How to Wire a Stair Light Switch

Turn off power to the existing stair light switch. Remove the screws holding the switch's cover plate in place. Remove the screws holding the light switch to the electrical box. Hold a circuit tester to the wires to ensure it is safe to work with the electrical wires.

Set up a ladder underneath the light fixture. Back out the screws and lower the light fixture from the electrical box. Twist off the wire nuts connecting the light switch's wiring to the light fixture's wiring and separate the wires. Loosen the grounding screw holding the grounding wire to the electrical box and remove the wire. Set the light fixture aside.

Return to the light switch. Loosen the screw terminals and unwrap the existing wires from each terminal. A black, white and ground wire run from this light switch to the light fixture. Pull on those wires to remove them from the wall.

Run a fish tape from the light fixture's electrical box to the second light switch's (the switch you are adding) electrical box. Attach an electrical cable to the fish tape's hook and reel the wires toward the light fixture.

Pull the cable about 6 inches into the light fixture's electrical box. Cut the electrical cable's outer casing with a utility knife. Peel the casing and the paper insulation back and cut off both. Separate the three wires found inside. Strip 1/2 inch off the end of each wire with wire strippers.

Secure the black wire to the light fixture's black wire with a wire nut. Do the same with the two white wires. Wrap the ground wire around the terminal screw in the electrical box and tighten the screw to hold the wire in place. Mount the light fixture to the electrical box with the screws you removed in Step 2.

Return to the second light switch's electrical box. Send the fish tape from this box to the first light switch's electrical box. Attach a three-way cable to the fish hook and reel it from the first box to the second box. Extend it about 6 inches into the box.

Remove the outer coating and paper insulation from the ends of the cable at both the first and second electrical boxes. Strip 1/2 inch off each wire in both boxes.

Start wiring at the second light switch's electrical box. Connect the two white wires with a wire nut. Connect one black wire to a screw terminal on the three-way switch's left-hand side and the second black wire to the second screw terminal on the same side. Connect the red wire to the screw terminal on the right-hand side. Cut two 4-inch pieces of grounding wire and strip both ends. Attach one of the wire's ends to the electrical box's grounding screw terminal and hold the other end together with the two grounding wires (one from the light fixture and one from the first light switch). Twist a wire nut over the grounding wires.

Return to the first light switch (the switch with the wires coming in from the main power supply). Attach the black power-supply wire to one of the left-hand side screw terminals on a three-way outlet and the second light switch's black wire to the second screw terminal on the same side. Attach the red wire to the screw terminal on the switch's right-hand side. Twist a wire nut over the two white wires to hold them together.

Attach the second 4-inch grounding wire from Step 9 to the electrical box's grounding screw. Hold the other end together with the two other grounding wires (one from the power supply and one from the second switch) and twist a wire nut over them.

Push both light switches into their electrical boxes. Secure them in place with the provided screws. Place a cover over each light switch and secure them in place with the provided screws. Restore power from the main power supply.