Background on what I'm trying to do:

This question appears somewhat similar in concept to Can I control a bathroom extractor fan independently from the light? however, in my case, I believe the wiring was originally separate and was later retrofitted to be controlled together, so running new wires and installing double switch

A bathroom has a triple switch plate with two switches that are redundant...if either one or both are on (pointed up), BOTH the mirror lighting and the ventilation fan run. (The third switch controls a ceiling light). We'd like to control the fan and mirror lighting independently so we can install a dimmer for the lighting as well as have the option to run the fan without the lights during the daytime.

Although installing a dimmer switch in the other bathroom where the switches had not been paired together was straightforward, un-pairing the fan and mirror lighting switches correctly before installing a dimmer seems essential to prevent damaging the fan.

The real question:

How would the wiring for an existing bank of switches be modified to pair two of the switches? I'm hoping to see a wiring diagram for what a typical setup for a set of switches like this would be before coupling, and a diagram showing how the wiring is modified to couple the switches.

Edit: here's a photo of the switches box: enter image description here

and because it's really hard to see what's going on in there, here's a totally unprofessional diagram of the wires that are easy to trace/look relevant. The little boxes in the center of the switches on my diagram are the holes on the back of the switches, and the little boxes on the side of the switches are the screws. The yellow things are wirenuts and each has a ton of wires coming in and out and its hard to follow what goes where since everything is stuck together with paint. but it looks to me that the wire indicated in orange should go to the hole on the back of the middle switch and the wire indicated in red should be removed?

enter image description here

edit2: after loosening the screw terminal on the left switch it appears the orange and red wires in my diagram are actually a single wire that had an inch of the insulation stripped to wrap around the screw terminal. Would electrical tape adequate for replacing the missing insulation? Or does it need heat-shrink tubing or to be cut and covered with a wire-nut?

  • When you say either or both are on, do you mean up? Or do you mean when either is switched from up to down, the light and fan go on?
    – bib
    Commented Oct 23, 2013 at 22:50
  • Also, can you (safely) get a picture of the cables in the box?
    – bib
    Commented Oct 23, 2013 at 22:51
  • 1
    Please provide either a picture of the current wiring, or some type of diagram that shows all the wires in the box, where they are connected, and where they go. There is no way to definitively answer this question as written, there simply is not enough information provided.
    – Tester101
    Commented Oct 24, 2013 at 9:57
  • the wiring is a real rats nest, at this point I'm more interested in the typical setup of how wiring is changed to couple or uncouple a pair of switches Commented Oct 24, 2013 at 15:44
  • 1
    The switches being "coupled" is a mistake somebody made, it is not a proper wiring technique. As for what the proper wiring should look like, see the first image in this answer. More specifically, the proper wiring should look something like this
    – Tester101
    Commented Oct 24, 2013 at 16:30

2 Answers 2


IF you are positive the wiring was originally separate, 1 cable to the mirror light,and one cable to the ventilation fan, Then yes, you can uncouple the leads at the box where they originate. You are gonna have to identify which wires are "hot" and which wires are switched, you can do this with a simple voltmeter. The reason for this is that you Only want to separate the wires that feed your fan and your mirror light. You dont need to separate the Hots; The 2 switches are still gonna share the "hot", but only one switch will independently control each device. But if the light and fan have been hooked up together at another location, like in the junction box behind you mirror light, that is a different story.


This is what the wiring should look like...

Three Switch Wiring

Notice the incoming ungrounded (hot) conductor is spliced, to supply power to each switch. The ungrounded (hot) conductors from the fixtures, are connected to the other side of the switches. The grounded (neutral) conductor is spliced with the grounded (neutral) conductors leading to the fixtures, and all the grounding conductors are tied together and to the box (if it's a metal box).

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