How can I add a light fixture to a three-way circuit by connecting to the switch?

Question

I would like to add a new light fixture to a 3-way circuit in my stairway. The new light will be closer to the second switch and therefore would like to wire it from there (the other fixture is in the ceiling of the second floor - so pretty far away).

The current circuit is as the following diagram:

_{(source: buildmyowncabin.com)}

How can I connect my second fixture on the second switch? Can I use the ground of my romex between the two 3-way switches to pass the neutral for my light?

Answer 1

1. Re-designate the wires between switches to be as follows:

   • Black = always hot
   • Red = switched hot to the light
   • White = neutral
   • Green/bare = ground, as always

2. Find and obtain "smart switches" which support this wiring scheme. They would need some wireless or power-line method to communicate with each other.

One option that would work slick for this is neutral required motion sensors, since they would not need to communicate with each other. They need to be "neutral required" types because "don't need neutral" types rely on leaking currrent through the turned-off lamp, and this won't work at all if the other motion sensor turns the light on. It also doesn't play nice with LED/CFLs.

Answer 2

Referring to the OP diagram, the black wires connected to the top left terminal on the three-way switches are the common hot wires. These receive un-switched power from the line and deliver switched power to the load. The other two terminals are "travelers". Their wires pass live current between the switches. When we add lights to this circuit we don't disturb the travelers. Rather, we insert an additional load between the line and the grounded return.

Specifically, to insert a second light at the second switch we insert it between the hot wire from the line and the common hot terminal of the second switch.

• Hot Common: Black wire from second light is connected to common terminal on second switch (top left in OP diagram). White wire from second light is re-identified as hot (black stripe) and connected to black wire entering second switch box.
• Hot Travellers: Same as OP diagram. Red wire entering second switch box is connected to either lower terminal on second switch (e.g., lower left in OP diagram). White wire entering second switch box is re-identified as hot (black stripe) and connected to other lower terminal on second switch (e.g., lower right in OP diagram).
• Return: Same as OP diagram. White grounded wire entering first switch box is connected to white wire from first light.
• Ground: Green/bare grounding wire from second light is pig-tailed with green/bare grounding wire entering second switch box and a green/bare grounding wire tied to grounding terminal on second switch.