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. 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.
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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 .
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.
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 CONNECTION for 2-WIRE RECEPTACLE CIRCUITS - 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.
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. 
Arc fault protection - AFCI's: Beginning in 2002 the NEC also required arc fault protection for electrical outlets for bedrooms. 
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.
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. Critique and content suggestions are invited. Credit is given to content editors and contributors.
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
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.
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.
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.
At above left is 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
Thanks to frequent InspectApedia contributor also cited at ABOUT InspectApedia.com
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.
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(Aug 10, 2015) M Schultz said:
Can GFCIs be used instead of a AFCI?
Can AFCIs replace GFCI?
Why not just have all AFCI breakers or all GFCI plugs?
These devices provide quite different safety protections and cannot be substituted for one another.
GFCI's detect a fault or short from hot to neutral or ground.
AFCI's detect arcing in the electrical receptacle.
I pose that the reason that there is not a combined device in widespread use is cost.
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