How do I determine if the power source is going right into the 3-gang box of switches, or directly to one of the fixtures (1 vanity light and 1 light/exhaust fan combo)? What is the best way to install new light fixtures (say over the shower and/or bathtub), taking power from the light of the light/fan combo?

Essentially, we are looking to have the current switch that turns on the light of the light/fan fixture also turn on the new lights. The other two switches will remain connected to their respective fixtures (the exhaust fan and the vanity lights). Thanks!

  • If you're able to post clear photos of the cables and connections at each location we can probably help. Otherwise, get yourself an inexpensive multimeter and start investigating.
    – isherwood
    Commented Dec 4, 2017 at 14:46
  • 1
    When saying over the tub / shower are you aware of the code requirements? Many folks place them at the edge because listed damp/ wet location ones are ugly. NEC 410.10.A & D
    – Ed Beal
    Commented Jan 5, 2018 at 18:03

2 Answers 2


OP seems to have done a scarper but for anyone else reading this:

The one sure way to find where the power enters a circuit is to completely disconnect each cable and drop a test load across its wires.

I detach the cables one at a time and re-connect each one before going on to the next. This saves having to mark everything first -- although it does involve a lot of running back and forth to the circuit breaker panel.

My favorite test load is a neon indicator lamp on a short piece of lamp cord with two insulated alligator clips. But almost any kind of lamp can be pressed into service.


To find the source run, you will need to see how the switches are wired, or looking up in the ceiling. As for adding a light, permit will be needed, water safe light fixture, and just pull a wire from the existing fixture to your new hole and wire it up. Of course power off for this install.....

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