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I have a 4-switch junction box where three of the switches are on one circuit and the fourth switch is on its own circuit. I need to identify the neutral wire with the 4th switch to install a smart switch. How do I do this? Not sure if this is important, but the 4th switch is also a 3-way switch.

The house is new construction built in 2017.

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  • Always helpful to include pictures of the wiring in question. If you can add sharp, focused pics of the wiring attached to the switches and the wiring in the back of the box (even better if you can get 1 or 2 showing the whole things, so people can piece it together in their heads) that is incredibly helpful in formulating an answer. – FreeMan Sep 29 '20 at 15:25
  • That's fair, I understand pictures are helpful. I was just wondering if there was a standard approach to identifying the neutral wire without specifics of the outlet/box. – BigBrownBear00 Sep 29 '20 at 16:03
  • @BigBrownBear00 -- nope, we'll need photos of the insides of the boxes to help you further – ThreePhaseEel Sep 30 '20 at 0:29
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You may or may not have any neutral wire in the box, depending how it's wired.

With switch loops (which you can't do legally now, but could for a long time) there's quite often no neutral. Assume any white wire connected to a dumb switch is hot from a switch loop.

Otherwise, with dumb switches, look for a bunch of white wires not connected to the switches, but connected to each other.

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  • The house was built 3 years ago. Pretty sure there is neutral in every junction box, per code. – BigBrownBear00 Sep 29 '20 at 14:38
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    Such information is relevant and should be edited into the question. Pictures of the box in question showing the wiring can also be helpful. – Ecnerwal Sep 29 '20 at 14:42
  • look for the two white wires pigtailed together and NOT tied into the Dumb switch... that is likely the Neutral and the smart switch will add a third white wire tying into it. – mark f Sep 29 '20 at 15:14
  • two tied together? There are four switches in this box so there will definitely be more than 2 neutral wires in there. – BigBrownBear00 Sep 29 '20 at 16:04
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    Ok, I re-read the question. you should have two separate neutral pigtails since you have two separate circuits. one will have two wire and the other will have 4 wires. – mark f Sep 29 '20 at 16:25
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With 3/4 way circuits, you often don't get a choice

A smart switch typically needs always-hot, switched-hot and neutral all in the same box obviously. That often does not exist in 3-way / 4-way topology.

"Modern home. Every box must have neutral" -- #1 not every box: some 3-way switch locations are exempt. #2 Even if neutral is present, if always-hot is not present it defeats the purpose of having neutral.

Poaching neutral is Right Out. You cannot steal neutral from another cable (and certainly not another circuit). That will cause current imbalance which will cause all manner of practical and regulatory havoc.

The entire 3-way circuit needs to be rethought

When introducing smart switches to a 3/4-way circuit, one must map the circuit and see where it's possible to put the smart switch master. This may require using matching smart-switch remotes if you want full control at your preferred location.

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B/c the neutral wire(s) are shared across the same circuit, the 3 switches that share a circuit all had their neutral wires capped together. The single switch on its own circuit has a separate cap for its neutral wires. Visually, it was easy to see all the neutral wires twisted together and that is how I could identify which neutral went with which circuit/switch.

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