How to Install a GFCI Outlet: A DIY Guide

A GFCI outlet, officially known as a ground-fault circuit-interrupter receptacle, helps to prevent electrical shock in wet areas, like kitchens, bathrooms and the outdoors. GFCI protection is required by the National Electrical Code in these areas of the home as well as in laundry rooms, garages, basements and crawl spaces. Replacing an existing standard electrical outlet with a GFCI receptacle is an easy way to improve safety and can be an efficient option for bringing a new installation up to current code standards if you're remodeling or just updating some wiring. Always check with the local code authority for requirements specific to your project.

Wiring a GFCI outlet is no more difficult than connecting a standard outlet, but you need to choose one of two different wiring configurations for the GFCI. Use the single-location configuration if you want to protect only the new GFCI outlet, and use the multiple-location configuration if you want to protect the GFCI outlet and the rest of the outlets and other devices on the "downstream," or load side, of the circuit. The load side is fed by the power coming out of the GFCI receptacle as opposed to the line side, which lies between the circuit breaker (in the home's service panel, or breaker box) and the GFCI receptacle. (See below for more details on single-location and multiple-location wiring.)

Choosing the wiring configuration for a new GFCI outlet is easier than it may seem. If you need just one GFCI outlet and there are no other outlets on the same circuit (for example, in a bathroom that has only one outlet), use the single-location wiring configuration. In rare cases, you may want to add GFCI protection at one outlet location and not at another outlet on the same circuit; this situation also calls for single-location protection.

Multiple-location protection is commonly used where two or more outlets on the same circuit require GFCI protection, such as in a kitchen, garage or basement or on multiple outdoor outlets on the same circuit. By installing a GFCI receptacle at the first outlet location using multiple-location wiring, you provide GFCI protection to all of the other outlets. The GFCI will protect itself and the other outlets, including standard (non-GFCI) outlets. Keep in mind that with this configuration, if the GFCI trips, it will shut off the power to all of the outlets, and you'll have to reset the GFCI to restore power. Also remember that the GFCI does not protect any outlets between itself and the service panel; the only way to do that is to install a GFCI circuit breaker.

A pigtail is a short length (about 6 inches) of wire that is joined to multiple circuit wires to create a single point of connection for all of the wires. When using the single-location configuration on a GFCI outlet, you must connect only one neutral, hot and ground wire to the outlet, and you cannot connect more than one wire to a single screw terminal with any type of outlet. Therefore, if there are two cables in the electrical box, you'll have two neutrals, two hots and two or more ground wires (there may be a third ground wire if the electrical box is metal, which must be grounded).

To create pigtails, you'll need about 6 inches of NM-B (nonmetallic) electrical cable that matches the cables in the box, or you can buy individual pieces of type-THHN insulated wire of the same gauge as the cable wire. Pull the wires out of the cable sheathing and strip about 3/4 inch of insulation from each end. Join one end to the matching wires in the circuit cables — neutral to neutral, hot to hot, ground to ground — using twist-on wire connectors (wire nuts) or push-in connectors (follow the connector manufacturer's instructions for how much to strip the wires). Shape the other end of the pigtail into a hook for connecting to the outlet's screw terminal.

When wiring a GFCI for multiple-location protection, the power must come into the "line" terminals and go out through the "load" terminals. Therefore, you need to identify which cable in the box is the line, or source, cable. If you don't know which cable is which, you can perform a simple test to identify the line cable. This is a live-wire test that can give you a painful shock if you're careless, so pay attention and be sure not to touch any wires with your hands during the test.

You conduct the test after the power has been shut off and you've removed the old outlet and separated all of the wires outside the box. Make sure none of the wires is touching one another or anything else. Restore power to the circuit by switching on the circuit breaker. (Make sure everyone in the house knows you are conducting the test so no one goes near the wires.) Touch the tip of a noncontact voltage tester to each of the two hot wires at the outlet box: The one that registers voltage is the line wire. Turn off the power again and then label the line hot wire and line neutral wire (the white wire in the same cable as the line hot wire) with masking tape; these are the wires you will connect to the "line" terminals on the GFCI outlet.

If you live in an old house that has circuit cables with no ground wire, it's possible that there is a ground connection if the outlet box is metal and is connected to metal conduit, but such ground connections are often unreliable. If the box is not metal or there is no metal conduit, there is no ground connection present. In any case, the GFCI outlet will still provide ground-fault protection, but it does not provide a true ground. A true ground is needed for surge protectors and other electronic devices.

GFCI outlets are sold with little stickers that say "no equipment ground" or something similar. When installing a GFCI on an ungrounded circuit, place one of these stickers on the outlet cover plate. This will remind you that the circuit is ungrounded so it's not suitable for surge protectors or other equipment that requires a ground.