I have a dual switch in my bathroom with two separate controls -- one for the fan and one for the light. It has the following four contacts:

  1. Ground (duh!)
  2. Two "common" ones, which seem short-circuited together -- only one is currently wired.
  3. Connector for device A (fan)
  4. Connector for device B (light)

I want to upgrade that to a switch smart enough to turn the fan on and off automatically, depending on humidity. All such switches are marked with "Neutral Wire Required" -- but I thought, I have it, because our wiring is only 7 years old.

The new switch has 6 connectors:

  1. Green for ground (duh!)
  2. Black "hot"
  3. Blue "hot" -- to be tied together with the black and to the "hot" wire
  4. Red -- for the light
  5. Brown -- for the fan
  6. White -- for neutral

Do I have any chance of wiring this device instead of what I currently have? For example, in the same 4-gang box, there is also a power-outlet, which does have a neutral wire. But it is on a separate circuit (because lights are separate from outlets) -- so I can not use it, can I?

Is there anything else I can do, other than to return the new switch?

  • Your hunch is right. You need the neutral to power the sensor electronics. The neutral in the adjacent socket will probably be on an RCD / GFCI. It might not trip if you wired the sensor neutral to it but you would be pushing it closer to its tripping point. e.g., If it trips at 30 mA and your sensor returns 5 mA through it continuously then you are that bit closer to nuisance trips.
    – Transistor
    Aug 14, 2017 at 6:03
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    Borrowed neutrals are best avoided. They can trip GFCIs (RCDs) unpredictably. They also mean that even if you think you have isolated a circuit by turning off the breaker, it is still connected through the neutral to another circuit. If you have two-pole breakers that disconnect hot and neutral, then the apparently disconnected circuit can still be live.
    – Simon B
    Aug 14, 2017 at 10:42
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    Can you post photos of the inside of the box? Aug 14, 2017 at 11:41
  • 1
    @Tester101: I'm in Ireland. OP doesn't state location. Regulations are location specific. Sockets are not allowed in bathrooms here other than shaver sockets with isolating transformers (last time I checked).
    – Transistor
    Aug 14, 2017 at 11:41
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    This doesn't answer the question, but have you considered using a fan with a humidity sensor instead? This way the humidity detection happens at the relevant point (at the ceiling) instead of somewhere off near the vanity cabinet/door. Panasonic, Broan, etc. have those models. You can wire them without a switch for automatic operation. Aug 14, 2017 at 11:42

1 Answer 1


There are two main ways of wiring a lighting circuit:

  1. The wiring for the circuit goes to the switch. From there a live and a neutral are teed-off. The teed-off live is switched, and teed-off neutral unswitched, and then run from the switch to its relevant light fitting.

  2. The wiring for the circuit goes to the light fitting. From there a live and a neutral are teed-off. The teed-off live for that fitting is then run to the switch and back, the teed-off neutral connects directly into the fitting. - This is the setup you have

It sounds like the new switch you've bought requires setup 1. There is absolutely no problem mixing these two setups. You could run a new cable from the fitting, taking the live and neutral together to the switch, and get rid of the existing cable to the switch.

Running the new cable may or may not be easy. In my house I can access the light fitting from above, as its in the loft. I can then find where the cable drops down into the wall to reach the switch, and feed a new cable through there (or tie it to the existing cable and pull through).

  • Oh I'm sorry, I see what you said there, it was a bit confusing. Nevermind. Aug 14, 2017 at 16:35
  • @AndyT, there is a white wire in the box -- the power socket uses it. But it is on separate circuit...
    – Mikhail T.
    Aug 14, 2017 at 18:19
  • @MikhailT. right but you don't want to use that one. Assuming your current setup matches #2 in this answer, you will need to rewire the fixture so a neutral goes to the switch box from the lighting circuit.
    – Doktor J
    Aug 22, 2018 at 15:20

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