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I have a bathroom with a switch for a fan, and another switch for a light. I replaced the light switch with a decora switch. For the fan, I'm using a smarter switch that has buttons for setting the on-duration of the fan (15, 30, 60 min). There are two switches, but a bundle of 4 neutral wires tied together.

What are each of the 4 neutrals for if there are only 2 switches? The reason I ask is I did a test whereby I connected the neutral wire of the timer switch to one of the neutrals coming into the box. And it worked. The other 3 neutrals were tied off together separately. When I did this, the light didn't work anymore.

Can someone help me understand what's going on here?

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Generally speaking, all the neutrals get tied together and the hots (and which often includes the switched hots) go off to different places. A typical example, which is a plausible case for what is happening here, is 4 cables:

  • Incoming hot/neutral from panel.
  • Light switched hot/neutral.
  • Fan switched hot/neutral.
  • Outgoing hot/neutral to someplace else. (Could be receptacles in the same bathroom or something else.)

In this case, initially all the neutrals would be tied together and not connect to any of the switches. Then you add a switch (timer) that requires neutral. The correct thing to do is to add a short piece of white wire (a.k.a., a pigtail) and add that to the existing 4 neutrals (may need a new, larger, wire nut) and connect it to the switch. However, if you instead simply "use one of those handy extra neutrals", something will stop working. Based on "the light didn't work anymore", you pulled the light's neutral, which you can verify by seeing if it is paired in a cable with the switched hot coming from the light switch.

Easy enough to fix - just use a wire nut to connect the 3 neutrals that are still in a bundle, the neutral going to the timer, and a new pigtail which will connect to the timer.

Note that you are in a good situation. Many older houses have switch loops that don't include a neutral, which makes adding modern timers/smart switches/etc. a bit more complicated.

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  • That makes a lot of sense. I didn't imagine lights had a neutral but it stands to reason this is necessary to complete the circuit. Same for the fan. Thanks so much!
    – hernsl
    Commented Feb 15, 2022 at 0:01
  • @FreeMan Not quite. Every electricity device needs at least two current carrying conductors to complete a circuit. For US 120V devices that means hot + neutral. For US 240V devices (e.g., water heater), that means two hots on separate legs. Commented Feb 15, 2022 at 15:32
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    D'oh! I've not messed with 240, so it's not in my thought process.
    – FreeMan
    Commented Feb 15, 2022 at 15:36

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