The wiring of three-way switches is certainly more complicated than that of the more common single-pole switch, but you can figure it out if you follow our diagrams. With a pair of three-way switches, either switch can make or break the connection that completes the circuit to the light. The whole project can be completed in a few hours if you don’t have to do any drywall removal and repair.
To add the switch, you’ll use one of two wiring diagrams, depending on whether the power comes to your light switch first (the most common situation) or to the light fixture first. Either way, complete these five steps:
- Turn off the correct circuit at your electrical panel.
- Add an electrical box for the second three-way switch in the basement. It’s likely you’ll also need to replace the existing switch box with a larger one to accommodate the extra wires for the three-way switch.
- Feed a length of 14-3 type NM cable (or 12-3, if you’re connecting to 12-gauge wire) between the two boxes. The 14-3 cable has three insulated conductors: white, black and red (plus a bare ground wire).
- Connect the wires to the new three-way switches with ground screws using one of the two wiring diagrams (Fig. A or B). On the switches, the common terminal will be identified by a label and/or the terminal screw will be a different color than the other two.
- Make sure to wrap black electrical tape around the ends of all white wires that are used as travelers between the three-way switches. If you have the setup shown in Fig. A, also wrap black tape around the white wire from the switch to the light. This way, both you and others will know these wires are “hot” and not neutral like most white wires.
Caution: If you have aluminum wiring, call in a licensed pro who’s certified to work with it. This wiring is dull gray, not the dull orange that’s characteristic of copper.
Figure A: Power to Light Fixture
This diagram shows how to wire the switches and the light when the power is coming to the light fixture.