In my bathroom I have a 3-pole dimmer switch (1st picture) and an extractor fan switch across the room (2nd picture) that is powered through the light switch. The extractor fan switch works as expected, but only if the light switch is also on. If the light switch is dimmed it also slows the extractor fan. Can I easily make it so that the extractor fan will turn on and off regardless of the light switch position?

Thanks in advance.

Chema Light switch enter image description here

  • Hello, and welcome to Home Improvement. A picture or diagram of the wires inside each box would really help. – Daniel Griscom Feb 18 '17 at 12:34
  • The fan switch gets its power from the dimmer switch. It may be that a mistake was made in the wiring. I would pull the switch(s) and identify the hot feeding the dimmer, hopefully it is in the box with both switches. I would remove the feed to the fan switch and supply the hot directly to the fan. Photos will help us identify the wiring but some basic tools like a non contact voltage tester may be needed. Whatever you do don't pull the wiring apart without marking the wires because if wires get mixed up it can take some time and a meter to figure out how they are wired. – Ed Beal Feb 18 '17 at 15:16
  • Can you post photos of the insides of all boxes involved? – ThreePhaseEel Feb 18 '17 at 16:07
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    What exactly do you mean by 3-pole? That has a meaning, but I super doubt it is the meaning you intend. – Harper Feb 18 '17 at 20:03
  • Thanks guys, I've added photographs. I only mentioned that it was a 3-pole switch in case that meant it could have be wired incorrectly in such a way that it operated as described. Upon opening it up, I'm guessing that if I run the black wire in and out of the same terminal on the light switch that would effectively bypass the switch on it's way to the fan switch, correct? – Chema Caillet-Bois Feb 21 '17 at 0:19

Based on your description, it seems that your fan is wired in one of two ways:

  • The fan switch is on the ungrounded conductor (hot, black wire typically) after passing through the dimmer switch
  • The fan switch is on the grounded conductor (neutral, white wire typically) coming out of the fan fixture

Always switch the circuit breaker off before attempting any wiring changes and use a circuit tester to ensure the power is off

I attempted to use the solid black lines to indicate black insulated wires. Typically, those represent the ungrounded (hot) conductor. The black lines with a white stripe are white wires, typically the neutral (grounded) conductor. Double check your arrangement to make sure you know which wires are which for your case because someone might not have followed convention.

The former case is ideal because it would make rewiring easier (and means that there's never a chance for the fan fixture to be live even when its switch is off).

Case 1

Here's a diagram showing the status quo and how you would want to rewire it:

Ideal Wiring Case

The red circle indicates a wire splice that needs to be moved upstream of the dimmer switch. You may find a wire nut on the load side of the dimmer switch, where one wire comes from the dimmer, one goes to the light, and one goes to the fan. Moving the wire coming from the fan so it's spliced on the wire coming from the circuit breaker should then isolate the fan and light.

Case 2

Based on the pictures, it seems that you may have the less ideal wiring case. This is dangerous because if the dimmer switch is on but the fan switch is off, the fan fixture is still energized and could represent a hazard if a short occurs.

Here's a diagram, as with case 1, showing the current and rewired circuits:

Non-Ideal Wiring Case

As before, the red circle indicates a splice that you will need to move. In fact, the final wiring diagram looks nearly identical because the desired outcome is the same no matter how you start. The only difference here is that you will need to add a wire splice where you removed the fan switch from the neutral. In essence, when you remove the switch, you need to close the circuit with a wire splice or you won't have a functional fan at all.


Here's what I think happened. The light was installed first. Then, someone came along and installed the fan, and simply tapped into the wiring at the light. This means that the Fan and the light came on together (Which was common at one time). At some other time, someone who wanted to poop quietly, installed a switch for the fan, and simply interrupted the existing wiring.

The image below, drawn with exceptional care in my high tech electrical drafting software shows in black and grey, what you have. The red line is what you should have. (Electrically speaking - Implementation and Code is a whole different story.) That is, you want to disconnect the black wire running between the light and the switch, and rewire it via a pigtail to the black wire before the dimmer. (HOWEVER!!!! That's a real wiring mess, and it actually shouldn't be done that way -- This is purely an electron flow discussion -- it's a jury-rigged solution)

This is non-trivial, and shouldn't be attempted without knowing what you are doing.

What I would do is turn the power off, and open up the boxes where the light and the fan are installed, and disconnect the fan from the light.

If you are lucky, (and you just might be, given the quality of the wiring!) the cabling is not stapled, and you just might be able to fish it down to the switch box.

If you are even luckier, you'll be able to run a short piece of Romex from the dimmer box to the switch box.

In the dimmer box, you'll wire all three whites into one nut. Green/Copper to ground (if it exists).

Disconnect the hot black (the one that goes into the braided bundle) and wire-nut it to the black in your new romex, and a short (6 in) piece of black that you've salvaged or pulled out of your excess romex (Use the right guage!) This short piece will go back into the dimmer.

What you have (ignore red) enter image description here

What you want: enter image description here

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