Things You'll Need
Single-gang electrical box
14-2 conductor with bare copper ground electrical cable
GFI (ground fault interrupter) outlets sense amperage imbalances between the hot and neutral wires of an electrical circuit. The design of the GFI includes hot and neutral "Line" terminals in which the power supply is connected, and "Load" terminals where additional electrical devices, such as other outlets or lighting, may be connected. For example, a lighting fixture may be controlled by a dimmer switch with power supplied by the "Load" terminal of the GFI. You can connect a dimmer switch with basic electrical circuit knowledge and the proper tools.
Turn off power to the GFI outlet. Test for absence of power with a voltage tester. Place one probe in each slot above the GFI and then below to verify that no power is present.
Remove the GFI cover plate screws with a flat-head screwdriver. Set the cover plate aside.
Remove the two GFI bracket screws with a Phillips screwdriver. Pull the GFI outlet from the electrical box as far as its wiring will allow.
Install a single-gang electrical box for the dimmer switch according to your particular application. For example, new construction requires a different box installation process compared to existing wall installations.
Cut a length of 14-2 with copper ground cable with the wire cutters. Make the cut long enough to reach from the GFI Load terminals to the dimmer switch box. Allow 4 to 6 inches of cable to extend out of each box.
Cut another length of 14-2 cable long enough to reach from the dimmer switch box to the light fixture. Leave 4 to 6 inches outside of the switch box and at the fixture.
Remove 4 inches of cable sheathing from each of the cable ends, using a utility knife.
Strip 3/4 inch of insulation from each of the black and white wires, using wire strippers.
Twist the stripped ends of the 14-2 black and white wire at the GFI into an open-end loop with needle-nose pliers.
Connect the black wire to the brass terminal screw on the Load side of the GFI and the white wire to the silver terminal screw. Keep the open end of the loop facing to the right. Tighten the Load terminal screws clockwise with a Phillips screwdriver.
Cut a 6-inch length of bare copper wire from stock. Bend an open-end loop on one end of the wire using needle-nose pliers. This length will be the ground pigtail wire.
Disconnect the bare copper ground wire from the green ground post of the GFI. Cut the loop off from the end of the wire using wire cutters.
Connect the looped end of the pigtail ground wire to the green grounding post of the GFI and tighten the post screw.
Connect the bare copper ground pigtail, the ground wire from the power supply and the ground from the dimmer switch box together with a wire nut. Turn the nut clockwise by hand until the connection is tight. If more pressure is needed, continue turning the nut using combination pliers.
Install the GFI outlet back into its box and secure the bracket screws with a Phillips screwdriver. Attach the cover plate and secure with the cover plate screws. Tighten the screws with a flat-head screwdriver.
Connect the two stripped white wires at the dimmer switch box with a wire nut. Hand-tighten by turning clockwise.
Connect the two bare copper wires at the switch box with a wire nut.
Connect the black wire from the GFI to one of the two dimmer switch wires. Connect the black wire from the light fixture to the other switch wire, using wire nuts.
Wrap all the insulated wire nut connections with electrical tape, winding the tape clockwise overlapping from the nut onto the insulated wires.
Install the dimmer switch into the electrical box with the bracket screws included with the switch. Attach the cover plate with the provided screws. Tighten all screws clockwise with a screwdriver.
Connect the black wire from the dimmer switch to the black wire of the light fixture. Connect the white wire from the switch to the white wire of the fixture using wire nuts. Tighten the wire nuts clockwise and wrap with electrical tape.
Connect the bare copper ground to the ground wire of the light fixture with a wire nut. Secure the light fixture according to the specific application requirements provided by the manufacturer.
Restore power and reset the GFI outlet by pressing the "Reset" button.
Electrical devices connected to the Load terminals of a GFI are protected by the outlet. Consequently, if the GFI is tripped, the added devices will also shut off.