Electrical Outlet Installation Basics for Homeowners InspectAPedia® -
How is an electrical outlet or receptacle wired in a home?
Where do the hot and neutral wires and the ground wire get connected on an outlet?
What are the wiring requirements and details for an electrical outlet?
How does the National Electric Code guide homeowner repairs to the electrical system?
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This article answers basic questions for homeowners about how to 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 lots of details to get right if you want to be safe. Readers of this article should also see Electrical Code Basics, ELECTRICAL DEFINITIONS and also SAFETY FOR ELECTRICAL INSPECTORS. Our photo at page top is not an example of a proper electrical outlet installation.
Do not attempt to work on your electrical wiring, switches, or outlets unless you are properly trained and equipped to do so. Electrical components in a building can easily cause an electrical shock, burn, or even death.
Even when a hot line switch is off, one terminal on the switch is still connected to the power source. Before doing any work on the switch, the power source must be turned off by setting a circuit breaker to OFF or removing a fuse. See SAFETY FOR ELECTRICAL INSPECTORS and Electrical Wiring Books & Guides
Electrical Outlet Wiring Advice for Homeowners & DIY Repairs
How to add an electrical outlet in a home
First of all, why might we need to add an electrical outlet in a building?
Convenience: we'd like to be able to plug in more electrical devices at some spot, say at a home office;
Safety: we need to plug in more devices at some location and we don't want to risk tripping someone or starting a fire by using extension cords;
Function: we need to plug in more devices at some location in a building, or we need to plug in a new high-current using device in a building, and we find we keep blowing fuses or tripping a circuit breaker; in this case we want to add a whole new circuit, from electrical panel to the location where we need the outlet, so that we can deliver adequate electrical power without blowing fuses or tripping circuit breakers.
The illustration at left shows the typical wiring of an electrical outlet or "receptacle", courtesy of Carson Dunlop.
Safety warning:
This sketch and these suggestions are absolutely not a complete guide to safe wiring.
This outline of how an electrical outlet is installed demonstrate that while mechanically, with a few simple tools, electrical wiring of an electrical receptacle is not difficult, there are quite a few details to get right in order for the electrical receptacle to be safely installed.
Here are a few examples, not to discourage you but to emphasize that a superficial "just wire it up and see what happens" approach is a bad idea:
The electrical circuit must be properly protected by the right fuse or circuit breaker - 15-Amp on #14 wire, 20-Amp on #12 wire if the wiring is copper.
The electrical wire used for the receptacle circuit must be the proper type in size (thickness or gauge) and number of conductors for the ampacity of the electrical circuit (typically #14 wire for a 15-amp circuit and #12 wire for a 20-amp circuit when copper wiring is being used) .
The electrical wire must have the proper number of conductors. In modern electrical circuits used to wire receptacles (electrical outlets) of the simplest sort, we expect to see three wires: a black "hot" wire, a white "neutral" wire, and a bare ground wire.
Older two-wire electrical circuits, such as the two circuits depicted at the right of our sketch (courtesy of Carson Dunlop) 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.
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.
The electrical circuit wire must be properly routed and secured between the electrical panel and the receptacle location, and must be properly secured at the junction box that is to hold the electrical receptacle.
We secure wires at intervals specified by the electrical code so that it doesn't sag, hang, pull on itself, and become damaged or unsafe. The two sketches below, courtesy of Carson Dunlop, show examples of routing electrical circuit wires through a wood stud wall and through a wall supported by metal studs.
The electrical circuit wire must be protected from damage. We don't route electrical wires where they can be punctured by someone driving a nail into a wall to hang a picture. Wires running in walls anywhere from floor level to seven feet above the floor (U.S.) or five feet above the floor (Canada) must be protected from nails driven through walls. We also don't route wires too close to places where the wires can be damaged by heat from a heating appliance or chimney, flooded, etc. as you'll see depicted in the two Carson Dunlop sketches below. Thanks to Steve for pointing out erroneous illustration link details.
Splices and other wire connections in the electrical circuit must be made properly to be secure and protected against short circuit in the electrical panel, in any junction boxes used for splices, and at the electrical receptacle (discussed more below).
Back-wired electrical receptacles: Although this sketch, courtesy of Carson Dunlop,shows push-in type terminals used on some electrical switches and receptacles (upper right in the sketch), and although those connections are legal, we don't trust them and we don't use them.
That's because the push-in terminals on some electrical outlets and switches are less reliable than the side screw terminals.
Studies have shown (Aronstein) that back-wired electrical receptacles whose wires are simply pushed into a round hole at the back of the receptacle have a very small contact area between the receptacle and the wire (just the very edge of a flat spring), and that this contact can be damaged, especially if the receptacle is re-used. On older versions of push-in terminals the hole for the wire would admit a #14 or a larger #12 wire. If an outlet was first wired with a #12 wire (which bends the contact spring farther open) and later re-wired to a #14 wire, the connection was particularly unreliable.
Some if not all newer electrical outlets with push-in terminals have been modified to use only the smaller #14 sized wire opening to prevent this failure. But we still prefer the increased contact area and increased contact security that comes from being able to tighten a screw.
A screw-terminal (upper left in the sketch) is a more reliable connection for wiring at an electrical outlet. Some newer electrical outlets have a back-wire push-in type terminal which uses a screw to securely pinch the wire in the receptacle - these are fine.
The splice at lower left in the sketch depicts using a twist-on connector - the right way to make a wire splice inside of an electrical junction box.
The proper sized and type of junction box must be used to house the electrical receptacle, must be properly secured in the wall, and must be located at the proper height from the floor.
If the junction box is too small you face several problems such as crowding which forces wires to be jammed and bent into the box, increasing the chance of a loose connection or damaged wiring, and worse, arcing.
If the clearance between the metal box and other live electrical parts is too small, arcs and short circuits could occur, as we depict in this sketch courtesy of Carson Dunlop.
It's likely to be permitted and safe to install a larger sized junction box than you need for an outlet or switch or wire splice, but you may need to purchase a special adapter-cover for the box in order to properly secure an electrical outlet therein. Installing a junction box that is too small is illegal and unsafe.
Don't damage electrical wires: When installing, securing, or stripping insulation from wiring in order to make a connection, the wire and its insulation must not be nicked or damaged, as we show in the sketch at left, courtesy of Carson Dunlop.
Securing: the electrical wire entering the junction box must be properly secured using the right connector or clamp.
Grounding: the junction box and electrical receptacle must be properly connected to the building grounding system - not shown in our sketch. The incoming ground wire is connected to both the ground terminal on the electrical receptacle (usually a green screw), and if the junction box is metal (not plastic) the ground wire is also connected to the metal junction box itself, usually by a special green screw that connects to a tapped threaded hole on the junction box back side, or by a grounding clip that secures the ground wire to the edge of the metal box.
Don't rely on the connection between the electrical outlet's steel mounting strap and the steel screw openings of the junction box to provide the ground connection. That's not a legal ground and it's unreliable. Use the a ground wire and ground screw on the receptacle itself to be sure that this important safety feature is correctly installed.
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).
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.
The hot and neutral wires must be connected to the proper terminals on the electrical receptacle.
The "hot" or "live" black wire (or red wire) is connected to the brass-colored screw terminal on the electrical receptacle, and the "neutral" white wire is connected to the silver-colored screw terminal on the electrical receptacle.
Reversed polarity on an electrical outlet is dangerous. If you accidentally reverse these wires the device you plug in to the receptacle may work but it is unsafe and risks a short circuit, shock, or fire.
An electrical outlet must be properly located on the wall, according to local electrical codes and the National Electrical Code. Examples of proper electrical outlet locations are shown in our two sketches below, courtesy of Carson Dunlop and in the following list of electrical outlet location requirements:
Tub-Shower clearance: Keep electrical receptacles at least three feet (one meter) away from a tub or shower.
Electric baseboard clearance: Keep electrical outlets offset above and to the side of vanity sinks, not right over the sink (left hand sketch below)
Electric baseboard clearance: Keep electrical receptacles off to one side, not right over electric heating baseboards to avoid overheating and possibly melting electrical cords draped over the heater (a fire risk) - see photo at below right, courtesy of Timothy Hemm.
Outlets in floors, countertops: Generally we do not mount electrical outlets flush in countertops or floors, though in some codes and jurisdictions the inspector may require that special (protected) floor-mount electrical receptacles be installed in order to meet the requirement that electrical outlets are available within six feet in any direction along a wall, and where no "wall" is available to install such receptacles (such as along a sleeping loft).
Garage electrical outlet location: In the garage electrical outlets should be 18" or more above floor level.
The electrical receptacle must be properly screwed to or mounted in the junction box, and the extra length connecting wires carefully pushed back into the junction box so as to avoid crimping, damage, etc.
An electrical receptacle cover plate must be installed over the finished receptacle. We like plastic cover plates better than metal as they reduce the chances of a cover plate becoming electrically "live" and thus unsafe.
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.
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Timothy Hemm has provided photographs of various electrical defects used at the InspectAPedia TM Website. Mr. Hemm is a professional electrical inspector in Yucala, CA.
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
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)
Special thanks to our reader Steve who pointed out errors in our illustrations 200
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