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70 year old house with a breaker panel that was upgraded from fuses about 20 years ago. A couple new circuits were added at that time for the kitchen and laundry room, but the wiring in this particular situation is all the original.

Replacing a ceiling fixture, I turned off the breaker for the circuit that the fixture is on and removed the old fixture. There are four white neutrals bundled together into a old rusty wire nut (I have a new red one ready to go,) and a single black hot wire. This box is controlled by a single switch.

I removed all the wire nuts and old mounting bracket, and was screwing in the new mounting bracket - nudging the bundled neutrals around in order to do so - when there was a tiny spark on the neutral tips and lights on a nearby circuit flashed. Happened so quickly I wasn't sure I hadn't imagined it.

I jostled the neutral bundle around a bit and no reaction, so I continued. When I went to pull the neutral bundle through the new bracket in order to get ready for the new fixture wire, a definite spark flew and the lights on that nearby circuit completely went out. Wiggled the neutrals some more and power was restored to the other circuit.

Promptly put the nuts back on the exposed wires and let it be. Time to call a pro but what would be happening here? I've been in this house for 30 years and have personally replaced all the fixtures, plus most switches and outlets at one point or another and I have never run into an issue like this. Am I correct in guessing that the neutrals on this circuit are in contact with a hot wire on that adjacent circuit - either by loose connections, or worn insulation?

Thanks, Jamie

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It sounds like you have a neutral shared between loads on two (or more) circuits. This can be hazardous for a couple of reasons.

Firstly because when a neutral that is supplying a load is disconnected it will become live. So you test for dead with everything connected and it looks fine, but then you disconnect the neutrals and suddenly you have a live wire in the box.

Secondly because the neutral conductor may not have adequate overload protection.

There is a legitimate construction known as a multi-wire branch circuit where two separate hots share a neutral, but extra precautions are needed to make it safe. Firstly the two hot lines must have "common maintenance shutoff", either with a double pole breaker or with the two breakers handle-tied together. Secondly the two hot lines must be on different "legs" of the supply to prevent overloading of the neutral conductor.

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    Well said, yes I suspect it is a MWBC that was not properly handle-tied. You handle-tie them so you de-energize the neutral so you can do maintenance like this. – Harper - Reinstate Monica Nov 11 '20 at 5:34
  • Thank you for the responses. So in the short run if I turn off both circuits, and connect and install the new fixture, I'm good to go to get light into that hallway. Meanwhile, I make a note on those circuits in my panel for future reference, and find an electrician for a permanent solution? – jmea Nov 11 '20 at 12:48
  • Can you tell if the two breakers are on different legs? if they are on the same leg then you need to avoid putting large loads on the circuits in question until you can get things sorted out. – Peter Green Nov 11 '20 at 14:42
  • Peter, I had to use the Google for that. Presuming the top answer (If two single-pole breakers are on the same side and are stacked one on top of the other, they will connect to different legs.) is correct, then these circuits are on different legs. I spoke with a friend who is a commercial electrician and he said it is extremely common in this area with houses of my vintage, for a neutral to be shared by two hots on different circuits. He offered to help, and also gave me the go-ahead to finish, along with some careful guidance about resecuring the neutrals. Thanks for the helpful comments! – jmea Nov 12 '20 at 12:42
  • Yes separate single pole breakers placed next to each other in seperate slots will end up on different legs, be aware though that a tandem breaker (a pair of breakers that fits in a single slot) has both breakers on the same leg of the supply. – Peter Green Nov 13 '20 at 22:37

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