How to Use a 2-Wire Romex Between Three-Way Switches

Turn off the power to the circuit on which you'll be working.

Unscrew and remove the faceplates over the switches. Remove the mounting screws and pull the switches out of the junction boxes so you can see the terminal screws. Loosen the terminal screws and remove the wires. Set the switches aside.

Feed your 2-wire Romex through the same conduit as the existing wire between the switches. Cut the Romex so each junction box has six inches of wire in it. Separate the ends of the new wire. Strip off the last inch of the insulation around the wires. Place a piece of black electrical tape on the white insulation and a piece of red electrical tape on the black wire. Do this in both junction boxes.

Twist the two white wires in the first junction box together and secure them with a wire nut. Note that these are the pure white wires remaining, not the wire you just marked. Repeat in the second junction box. Twist the bare ground wires together. If one isn't already in the junction box, cut a length of bare copper wire about three inches long from the 2-wire Romex. Twist that together with the ground wires and secure them with a wire nut. Repeat in the second junction box.

Find the black wire that connects the two junction boxes. Tape each end of that wire with electrical tape so no bare copper is exposed.

Bend the ends of the three-inch length of ground wire, the remaining black wire and the tape-marked Romex into hooks. Hook the ground wire to the green terminal on the three-way switch. Hook the black wire to the dark-colored screw. Hook the red-marked wire to the light-colored screw on the right and the black-marked wire to the light-colored screw on the left. Tighten the terminal screws. Tape over the connections with electrical tape. Repeat for the other switch.

Remount the switch in the junction box and replace the faceplate. Repeat for the other switch. Restore the power at the circuit box.