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I installed a Leviton smart switch in a 3-way circuit and the corresponding coordinating switch in the picture (right switch). The lights function as they should using both switches.

However, I recently was about to install another switch on left side of the box, when I looked at the coordinating switch and discovered that I probably wired it incorrectly. I believe I erroneously connected the neutral terminal on the coordinating switch to the neutral of the other circuit.

The circuit on the right only has one cable coming in with 3 wires: red, black and white. I believe the red is a traveler, black is the line/traveler and white is the neutral. The instructions for the switch had me capping off the black wire.

In this case, would I need to connect the neutral of the coordinating switch to this yellow wire cap with the the black and white?

2 gang box

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You can't take neutral from other cables.

NEC 300.3.

Neutral can only be taken from cables that already have wires going to the switch. This is a big problem on the far end of a 3-way, where neutral does not exist.

As such, the wiring arrangement of the 3-way switch needs to be mapped, the use of those wires reconsidered, and the location of any smart switches and remotes carefully chosen. For instance you can see that the other 2 wires in the 3-way cable are basically wasted.

Generally, the smart switch "master" (which actually switches power to the lamp) needs to be at the closer location, not the far spur like this. The most you can have here is a smart switch remote.

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  • Thank you, that makes sense. If I were to leave this spur switch disconnected (I don't use it much, anyway), what would be the best way to terminate the unused red wire?
    – user1259832
    Commented Jul 7, 2022 at 20:09
  • @user1259832 cap it off with a wire nut.
    – Harper - Reinstate Monica
    Commented Jul 7, 2022 at 21:43
  • off topic, but the US electrical codes keep amazing me: why is it even allowed to have multiple circuits in the same box?
    – Pelle
    Commented Jul 8, 2022 at 13:58
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You have a black and a white coming in from the same place as the red and not connected to anything except each other.

If there's a neutral at the other place you can use that white to connect to the neutral. leave the black unused, (capped at both ends) connect whatever was connecting to the other end of the black and white to each other.

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