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Do I need to get an electrician to sort this out?
November 12, 2012 8:10 AM   Subscribe

Home electrical wiring with voltage between neutral wires - is this normal?

Our home in the US was re-wired shortly before we bought it, within the past ten years. I was working on one outlet (call it outlet A) to possibly add an outlet nearby and had turned off the power to it at the breaker. The power remained on at a nearby outlet B with an adjacent breaker in the panel (but the breakers are not tied together).

I tested the outlet I was working on and found no voltage between the hot and neutral, hot and ground, or neutral and ground. On disconnecting the neutrals from the screw terminals, I was startled to see a spark. The voltage tester lit up when put across the two neutrals at the screw terminals. This voltage disappears when the circuit for outlet B is also turned off.

The box for outlet A has three strands of Romex coming in. The receptacle has two white neutral wires at the screw terminals and a third pushed in to the back of the receptacle. It looks like the third hot wire is tied in with the others using a wire nut.

I wonder if a shared neutral situation between the circuits for outlet A and B could explain the spark I saw, or if there is some other sort of legitimate wiring scheme that would explain this, or if we should get an electrician out to get it fixed.
posted by exogenous to Home & Garden (6 answers total) 1 user marked this as a favorite
 
So if A and B share a neutral, and there's anything plugged in to circuit B, then separating the neutral across A could indeed cause voltage across the neutral at A. Circuit goes out B hot, goes through nightlight or wall wart or whatever, comes back through shared A & B neutral. Neutral is bridged across the A socket (rather than pigtailed, my preference), which means that when you open neutral there you suddenly have 110v potential through the nightlight/wall wart/whatever.

So, yeah, sounds like you've got a shared neutral circuit, and B is the other leg downstream.

I'm just a yahoo do-it-yourselfer, but on shared neutral circuits I like to tie the circuit breakers together so that when you turn one off it's obvious that the other gets turned off too. I also tie the neutrals together in the socket box and run a pigtail to the socket, so that the continuation junction is made with a wirenut and not with a screw on the side of the socket.
posted by straw at 8:26 AM on November 12, 2012


I should that the light plugged in to outlet B went out when the neutrals were separated at outlet A.
posted by exogenous at 9:25 AM on November 12, 2012


Bingo.

Sounds like a shared neutral daisy-chained through socket A. If you've got space in the box, I would join those with a wire nut and pigtail the socket. No matter what, I'd make note in the breaker box that those two share a neutral, and if they're adjacent I'd run a tie through the handles so that if you turn one off you have to throw the other.
posted by straw at 10:20 AM on November 12, 2012 [1 favorite]


They also make a cap you can use to attach two adjacent circuit breakers so one turns off with the other.
posted by zvs at 10:31 AM on November 12, 2012


Oh, and just because I'm paranoid: if you don't trust the wiring I'd find a way to check to make sure that indeed the hot in A and B are out of phase. If the breakers for A and B aren't adjacent in the box, they should be. Since you're handy with a voltmeter: If you don't feel comfortable moving them, then if it were my house, I'd plug an extension cord into socket B and check B hot against against A hot to make sure that there was 220v between them, confirming that they are separate sides.

If they aren't, for some reason, then putting a load like space heaters on both A and B could leave you drawing 30A through the shared neutral. That would be suboptimal, likely creating an unintentional third space-heater...
posted by straw at 4:11 PM on November 12, 2012 [1 favorite]


I just measured 230 V between hots on the two outlets so I think it's good. Thanks, straw.
posted by exogenous at 3:41 PM on November 17, 2012 [1 favorite]


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