I want to swap out a light switch with a sensor switch in my boiler room but I'm not sure how to wire it. I took out the old switch expecting to find 3 wires connects: a metal ground, a white return, and a black hot. Instead, all of the White returns are capped together in the box and two black wires are connected to the switch. original switch

I tried wiring the sensor in the same way and it was on for a while but wouldn't switch off when I pressed the plate under the sensor. Then, it did go off and now won't turn on any more. Here's what the new switch looks like: -- photos deleted because I don't have enough karma to post more than 2 links --

And, this is what the inside of the box looks like: inside of box

My guess is either the sensor switch is defective or I wired it wrong and it's defective now ;p

Any help in understanding this wiring would be appreciated. The whole house is like this and I want to replace this switch and put a dimmer in the nursery. I've just never seen a setup with what looks like 2 hot wires and no return.


  • 1
    Since neutral is in the box, you can get a better grade motion sensor that takes a neutral wire. This will play well with all CFL and LEDs. Jan 14, 2017 at 7:42

1 Answer 1


Light wireBasically the only "HOT" is the black coming into the box from the Main. The switch is an interrupt to the light, which uses the electricity. So the way they have it set up is the switch is just passing the Voltage through from black to black when on, and creating an "Open" when it is off. At which point the light (or a plug outlet) would be off; this is where the actual Black/White setup in your mind really matters.

  • Are both black wires the same then? (i.e. I can connect either wire to either terminal on the switch)
    – user341493
    Jan 14, 2017 at 4:05
  • They are not the same; but for a simple switch yes you can have them on either terminal of the switch.
    – Edward
    Jan 14, 2017 at 4:07
  • One black wire is always-hot. The other is switched-hot. Neither one is neutral. Jan 14, 2017 at 7:40
  • 2
    Right, and his "sensor" switch may require the "always hot" wire on a specific terminal to power it's electronics. Check which is which, using a tester and then check your installation instructions.
    – DaveM
    Jan 14, 2017 at 16:32

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