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I am installing a smart 3-way light switch which requires a neutral at both switches. One of them has the neutral, the other does not. But...I was able to pretty easily run a neutral from the bedroom light switch. The 3 way is out in the hall so it's on a different circuit. Got it all wired up and the hallway switches work great. But, now the bedroom circuit trips. Even when it trips, the 3 ways still work fine. Is there any solution I can do at the breaker box to bring this to code and stop the bedroom circuit from tripping? I don't understand what is happening as I thought all the neutrals went back to the same place. Thanks for any helpenter image description here

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    Does the bedroom breaker have a TEST button on it? What make and model are the 3-way switches in question? Can you post photos of the insides of the boxes involved please? Oct 20 at 23:00
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    Yes, the bedroom breaker has a test button, the hallway breaker does not. The hallway box just has hot and two travelers. The bedroom is a double box with two switches, one for closet and one for the overhead. I was able to drill through the back of the bedroom box into the hallway box and run a neutral through. Not sure how to post photo, will upload shortly. Edit: posted a photo of the bedroom box, the loose wire is the neutral I ran which I have disconnected currently.
    – Drew Reed
    Oct 20 at 23:16
  • @DrewReed -- can you get us a photo looking into the back of the bedroom switch box please? Oct 21 at 0:22
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The reason this trips the breaker isn't really relevant, since you're not allowed to do this thing.

You can't steal neutrals from other boxes

NEC 300.3 all related wires must be together in the same cable. Further, all currents must be equal and opposite in any given cable. In your case power is coming off one cable and returning on your pirate neutral wire, so currents will be unequal/imbalanced both in the cable and the pirate neutral. That will throw large amounts of EMF which can cause a variety of problems including arcing.

Most especially, neutral cannot be stolen from other circuits. Neutrals don't have fuses and nothing prevents an overload on a neutral except that the breaker supplying its partner hot would trip.

Your logic of "I've seen the service panel, I know all neutrals and grounds are all connected THERE" does not change that fact that we're not "there" and we are actually "Here". The distance is important. The wires are run separately for many reasons.

You can't just run single wires through walls

Any wire must be contained inside one of the wiring methods listed in NEC chapter 3. So the idea of having a bare neutral run between boxes through the wall, not in an approved conduit or cable system, is "made of nope".

I know neutral seems harmless to you because you heard it's the same voltage as ground. However, that's only true when everything is working properly. At other times neutral can have line voltage on it. Which is why it is insulated.

Just re-task the existing wires in the 3-way cable.

Almost every smart switch/3-way installation has to deal with things like this, since most smart switches need always-hot and neutral, and those two are never present in typical 3-way wiring. 3-way switch manufacturers have ways, listed in the UL approved instructions, to deal with these problems.

For instance, old switches need 2 travelers. Smart switches do not, but they do need always-hot and neutral at every switch location. So typically you re-task the black and white wires to be always-hot and neutral, and the red wire is used for data communication between switches, or simply becomes a spare if the switch uses wireless or power line communication.

So consult your instructions, and/or ask us, and we can figure out how to wire a smart switch with the wires you've got in the wall.

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You are NOT allowed to do that, for several reasons:

  1. If both circuits happen to be on the same leg/phase of your utility service, you can overload the neutral back to the panel by a factor of two, because neutrals don't have overcurrent protection. This can set your house on fire.

  2. The hot(s) and the corresponding neutral for for a circuit must be contained in the same cable or conduit, so that their magnetic fields cancel out. When you violate this rule, you can induce voltage and current in nearby pieces of metal, cause heating, and cause vibration in your electrical connections. This can also burn your house down.

  3. As you've noticed, if one or both circuits is on a GFCI or AFCI, they will detect this and shut off. This will prevent some deadly and house-burning situations, but obviously isn't workable long term.

Your options for having functional electrical and not burning down your house:

  • Get neutral from the proper circuit. May involve pulling a new wire in conduit, or running a new cable with an extra wire.
  • Find a smart switch that doesn't need neutral. They're rare, but some exist.
  • Go back to a non-smart switch.
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I ended up getting it to work with only one smart switch. A single Kasa 3-way smart switch senses whether the light is on or off and switches properly. Pretty sure that the second 'slave' switch simply operates as a normal 3-way switch, except that it also senses load and lights the led ring when the light is off. Not sure why this isn't mentioned in any documentation.

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  • Do note that technically your two boxes are now in violation of code because they are no longer installed in accordance with the manufacturer's instructions. The instructions do not allow for modification of the box by drilling holes in them for single wires to escape. You are most likely OK for now, but a particularly thorough home inspector at the time of sale might just pull random switches/outlets to look for wiring issues and may discover this.
    – FreeMan
    Oct 21 at 16:24
  • You mean they'd have an issue with a hole in the box? Now we're getting silly.
    – Drew Reed
    Oct 21 at 20:37
  • It may be silly (and I'd agree), but, AFAIK, that is the case. You're not allowed to drive a screw through the side of a plastic box to mount it to a stud, and you're not allowed to drill a hole through the back to run wiring. Even though there are holes top & bottom with integral wire clamps to hold cables...
    – FreeMan
    Oct 21 at 20:50

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