I'm in USA. I'm changing an old bathroom light/exhaust fan out. The image below is of the ceiling, and the connection to the light/fan. ceiling Image of how switch is wired. Not of actual switch switch


Single Pole Light Switch

-A black wire on the top screw, connection uncertain

-A white wire on the bottom screw, connection uncertain

Light/Fan Unit

-A black wire, connection uncertain A white wire, connection uncertain

Wires in the ceiling

-A black wire, known connection to the light switch, unknown to which

-A white wire that has been tested hot, known connection to the light/fan unit, unknown to which


i would like to replace the switch with a new switch, to take advantage of the independent fan and humidity sensor controls, like pictured below. new switch

switch two and three only work when switch one is turned on. can i rewire it to be simply hot(white) to switch, and neutral(black) to fan/light neutral? picture here. proposed change

Thanks kindly for any and all help. I appreciate it.

  • 2
    Your diagram and description are both confusing. Please revise to list cables, wires, and connections in a more structured way. Grounds are probably irrelevant. Photos might help. Also make sure your question is a question. It starts out as one but ends up as a rambling statement.
    – isherwood
    Commented Apr 10 at 20:56
  • Welcome. Please take the tour so you know how this site works.
    – isherwood
    Commented Apr 10 at 21:04
  • Plaes edit in pictures of the actual wiring as presently connected at the fan, switch, and whatever other locations it's accessible. What you have described and drawn does not add up.
    – Ecnerwal
    Commented Apr 10 at 21:22
  • 2
    You have three wires labeled "connected to switch", but only two wires are actually connected to the switch? Is there only one switch? Why do you think the three labeled wires are connected to the switch? Two wires in the ceiling include the description "unknown to which" - what does that mean?
    – Mark
    Commented Apr 10 at 22:35
  • 1
    @Mark the third wire, ground, is connected to switch, but i was told it was likely irrelevant so i didn't include. there is only one switch. i know they are connected to the switch. the ground i understand. "unknown to which" i do not the connection that is being made. the confusion for me is for the other two "connected to switch", they used loose red wire to extend them, before then connecting them to romex 14/2. due to walls in the way it can't be traced directly. adding to the confusion, is that the white in the ceiling is hot and goes directly to the light/fan unit. Commented Apr 10 at 23:02

1 Answer 1


I think there's enough information to answer your question, though you'll have to verify a few things. You won't be able to do this using the existing wiring, for several reasons:

  • You can't have individual wires running loose inside the walls. They have to be contained in cables (i.e. Romex) or run in conduit.
  • You can't have wire connections hidden inside the wall. Connections have to be inside an accessible electrical box.
  • It doesn't look like you have a ground at the fan or switch (unless there's another cable entering the switchbox).
  • Finally, you don't have enough wires running between the fan location and the switch to control the fan and light as you want.

I've marked up your photo to identify the apparent purpose of each of the existing wires based on your comments.
enter image description here

This is where you'll need to do some verifying with your multimeter.

  • The source on the left appears to be where power comes into this part of the circuit. Turn the power off and disconnect the two wires attached to the black and white wires labelled as source. Also disconnect the red wire at the bottom labelled as "switched power from switch" and the black wire labelled as "ground to switch".
  • Find a good ground that runs back to your breaker panel (a known grounded outlet for example) and turn the power back on.
  • measure the voltage between the white wire and ground, and between the black wire and ground. Do this twice - with the switch in both on and off positions. You should have 120 volts on one wire and 0 volts on the other regardless of which position the switch is in. Note which wire is hot (hopefully it's the black one).
  • Measure the voltage between ground and each of the wires from the switch, with the switch in both positions. You should have 0 volts on both wires in both positions.

If these all check out, you've confirmed the source of the power, and that nothing is hot in the switch loop when it's disconnected from the source.

  • Turn the power off again and connect the red "always hot to switch" wire to whichever of the source wires was measured as hot.
  • Turn the power back on, and measure the voltage on the "switched hot from switch" wire with the switch in both positions. You should see 120 volts with the switch on, and 0 volts with the switch off.

If these steps check out, then you've confirmed the switch loop wiring.

To make this work, you need to wire it like this:

enter image description here

You can see that you don't have enough wires between the fan and the switchbox. You need:

  • Always hot supply to switch
  • Switched hot to light
  • Switched hot to fan auto
  • Switched hot to fan manual
  • Neutral - won't be used, but needed to meet code. Capped off in switchbox.
  • Ground (not shown in diagram, but needed in every box)

I see a flexible metal conduit passing through the opening in your ceiling. If your house is wired with conduit, you can install new conduit or extend the existing conduit from the switchbox and pull new wires through it. You'll need to make sure it's a continuous run from the switchbox to the fan box, with no connections or individual wires hidden inside the walls.

If you use cable instead of conduit it will get more complicated because there are too many conductors for normally available cable. I'm pretty sure you would have to run a 14/2 to bring the hot and neutral from the source into the switchbox, and a 14/4 to carry the three switched hots and a neutral back to the fan box. I believe the neutral would have to run down and back if two cables are used because the supply and return current have to be enclosed in the same cable.

You will also need to be sure the switchbox is large enough because you're adding a lot of wires to it.

Finally, you'll need to follow the source wiring back to the box it originates in, and run new conduit or cable between there and the fan box in order to eliminate the existing individual wires and splices that are hidden within the walls, and provide a ground to the fan and switch.

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