GFCI outlets reduce the danger of deadly shock from faulty plug-in cords and devices. A GFCI (ground fault circuit interrupter) is a special type of outlet that detects dangerous ground faults and immediately turns off the power to stop shocks. You can replace almost any electrical outlet with a GFCI outlet. Correctly wired GFCIs will also protect other outlets on the same circuit.
While it's common to find GFCI outlets in bathrooms and kitchens, the electrical code also requires GFCIs in unfinished basements, garages, most outdoor receptacles and places where construction activity occurs. We'll show you how to replace a standard duplex receptacle with a GFCI and wire it to protect other outlets. (For more information about wiring outlets, see Wiring Switches and Outlets).
You'll need a screwdriver, a wire cutting and stripping tool, and an inexpensive voltage tester. We also recommend you add a GFCI tester to your tool drawer. GFCI testers are available at home centers and hardware stores and are a handy device for troubleshooting standard outlets as well as GFCIs.
Before you start, locate the circuit breaker or fuse that controls the outlet you plan to replace and shut off the power to the circuit. Plug a lamp, radio or the GFCI tester into the outlet to test for power and make sure it's off. Then unscrew and pull out the old outlet and count the number of wires in the box. Calculate the minimum box size required for all of the wires plus the GFCI (see “Required Box Size,” below). If the existing box is large enough, follow the steps in Photos 1 – 4 to replace the outlet with a GFCI. Replace an undersized box with a new one of adequate size. (For more information on box replacement, see What You Should Do With Crowded Electrical Boxes).
Using a GFCI to protect additional outlets on the same circuit breaker or fuse can be tricky. Don't do it unless you know exactly where the wires go. In the workshop, we had the advantage of being able to visually trace the wires.
Test all the GFCIs in your house at least monthly by pressing the test button or using your GFCI tester. If an outlet fails to trip, replace it with a new one.
Required Box Size
To figure the minimum box size required by the National Electrical Code, add:
1 – for each hot and neutral wire
entering the box
1 – for all of the ground wires combined
1 – for all of the cable clamps combined (if any)
2 – for each device (switch or outlet—but not light fixtures)
Multiply the total by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box size required in cubic inches. Plastic boxes have their volume stamped inside. Steel box capacities are listed in the electrical code.