Add to an Existing Three-Way Circuit
As long as the circuit can handle the extra load, run a two-wire cable to the new light, connected in parallel with the existing light.
I want to add a second light in a long hallway. The existing light is in between a pair of three-way switches. What’s the best and easiest way for me to add a light in this scenario?
Greg Lowe, via email, None
Brian Walo, a licensed electrician in Mt. Pleasant, S.C., replies: In this scenario, it’s probably easiest (and least confusing) to run a two-wire cable to the new light, connected in parallel with the existing light. This is accomplished by twisting the hot, neutral, and ground (from the two-wire cable) for the new light together with the existing light. However, you have to be careful that you’re not overcrowding the work box that holds the original light fixture by adding another wire to the setup, and that the circuit has the capacity to handle the extra load.
Most plastic work boxes have their capacity and the maximum number of conductors written on the side, but this may be tougher to figure out for existing boxes. The calculation for the maximum number of conductors in a work box depends on the volume of the box in cubic inches. If you’re at or near the maximum number of conductors allowed, you may need to install a bigger box.