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I have a set of hallway lights with two 3-Way switches. Currently the power source enters at one switch, then the wiring traverses through each of the lights and then terminates at the other switch (see 1st image). What I want to do is change the switch at the end of the hallway to a motion sensor and make the other switch a master ON (in Off position the motion sensor would determine state of lights).

The motion sensor requires a neutral which does not exist in that box currently. I think the below diagram would work (2nd image), but not sure it would be compliant. Once concern is that it would be possible for the lights to receive power from both the normal switch and the motion sensor switch at the same time. Would that be a problem? Thank you in advance!

EDIT: The reason I want to use the normal switch as a master ON is because there is a treadmill in that area and using it does not consistently trigger a motion sensor. I want the motion sensor for people walking in that area and the other switch to turn On indefinitely while using the treadmill.

Current Wiring Proposed Wiring

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  • Without commenting on the overall plan, I'd swap the red and black. The way you have it, red is always hot, and black is switched hot, counter to what is expected.
    – Chris Cudmore
    Commented Jan 4, 2019 at 20:56
  • Move the lights after the switches. Do the circuit P-S-S-L-L. This will let you use only /3 and /2. Otherwise you would need /4 which is hard to find.
    – Dan D.
    Commented Jun 4, 2019 at 16:10

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The original diagram has a problem

There's one issue with the original diagram -- it shows the two /2 NM cables between the lights being used in a way that can be considered a 300.3(B)/310.10(H) violation for cables in parallel. So, as long as all the wires between the two lights are run in a single cable, you are fine in doing what you propose -- powering a light from the same circuit via two parallel paths is not a Code violation, or a major issue otherwise, as long as folks know to turn things off at the breaker before servicing the fixtures.

I would take up Chris Cudmore's suggestion of making the black always-hot and the red switched-hot, by the way, though.

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