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I discovered this in the course of replacing an old ungrounded receptacle with a GFCI. I connected a run of lights controlled by three-way switches (two switches) to the load side. Afterwards, as soon as I flipped either switch, the GFCI would trip. It turns out that one of the lights in the run has its neutral routed back through the GFCI, but the other one is spliced to neutrals from other lights on the same circuit that are controlled by a different switch (diagram below). This gives two different routes back to the junction box upstream of the GFCI, which naturally makes it unhappy.

So first, is it ok to have a hot wire send part of its current back through a different neutral like this? If so, probably the simplest thing for me to do is just move the lights off the load side of the GFCI. If it's not ok, how should the wiring be changed?

Super-high-tech diagram. I used blue for the neutrals since the paper is white: Wiring diagram

  • Better drawing than most people post. See that art class in elementary school came in handy. 😉 – ArchonOSX Dec 27 '15 at 1:05
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No, this will cause inductive heating if it passes through anything ferrous. Like a Romex clamp, locknut, MC cable, or steel conduit. It is called splitting a neutral and is really bad workmanship. Depending on the current load, it could become hot enough to start a fire. Change it as soon as possible.

  • Ug. The previous owner of this house did a lot of DIY electrical work, but he apparently wasn't very good at it (this is about the fourth serious issue I've found). Any ideas for fixing it other than somehow getting a blue line from the "sun" on the right to the lamp in the middle (which would involve running new wires through walls)? – dlf Dec 27 '15 at 1:19
  • @dlf can you get the new wires into the wall cavity between the studs from either the attic or the crawlspace? – Craig Dec 27 '15 at 1:28
  • @Craig maybe. The light in the middle is at the top of the stairs to the basement, and the light on the right is in the basement. The basement walls, including the one that divides the stairs from the main room, are covered in wood paneling. In order to get new wires between the two lights, I'd probably need to pull that down. – dlf Dec 27 '15 at 1:43
  • Another solution would be to put the light on the right on a different switch. This would be a bit less convenient than being able to switch it on from the top of the stairs, but more convenient than burning the house down. – dlf Dec 27 '15 at 1:44
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    @ArchonOSX As a temporary measure until I decide what to do, would it be sufficient to sacrifice the light on the right by disconnecting its neutral from the bundle and capping it off? – dlf Dec 27 '15 at 2:43

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