How to identify shared/common Neutral wire?
While replacing a defective circuit breaker in my 1982 Calif house, I noticed what may be a shared neutral on 2 other breakers.
I am suspicious because viewed at the main panel, 2 hot wires (from 2 separate, single pole breakers) and 1 neutral were all going into one conduit (house was built with flexible metal “greenfield” conduit). Worse, the 2 breakers were on same phase or leg, so the shared neutral could be at risk for overcurrent.
Are there any tips or techniques for easy & reliable confirmation of shared neutrals?
Probably I’ll replace those 2 separate breakers with a single, 2-pole breaker — but only if I can be certain what “might” be a shared neutral, really is.
I’d much prefer to have dedicated separate neutrals on those 2 breakers, but even with the flex greenfield conduit it would be an arduous job to pull a new wire and then rewire 8-10 outlets — not worth it.
Replies
If you've got two hots (from separate breakers) and one neutral then that's a shared neutral. If the hots are on the same "leg" and the sum of the two breakers exceeds the current capacity of the neutral wire then that's a hazard. Somehow rejiggering to place the two hots on opposite legs would alleviate the overcurrent hazard (though there may still be a technical violation of code in there somewhere).
Of course, in practice the likelihood of simultaneous full-breaker-capacity current on both breakers is unlikely, save for someone running lots of electric resistance heaters.
Shared neutral
A shared neutral is OK if the two hots are on opposite legs. Put them on opposite legs so as to not down grade the system if you are reasonably confidient there are no issues down the line (i.e. they are not connected together in some outlet box.) Technically I think they have to both trip together in the event of an over load. By that I mean be on a two pole or ganged circuit breakers, but I wouldn't worry too much about that.
It is possible and legal in some cases for 120 and 240 volt outlets to share the same circuits. In my shop I have a 3 wire 20A circuits, for example, with both kinds of outlets. This is common in industrial applications but unnecessarily complicated in residential work.
Too bad there isn't some "tone detector" type of equipment available to trace electrical wires, like the telecomm wiring guys use ... but I was able to "brute force" verify the shared neutral.
Let's call the 2 separate breakers with suspected common neutral, "B1" and "B2". Conveniently I had receptacles off B1 & B2 prresent in the same room. Turned off power to whole house, disconnected the suspected common neutral back at the main panel, disconnected all appliances from B1 and B2, and then re-energized B1 & B2 circuits.
After measuring receptacles w/ a DMM, confirmed B1 & B2 were indeed on a common neutral and on the same phase/leg -- bad.
Found a Murray 2-pole Combination-AFCI breaker at Home Depot ($85) and rewired the two 1-pole breakers to the new 2-pole AFCI. The 2-pole AFCI is designed to work on 1 shared neutral. Minor inconvenience: both circuits will now always trip simultaneously.
Now the 2 circuits on common neutral have the hot on opposite phase/legs (good), and I have AFCI --- AFCI is what I wanted all along.
Happy at last :-)
tom999 wrote:Too bad there isn't some "tone detector" type of equipment available to trace electrical wires, like the telecomm wiring guys use ... but I was able to "brute force" verify the shared neutral....
Well, something like the Fluke 2042 cable locator would have been nice to have ... but seems to cost over $500 :-(
Circuit trackers.
There are circuit trackers for line voltage wiring.