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Neutral shared on small part of 2 circuits.

2 months ago

I have a two gang box at my door to my attached garage. One switch for garage light, one switch for kitchen ceiling light. Both are three way switches. The power for both these lights are on the same circuit 8, but are provided at the other 3-way switch for each. From the other switches, they used older 2 wire cable and used the black and white as travelers coming to the door at the garage. This introduced the issue that there is no circuit 8 neutral present at the garage door area switches. So what was done, is that a small circuit nearby, circuit 11, was ran into this box just for the purpose of providing a neutral for the 3-way switches. On circuit 11, this sharing of its neutral is about midway in the circuit, with an outlet and light on one side, and two outlets on the side. Circuit 8 has alot of stuff on it, but in this area (with the neutral provided by circuit 11), it just has the kitchen ceiling light and the garage overhead light on it.

My question is how dangerous or problematic this could be, provided the following info. I understand how MWBC or "shared neutral" circuits work, and this appears to be a variation of a partial shared neutral circuit, but with the neutral not in the same cable as the ungrounded hot conductors. Also, I understand that either circuits neutral wires could remain "hot" with the breaker off, unless both breakers are off, so I have marked my panel and drawings accordingly. Also, that this might introduce EM leakage from the wires. Lastly, the two circuits in question are on opposite phases of my panel, so I imagine the current would cancel out, just as in a normal "shared neutral" circuit.

I understand this is not up to code or ideal, but it is working, and I want to make sure it is safe as long as I know about it and let any electrician I hire know also.

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