A standard parallel blade design OR the older tandem design.
Hubbell, a prolific inventor of electrical receptacles, plugs, and also bulb sockets and controls, designed and patented a number of key electrical receptacles that are cited below.
Bossert, William F. 'Outlet-box for interior conduits.' U.S. Patent 754,414, issued March 15, 1904.
Hubbell Inc. HUBBELL CORPORATE HISTORY [PDF] Hubbell Inc., 584 Derby Milford Road Orange, CT 06477-4024 USA, Tel: (203) 799-4100 Website:
Hubbell Inc. HUBBELL ELECTRIC CO CATALOG 1906 [PDF] Western Electric Company, Eleventh York Sts, Philadelphia PA USA, retrieved 2018/12/20, original source https://ia600607.us.archive.org/14/items/ElectricalSpecialitiesCatalogueAndPriceListNo.9August1906/Cca49543WesternElectricCo..pdf
Below: illustration from Harvey Hubbell's Separable Attachment Plug patent from 1904.
[Click to enlarge any image]
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Excerpt:
I have devised the simple and novel separable attachment-plug which I will now describe, referring to the accompanying drawings forming part of this specification
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,251, issued November 8, 1904.
Excerpt:
Be it known that I, HARVEY HUBBELL, acitizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Separable Attachment-Plug, of which the following is a specification.
My invention has for its general object to produce a practical, durable, and inexpensive direct-acting separable attachment-plug that will avoid the objections raised by the underwriters to attachment-plugs of this type now upon the market and shall have for its special objects to produce a direct-acting attachment-plug in which ample current-carrying capacity shall be provided independently of locking-springs, which may or may not be used, and in which the possibility of arcing or sparkingin making the contact shall be wholly done away with.
With these and other objects in view I have devised a novel attachment-plug in which a base is provided with insulating-chambers to receive contact-plates and locking-springs, if used, and with contracted insulating-passages leading to said chambers, in which the end block is made independent of the base and is secured thereto, covering the chambers, and in which an insulating-cap is provided with knife-blade contacts which engage the contact plates after they have been passed through the contracted insulating-passages.
It should be noted that the object in making the base and end block independent is to permit the formation of the insulating-chambers in the base, which receive the contact-plates and the locking-springs, if usechsaid chambers being covered by the end block, and, furthermore, that by providing contact-plates of ample current-carrying capacity in the chambers independent of the locking-springs it becomes immaterial, so far as the currentcarrying is concerned, whether or not locking-springs are used.
My present construction therefore enables me to avoid certain of the most serious objections raised by the underwriters to attachment-plugs heretofore placed upon the market.
Hubbell, Harvey. 'Cartridge-fuse and fuse-block.' U.S. Patent 819,657, issued May 1, 1906.
Photo above: a round pin receptacle, possibly with ground and missing one mounting screw, possibly an early Hubbell-design round two-pin floor receptacle, courtesy of Bay State Home Inspections and discussed originally
This receptacle is also similar to George Thomas' receptacle and plug design from 1910 and cited below.
at OLD ELECTRICAL WIRING TYPES
Below: an excerpt from the patent illustration for Harvey Hubbell's multiple attachment plug patent issued in 1904.
Knife blade (flat) plug and receptacle connectors designed by Hubbell were produced as early as 1904 and became the dominant connector design used in the U.S. and Canada.
Hubbell, Harvey. 'Multiple attachment-plug.' U.S. Patent 776,326, issued November 29, 1904.
Excerpt:
I Harvey Hubbell, a citizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Multiple Attachment-Plug, of which the following is a specification.
My invention has for its object to provide a multiple attachment-plug which shall comprise a body adapted to engage floor, wall, or ceiling receptacles or other fixtures and a plurality of separable caps which shall be adapted to engage the body, so that a plurality of lights, groups of lights, fans, motors, heating, soldering, surgical, or other appliances may be placed in the circuit simultaneously or independently.
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Hubbell, Harry C., 'Portable Electrical Lamp', U.S. Patent 753,138, issued February 23, 1904
Excerpt:
I, HARRY C. HUBBELL, a citizen of the United States, residing at Brooklyn, in the county of Kings and State of New a portable electric lamp for miners use or for other purposes which will be safe, efficient, and durable.
Hubbell, Harvey. 'Attachment-plug.' U.S. Patent 783,275, issued February 21, 1905.
Hubbell, Harvey. 'Flush attachment-plug.' U.S. Patent 857,176, issued June 18, 1907.
Hubbell, Harvey. 'Self-closing flush-receptacle plate.' U.S. Patent 961,720, issued June 14, 1910.
Hubbell, Harvey. 'Pull-socket.' U.S. Patent 1,053,176, issued February 18, 1913.
Krantz, Hubert. 'Electrical outlet-box.' U.S. Patent 740,663, issued October 6, 1903.
Excerpt:
Be it known that I, HUBERT KRANTZ, a citizen of the United States of America, residing in the borough of Brooklyn, county of Kings, State of New York, have invented Improved Electrical Outlet-Boxes, of which the following is a specification.
The main object of my invention is to provide a simple and eificient electrical outletbox the face of which can be readily adjusted toward or from the body of the box as the particular place of use may require, and this without interfering with the wires in the interior of the box.
Thomas, George B. 'Electrical plug-receptacle.' U.S. Patent 952,961, issued March 22, 1910. - illustrated above.
Excerpt:
Be it known that I, GEORGE B. THoMAs, a citizen of the United States of America, and residing at Bridgeport, in the county of Fairfield and State of Connecticut, have invented a certain new and useful Improvement in Electrical Plug-Receptacles, of which the following is a specification.
My invention relates to plug receptacles and particularly to flush receptacles of the type described in Goodridge 7 54,863 the object of my -invention being to improve in several features the construction shown therein. 240VAC Electrical Receptacles
There are several common appliances or tools that require a 240-Volt-AC electrical receptacle and a number of 240VAC (or 220 VAC or 204VAC depending on where you live) electrical receptacle designs.
When wiring a 240V receptacle the circuit wires used will include
A black 'hot' wire (120V)
A red 'hot' wire (120V) normally from separate circuit that is on the opposite side or different elecrical phase in the electrical panel from the black wire
A grounding conductor wire
No neutral wire is typically connected to this device. The 240V power is achieved across the two individual 120V 'hot' circuits.
Examples are listed below.
Our photo of a 240V electrical receptacle shown here is a 20-Amp 240-volt electrical outlet that would be installed on a 20-Amp wall or window air conditioner circuit in a building.
This particular recetpacle, a Leviton product, is UL-listed for 20 Amp, 250 Volt, NEMA 6-20R, 2-pole, 3-wire circutis; it's further described as a narrow-body single receptacle, straight blade, commercial grade, grounding, side-wired, steel strap; available in ivory, white, brown. Similar products are sold in North America by Cooper, Eagle Wiring, and Pass Seymour.
240-Volt window or through-wall room air conditioner
240-Volt equipment such as small welders
Below are two additional configurations and types of 240 VAC electrical receptacles.
The white three-bladed 240-V receptacle below is a 20-Amp Leviton product that is a non-grounding type electrical receptacle. This is a 20A, 125/250 V, NEMA 10-20R, 2P, 3W, narrow-body single receptacle, straight bladeed, non-grounding, side-wired, steel strap device.
The orange 240 V receptacle below is a also a Leviton product, a flush-mounting twistlock single 20-Amp receptacle wired to provide an isolated ground and is described as a 20-A, 250V, NEMA L6-20R, 2P, 3W, flush mount locking receptacle, industrial grade with locking ground.
...
Continue reading at BACK-WIRED ELECTRICAL DEVICES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
HOW TO ADD an ELECTRICAL OUTLET - home
GFCI PROTECTION, GFCI CODES - ground-fault-circuit-interruptor receptacles (and circuit breakers) Suggested citation for this web page
ELECTRICAL RECEPTACLE TYPES at Inspect A pedia.com - online encyclopedia of building environmental inspection, testing, diagnosis, repair, problem prevention advice. INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION TESTING
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[1] ... Books Articles on Building Environmental Inspection, Testing, Diagnosis, Repair
Our recommended books about building mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Timothy Hemm has provided photographs of various electrical defects used at the Inspect A Pedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor [Website: /www.house-whisperer.com ] is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-873-8534 or by Email: johncranor@verizon.net
[3] NFPA - the National Fire Protection Association can be found online at www.nfpa.org
[4] The 2008 NEC National Electrical Code (ISBN 978-0877657903) Online Access LINK (you'll need to sign in as a professional or as a visitor)
[5] Special thanks to our reader Steve who pointed out prior errors in our illustrations.
[6] Simpson Strong-Tie, 'Code Compliant Repair and Protection Guide for the Installation of Utilities in Wood Frame Construction', web search 5/21/12, original source strongtie.com/ftp/fliers/F-REPRPROTECT09.pdf, [copy on file as /Structures/Framing/Simpson_Framing_Protectors.pdf ]. 'The information in this guide is a summary of requirementsfrom the 2003, 2006 and 2009 International Residential Code(IRC), International Building Code (IBC), International PlumbingCode (IPC), International Mechanical Code (IMC), 2006 UniformPlumbing Code (UPC) and the 2005 National Electrical Code.'
'Electrical System Inspection Basics,' Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'How to plan and install electric wiring for homes, farms, garages, shops,' Montgomery Ward Co., 83-850.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'Home Wiring Inspection,' Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
'Evaluating Wiring in Older Minnesota Homes,' Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
'Electrical Systems,' A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
'Basic Housing Inspection,' US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
...
Carson, Dunlop Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. The firm provides professional HOME INSPECTION SERVICES and also extensive HOME INSPECTION EDUCATION and home inspection-related PUBLICATIONS. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book illustrations from The Illustrated Home . Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The ILLUSTRATED HOME illustrates construction details and building components, a reference for owners inspectors.
Special Offer : For a 5 discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page 'Promo/Redemption' space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop Weldon Associates
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The HOME REFERENCE BOOK - the Encyclopedia of Homes, Carson Dunlop Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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Types of electrical receptacles: how to select the right type of electrical receptacle (outlet):
It's important to use 20-A rated receptacles if the electrical circuit is a 20-amp circuit. Don't install a grounded electrical receptacle plug on a circuit that has no electrical ground. Remember to install AFCI or GFCI devices where they are required.
This article explains how to match the receptacle type to the circuit type and use. We describe both 120V receptacles and 240V receptacles.
This article series describes how to choose, locate, and wire an electrical receptacle in a home. Electrical receptacles (also called electrical outlets or 'plugs' or 'sockets') are simple devices that are easy to install, but there are details to get right if you want to be safe.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need. Choose the Proper Electrical Receptacle Type
The proper type of electrical receptacle must be selected : some receptacles are rated only for 15-Amp circuits and must not be installed on a 20-Amp circuit.
20-Amp electrical receptacles may be designed to only accept plugs for 20-Amp appliances (which may have a different plug-spade configuration in which one of the plug terminals is twisted to be at 90 degrees to the other).
[Click to enlarge any image]
Some 20-Amp electrical receptacles are designed to accept either conventional plugs used by a 15-A appliance as well as 20-A plugs used by a 20-A appliance.
It's generally ok to plug a 15-A appliance into a 20-A circuit since that appliance is not going to overload the circuit in normal use. But the opposite is not true 20 Amp T Slot Receptacle Wall Plate
. If you plug a 20-Amp appliance into a 15-Amp circuit you are risking overloading the circuit and tripping the circuit breaker, blowing the fuse, or worse, overheating the circuit and risking a fire.
Below our photographs illustrate a 15-Amp grounded electrical (below left) and a 20-Amp grounded electrical receptacle (below right).
You'll notice that the heavier-duty 20-Amp electrical receptacle has that T-slot at it's wider connection opening - an easy way to identify a wall receptacle rated for 20-Amps - provided that the receptacle was properly matched to the wire size and the circuit breaker or fuse size.
Details about how to wire up an electrical receptacle are at ELECTRICAL RECEPTACLE CONNECTION DETAILS - where to connect black, white, red, green, ground wires . Two-Wire Electrical Receptacle Circuits
Older two-wire electrical circuits , such as the two circuits depicted at the right of our sketch above may provide only the hot and neutral wires and no ground wire.
If no ground wire or ground path is provided, it is improper and unsafe to install a grounding (3-prong) electrical receptacle on that circuit.
Our photo (above left) shows a conventional grounded three-prong electrical receptacle - the round hole is the ground connection - at the left end of the picture closest to my thumb.
At right in the photo is an ungrounded electrical receptacle. This is the right device to install if no ground is present on the electrical circuit.
You don't want to 'fool' a building occupant into thinking that a ground is present when there is not one, so you don't install a receptacle that has that third ground opening in its face.
Some older two-wire circuits which are covered with a flexible metal jacket ('BX' or 'armored cable' wire) may provide a ground path by means of the cable jacket itself.
We don't rely on it, and in event of certain short circuits it's unsafe: the exposed metal sheathing of the wire becomes live, risking a shock.
Details about how to wire up an un-grounded receptacle are at 2-WIRE RECEPTACLE CONNECTIONS - no ground
The illustration at above-left shows the typical wiring of an electrical outlet or 'receptacle', courtesy of Carson Dunlop Associates.
Click any image to see an enlarged, detailed view of electrical wiring details for 'plugs' or electrical receptacles. Choosing GFCI and AFCI Receptacles
Ground fault protection - GFCI's: The NEC also requires that only special ground fault circuit interrupter (GFCI) protected outlets can be installed in certain hazardous locations like kitchens, baths, garages, outdoors.
A GFCI-protected electrical receptacle includes circuitry that turns the electric power off at the outlet quickly should a ground-fault (electricity flowing to earth, such as through your hand and down a water pipe) be detected. [4]
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. [4]
AFCI's are similar to GFCI's discussed above, but they include an additional level of protection against fire by detecting small electrical arcing at a connection - a condition that can lead to overheating and fire.
As you can see from this US CPSC photo, you can add Arc fault protection to a home circuit by installing a special circuit breaker in the electrical panel.
By this means you can provide arc fault protection and thus improved fire safety for all electrical outlets on the circuit - for example in the building's bedrooms
In the FAQs (below) we discuss the importance of wiring the Line and Load terminals of GFCIs and AFCIs correctly.
See AFCI GFCI WIRING, TESTING SAFETY
and AFCIs ARC FAULT CIRCUIT INTERRUPTERS for details about these devices.
Readers of this article should also see SAFETY for ELECTRICAL INSPECTORS.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Question: Is it safe to plug a 10-Amp A/C into an outlet that is back wired?
is it safe to plug in an ac unit that runs 10 amps , into outlet that is backed wired, i had read that you don't like this method, the outlet is on third floor and is on a 15 amp breaker - Johnny B 5/2/12 Reply: Comparing Three Types of Backwired Receptacles: 20-A Clamp Type 15-ASpring Type Clamp Type Backwiring Devices
Johnny, that's an interesting question and one I'm scared to answer - by online posting one cannot assure the electrical safety of your building.
Details about back-wired electrical devices (receptacles switches) are at BACK-WIRED ELECTRICAL DEVICES.
Here we will illustrate three different types of electrical receptacles that can be wired from their back-side.
Our photo (left) illustrates a spec-grade 20-Amp, 125V rated electrical receptacle that looks as if it is 'back-wired' - in fact while a wire can be wrapped around the terminal screws on this device.
The screw is intended to be used to tighten a rectangular brass plate against a square metal nut (silver in color) that makes a very strong and positive connection over a good area of wire surface.
This receptacle is marked on its back surface as CU Wire Only - copper only . [Click images to see enlarged details.]
That said, I agree that older, spring-type back-wired electrical connections (shown at below left) are not as reliable as connections made under a screw or clamp, as the total contact area between the back-wire spring edge and the wire surface is minimal.
Nevertheless, on a 15-A circuit using 15-A devices such as receptacles, the circuit and its devices are rated and intended to be able to support the 10-amp load you describe, so long as the sum of all of the items plugged into that electrical circuit don't overload it.
Contractor-grade 15-A spring-type -connector back-wired electrical receptacles (below left) provide a single opening at each of the four terminals (two neutral wires, two hot wires) on the back of the receptacle (red arrow).
The yellow arrow points to a release spring that will allow removal of the wire, but we prefer not to re-use this type of back-wired receptacle. Tightening the screw at the main wire terminal (blue arrow) has nothing to do with the spring-clamp that is securing the back-wired terminal wire.
Some newer heavy-duty 15-A back-wired electrical receptacles (above right) o not rely on a simple spring-edge to contact the electrical wire, as we illustrate in our second photo (above right).
Rather, when the wire is inserted into a receiving hole on the back of the receptacle (either of the two red arrows).
When the terminal screw is tightened (blue arrow) that actually snugs up a clamp that contacts a much larger surface area of the back-wired wire.
That's a more secure connection mechanically. On this receptacle, instead on a single back terminal accepting a single wire, there are a pair of back terminal openings at each of the four terminal screws.
Follow-up:
Thank you for responding, my town home was built in 1999, not sure if that is considered newer or older, lights do dim though when i use 10 amp vacuum . - Johnny B.
Reply:
Backwiring electrical receptacles is a permitted installation and might be found in a 1999 home - but as we show above, there are two different approaches, the second of which is a better quality installation and is in our opinion more reliable. Older Antique Electrical Receptacles or 'Outlets'
At above and below are photos of an un-grounded electrical receptacle found in many older homes where the electrical circuit wiring did not include a grounding conductor.
These receptacles can still be purchased and are the only receptacle type that should be installed on an un-grounded electrical receptacle circuit as the absence of a third ground-prong opening makes clear to the user that no ground is present.
See FALSE GROUND at RECEPTACLES for a discussion of how this particular receptacle enjoyed giving the author a shock.
Above at right is an antique surface-mount plastic or bakelite wall receptacle or 'wall plug' or 'wall outlet' in common parlance. This installation is not just obsolete but improper and unsafe as you can see exposed wires poking through the wall trim below the receptacle. Property inspector Steve Smallman, Raleigh NC, who contributed this receptacle points out that
We expect the receptacle to have been designed for a 20A circuit based on the Tee-shaped slot
The receptacle has been heavily painted over, interfering with its use
The wiring exposed below the receptacle is stripped-back armored cable ('BX') without proper strain relief or protection.
If the actual circuit is powered by a 15A circuit breaker or fuse, the fact that it pretends to offer 20A is mitigated by proper fusing but could also cause trouble for the homeowner as it invites overloading.
Thanks to frequent InspectApedia contributor also cited at ABOUT Inspect A pedia.com
Steve Smallman, Raleigh, NC, Email: steve@stevesmallman.com, Website: http://stevesmallman.com/ Tel: 919-669-3639 - Quoting: Steve Smallman Property Inspections (SSPI) inspectors have performed or supervised over 25,000 inspections since we introduced home inspections to the Triangle area in 1980. Mr. Smallman, located in Raleigh N.C., is a contributor to InspectApedia.com and has commented on or provided information on plumbing traps, electrical wiring, commercial FPE electrical panels and DIY Tests of FPE equipment, roofing underlayment, and building exteriors.
Below are two versions of another antiquated and complex electrical receptacle.
I think that these electrical receptacles provide either 120V or 240V connections depending on the receptacle and slots used and on the actual wiring present. Comments are invited.
Below: an older Harvey Hubbell (1857-1927) design electrical receptacle designed to accept 120VAC wall plugs sporting either the 'modern' 15A standard parallel blade design OR the older tandem design.
Hubbell, a prolific inventor of electrical receptacles, plugs, and also bulb sockets and controls, designed and patented a number of key electrical receptacles that are cited below.
Bossert, William F. 'Outlet-box for interior conduits.' U.S. Patent 754,414, issued March 15, 1904.
Hubbell Inc. HUBBELL CORPORATE HISTORY [PDF] Hubbell Inc., 584 Derby Milford Road Orange, CT 06477-4024 USA, Tel: (203) 799-4100 Website:
Hubbell Inc. HUBBELL ELECTRIC CO CATALOG 1906 [PDF] Western Electric Company, Eleventh York Sts, Philadelphia PA USA, retrieved 2018/12/20, original source https://ia600607.us.archive.org/14/items/ElectricalSpecialitiesCatalogueAndPriceListNo.9August1906/Cca49543WesternElectricCo..pdf 20 Amp T Slot Receptacle Adapter
Below: illustration from Harvey Hubbell's Separable Attachment Plug patent from 1904.
[Click to enlarge any image]
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Excerpt:
I have devised the simple and novel separable attachment-plug which I will now describe, referring to the accompanying drawings forming part of this specification
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,251, issued November 8, 1904.
Excerpt:
Be it known that I, HARVEY HUBBELL, acitizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Separable Attachment-Plug, of which the following is a specification.
My invention has for its general object to produce a practical, durable, and inexpensive direct-acting separable attachment-plug that will avoid the objections raised by the underwriters to attachment-plugs of this type now upon the market and shall have for its special objects to produce a direct-acting attachment-plug in which ample current-carrying capacity shall be provided independently of locking-springs, which may or may not be used, and in which the possibility of arcing or sparkingin making the contact shall be wholly done away with.
With these and other objects in view I have devised a novel attachment-plug in which a base is provided with insulating-chambers to receive contact-plates and locking-springs, if used, and with contracted insulating-passages leading to said chambers, in which the end block is made independent of the base and is secured thereto, covering the chambers, and in which an insulating-cap is provided with knife-blade contacts which engage the contact plates after they have been passed through the contracted insulating-passages.
It should be noted that the object in making the base and end block independent is to permit the formation of the insulating-chambers in the base, which receive the contact-plates and the locking-springs, if usechsaid chambers being covered by the end block, and, furthermore, that by providing contact-plates of ample current-carrying capacity in the chambers independent of the locking-springs it becomes immaterial, so far as the currentcarrying is concerned, whether or not locking-springs are used.
My present construction therefore enables me to avoid certain of the most serious objections raised by the underwriters to attachment-plugs heretofore placed upon the market.
Hubbell, Harvey. 'Cartridge-fuse and fuse-block.' U.S. Patent 819,657, issued May 1, 1906.
Photo above: a round pin receptacle, possibly with ground and missing one mounting screw, possibly an early Hubbell-design round two-pin floor receptacle, courtesy of Bay State Home Inspections and discussed originally
This receptacle is also similar to George Thomas' receptacle and plug design from 1910 and cited below.
at OLD ELECTRICAL WIRING TYPES
Below: an excerpt from the patent illustration for Harvey Hubbell's multiple attachment plug patent issued in 1904.
Knife blade (flat) plug and receptacle connectors designed by Hubbell were produced as early as 1904 and became the dominant connector design used in the U.S. and Canada.
Hubbell, Harvey. 'Multiple attachment-plug.' U.S. Patent 776,326, issued November 29, 1904.
Excerpt:
I Harvey Hubbell, a citizen of the United States, residing at Bridgeport, county of Fairfield, State of Connecticut, have invented a new and useful Multiple Attachment-Plug, of which the following is a specification.
My invention has for its object to provide a multiple attachment-plug which shall comprise a body adapted to engage floor, wall, or ceiling receptacles or other fixtures and a plurality of separable caps which shall be adapted to engage the body, so that a plurality of lights, groups of lights, fans, motors, heating, soldering, surgical, or other appliances may be placed in the circuit simultaneously or independently.
Hubbell, Harvey. 'Separable attachment-plug.' U.S. Patent 774,250, issued November 8, 1904.
Hubbell, Harry C., 'Portable Electrical Lamp', U.S. Patent 753,138, issued February 23, 1904
Excerpt:
I, HARRY C. HUBBELL, a citizen of the United States, residing at Brooklyn, in the county of Kings and State of New a portable electric lamp for miners use or for other purposes which will be safe, efficient, and durable.
Hubbell, Harvey. 'Attachment-plug.' U.S. Patent 783,275, issued February 21, 1905.
Hubbell, Harvey. 'Flush attachment-plug.' U.S. Patent 857,176, issued June 18, 1907.
Hubbell, Harvey. 'Self-closing flush-receptacle plate.' U.S. Patent 961,720, issued June 14, 1910.
Hubbell, Harvey. 'Pull-socket.' U.S. Patent 1,053,176, issued February 18, 1913.
Krantz, Hubert. 'Electrical outlet-box.' U.S. Patent 740,663, issued October 6, 1903.
Excerpt:
Be it known that I, HUBERT KRANTZ, a citizen of the United States of America, residing in the borough of Brooklyn, county of Kings, State of New York, have invented Improved Electrical Outlet-Boxes, of which the following is a specification.
The main object of my invention is to provide a simple and eificient electrical outletbox the face of which can be readily adjusted toward or from the body of the box as the particular place of use may require, and this without interfering with the wires in the interior of the box.
Thomas, George B. 'Electrical plug-receptacle.' U.S. Patent 952,961, issued March 22, 1910. - illustrated above.
Excerpt:
Be it known that I, GEORGE B. THoMAs, a citizen of the United States of America, and residing at Bridgeport, in the county of Fairfield and State of Connecticut, have invented a certain new and useful Improvement in Electrical Plug-Receptacles, of which the following is a specification.
My invention relates to plug receptacles and particularly to flush receptacles of the type described in Goodridge 7 54,863 the object of my -invention being to improve in several features the construction shown therein. 240VAC Electrical Receptacles
There are several common appliances or tools that require a 240-Volt-AC electrical receptacle and a number of 240VAC (or 220 VAC or 204VAC depending on where you live) electrical receptacle designs.
When wiring a 240V receptacle the circuit wires used will include
A black 'hot' wire (120V)
A red 'hot' wire (120V) normally from separate circuit that is on the opposite side or different elecrical phase in the electrical panel from the black wire
A grounding conductor wire
No neutral wire is typically connected to this device. The 240V power is achieved across the two individual 120V 'hot' circuits.
Examples are listed below.
Our photo of a 240V electrical receptacle shown here is a 20-Amp 240-volt electrical outlet that would be installed on a 20-Amp wall or window air conditioner circuit in a building.
This particular recetpacle, a Leviton product, is UL-listed for 20 Amp, 250 Volt, NEMA 6-20R, 2-pole, 3-wire circutis; it's further described as a narrow-body single receptacle, straight blade, commercial grade, grounding, side-wired, steel strap; available in ivory, white, brown. Similar products are sold in North America by Cooper, Eagle Wiring, and Pass Seymour.
240-Volt window or through-wall room air conditioner
240-Volt equipment such as small welders
Below are two additional configurations and types of 240 VAC electrical receptacles.
The white three-bladed 240-V receptacle below is a 20-Amp Leviton product that is a non-grounding type electrical receptacle. This is a 20A, 125/250 V, NEMA 10-20R, 2P, 3W, narrow-body single receptacle, straight bladeed, non-grounding, side-wired, steel strap device.
The orange 240 V receptacle below is a also a Leviton product, a flush-mounting twistlock single 20-Amp receptacle wired to provide an isolated ground and is described as a 20-A, 250V, NEMA L6-20R, 2P, 3W, flush mount locking receptacle, industrial grade with locking ground.
...
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HOW TO ADD an ELECTRICAL OUTLET - home
GFCI PROTECTION, GFCI CODES - ground-fault-circuit-interruptor receptacles (and circuit breakers) Suggested citation for this web page
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[1] ... Books Articles on Building Environmental Inspection, Testing, Diagnosis, Repair
Our recommended books about building mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Timothy Hemm has provided photographs of various electrical defects used at the Inspect A Pedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor [Website: /www.house-whisperer.com ] is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-873-8534 or by Email: johncranor@verizon.net
[3] NFPA - the National Fire Protection Association can be found online at www.nfpa.org
[4] The 2008 NEC National Electrical Code (ISBN 978-0877657903) Online Access LINK (you'll need to sign in as a professional or as a visitor)
[5] Special thanks to our reader Steve who pointed out prior errors in our illustrations.
[6] Simpson Strong-Tie, 'Code Compliant Repair and Protection Guide for the Installation of Utilities in Wood Frame Construction', web search 5/21/12, original source strongtie.com/ftp/fliers/F-REPRPROTECT09.pdf, [copy on file as /Structures/Framing/Simpson_Framing_Protectors.pdf ]. 'The information in this guide is a summary of requirementsfrom the 2003, 2006 and 2009 International Residential Code(IRC), International Building Code (IBC), International PlumbingCode (IPC), International Mechanical Code (IMC), 2006 UniformPlumbing Code (UPC) and the 2005 National Electrical Code.'
'Electrical System Inspection Basics,' Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'How to plan and install electric wiring for homes, farms, garages, shops,' Montgomery Ward Co., 83-850.
'Simplified Electrical Wiring,' Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
'Home Wiring Inspection,' Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
'Evaluating Wiring in Older Minnesota Homes,' Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
'Electrical Systems,' A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
'Basic Housing Inspection,' US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
...
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Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book illustrations from The Illustrated Home . Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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The HOME REFERENCE BOOK - the Encyclopedia of Homes, Carson Dunlop Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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