Shared Neutrals

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jeff43222

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Minneapolis, Minnesota
I had a troubleshooting call in an old house yesterday involving lights and a receptacle all being dead. I went through my usual routine of cracking open the fixtures and looking for a loose connection. At the panel (C-H), everything looked OK until I cracked it open. It turned out that the dead lights and receptacle were on two separate circuits that shared a neutral, but the two breakers were on the same bus. I found two other instances of improper neutral sharing inside the panel. The problem was ultimately solved when I torqued the screw where the shared neutral was connected in the panel. I was a little surprised that this fixed it, as the screw was already pretty tight.

I explained all of this to the homeowner and moved the breakers around so that the circuits sharing neutrals were side by side. I'm curious if other ECs would have done the same. Technically, I don't know for certain which neutrals are being shared by which circuits, as I didn't install the wiring (maybe someone played mix-n-match with the neutrals downstream somewhere). I just went on the assumption that if two hots and a neutral are the sole wires leaving the panel through a given piece of conduit, then they are a multiwire circuit.
 
Re: Shared Neutrals

First, a pair of circuits can function perfectly, with both breakers on the same leg, and with a shared neutral. But if both are loaded heavily enough, that neutral wire will do double-duty, and may overheat. So your solution to the multi-wire branch circuit issue was exactly the right thing to do.

But I share your doubts about the "lights out" problem being "fixed" by torquing what seemed to be an already tight connection. My guess is that there might be another point somewhere downstream at which the shared neutrals come together, perhaps under a wire nut, and the wires are loose at that point. It might be one of those "ghost problems" that may show up again at any time. Was the entire house live when you torqued that screw, and did you notice the "dead lights" come on at the moment you were tightening? If not, then it might have been coincidental that the lights worked after you applied the torque, and after you then turned it back on. I offer this suggestion only in the event that you get a call-back from this customer.
 
Re: Shared Neutrals

I always try to err on the side of safety, so although I didn't see any evidence that the shared neutral was overloaded, I recognized the potential for it, which is why I wanted to put the breakers on opposite legs. Since a neutral doesn't have protection like a hot does, I felt it was prudent to do what I did.

As for the cause of the problem, my first thought was that there was a loose neutral somewhere in the chain of outlets. Finding the chain was the hard part, as the panel wasn't indexed very well (big surprise, I know). The first few outlets were totally dead, but eventually I found a light outlet that had a hot wire. I opened up most of the outlets and all of the switches in question, and I didn't find anything loose.

Now that I think of it, I also doubt torquing the neutral in the panel solved the problem. I first noticed things were working again when I came out of the basement room that housed the panel and saw the basement stairs light was now on. In addition to torquing the screw, I did move several breakers around, and I remember noticing that the breakers weren't held very snugly in place.

My new theory about yesterday's problem is that the breaker (or breakers) protecting the dead outlets yesterday was probably loose, and my shifting the breakers around around got things re-connected. I think I'll check for breaker looseness first the next time I get a call like this. That takes far less time than cracking open dead outlets in succession until the loose wire is found.

I told the homeowner that if I had that panel in my own house, I'd replace it, but I also told her that my perspective is a little different from hers since I wouldn't have to pay someone to replace it like she would.

[ August 18, 2005, 12:00 PM: Message edited by: jeff43222 ]
 
Re: Shared Neutrals

I usually find a wire tracer to be a lot faster than opening all the outlets.

Mark
 
Re: Shared Neutrals

George:
Have you tried locating anything behind walls, floors or ceilings like they claim it to be capable of doing ???
 
Re: Shared Neutrals

Sorry, lost the thread. Yeah, it traces behind drywall and thin cabinets (buried disposals). Haven't tried much of anything thicker. :)
 
Re: Shared Neutrals

With an old house, you really could be completing the circuit by leaving a switch (3-way/SP) in a different position inadvertently. You might try troubleshooting with a ground wire to a good ground. Shut off one of the circuits that share the neutral, and then look for the break.
 
Re: Shared Neutrals

Had the same problem once.Circuit kept coming on and off.Decided to look at the panel first when the lights were dead and sure enough it was a bad breaker relaced it have not heard since that was about year ago.I do remember the panel was in a damp finished basement suggested to HO some options but never heard I am sure with time it will happen again.
 
Re: Shared Neutrals

I have the Amprobe AT-4005 (About $500). Kinda pricey, but has been worth it in many cases. Mostly in terms of good will when I find buried splices without making swiss cheess out of the walls.

Mark
 
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