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#49151 02/28/05 03:10 PM
Joined: Jun 2001
Posts: 141
amp-man Offline OP
Anyone have a good way to identify multi-wire circuits at the panel?

Have a case where the service and panel were replaced in a house with old wiring (knob and tube), and the original wiring was brought to a j-box (or boxes). New conductors were spliced on, with several circuits converted to shared neutral circuits, and new unlabelled conductors brought into the breaker panel. A similar instance where original wiring was done in pipe and again, no labelling of conductors on multi-wire circuits.

So in both cases, I can't tell by inspection if adjacent breakers are separate circuits or if they share a neutral (unlike a situation where the wiring is done with NM and you can see the three conductors of the Edison ckt entering the can together).

Going back to the j-boxes to find (and then buzz out from j-box to panel) the multiwire ckts would be my last choice. Tight crawl below the house, and a couple feet of blown in insulation in the attic, and who knows where those j-boxes are.

One approach I see as workable would be to turn on loads on all circuits, measure amperage on each ungrounded conductor at the breaker, and on each neutral. Flip off a breaker and see on which neutral there is a corresponding decrease in current. This should tell me which neutrals are shared, and by which circuits. This aproach will be somewhat tedious, as the Square-D Homeline panel is cramped and it'll be tight to get an amp-clamp onto many of the neutrals.

Man, I do like commercial work where the panels generally have a lot more room in which to work!

Anyone have another technique, or any ideas I could try?



Joined: Jan 2003
Posts: 4,391
Cliff if need be I would turn off all the breakers and turn one on at a time and then find the neutral with a load on it.

I can't help but ask why you want to do this?


Bob Badger
Construction & Maintenance Electrician
Joined: Jun 2001
Posts: 141
amp-man Offline OP

I'm adding circuits to a panel. I'll have to use twin breakers and so replace a couple of 1" singles. There are spaces in the panel for twin breakers, but to get to the section of the busses that'll accomodate the twins, I'll have to move around some breakers.

I don't want to wind up with the two ungrounded wires of a multi-wire circuit on the same pole! I could just assume that everything is O.K. now (there aren't any problems evident and there are no signs of overheated neutrals), and just be sure to keep the ungrounded conductors on the poles they're now on.

But I'd like to identify the Edison circuits, just to make sure there's no miswire now that might become a problem later, although not because of anything I did.

Thanks for the suggestion.


Joined: Feb 2003
Posts: 687
If you have all the wires unhooked from that run do a continuty test. Note this only works if some sort of load is on that circuit. For older homes there is probibly too many things pluged in so this test will work.

You find your 1 nut. and 2 hots (1 phase) that have continuty together. If I have everything apart and am checking things I try to look to see too many have continuty together. Meaning a misswire with 3 hots for 1 nut.(1 ph), nutrals wires tied to ground, 2 nuts. tied together.

Some things should make sence like wire sizes of a network tend to be the same (but no garantees). Networks need to be in the same raceway.

Once you figure it out tape them together.

Try to figure out your networks before you move things around.


Joined: Nov 2002
Posts: 794
Likes: 3
Would this work?: Replace one of the old breakers with a twin breaker. The old wire that went to the old breaker now goes to one of the two outputs of the twin. The other output is avaliable for a new circuit. This should keep the old wire on the same phase. Then repeat for additional breakers. Once you're done with this you'd have a few twin breakers all with an old wire connected and all with avaliable outputs for new circuits. Do one at a time so the wires don't get mixed up. This should maintain proper phasing so the edison hookups stay correct. You won't know if you had to worry about this on all the circuits, but you'd save time spent on hunting them down, and still know that you didn't mess any up.

Does the panel have alternating phases on adjacent slots? If you move an existing breaker an even number of slots down, it should see the same phase. (assuming 120/240V house setup, not 3 phase). Thus you should be able to move 1 breaker to make room for a twin where the panel expects it.

You could measure the voltages on each circuit, make notes as to which read say 0V from the red feeder and which read 240V from the red feeder. Then when you finish moving things retake the readings and compare against your notes.

[This message has been edited by wa2ise (edited 03-01-2005).]

Joined: Aug 2001
Posts: 332
Can you replace a suspect breaker with a GFCI breaker? GFI breakers won't set if the neut is shared.

Joined: Jan 2005
Posts: 5,411
Likes: 1
Cat Servant
The only way I can think of....that Might work (haven't actually done it) is to use one of those old-fashioned "blinker buttons" (like they used to use for Xmas lights) on a 60W bulb on the circuit...then amp-clamp the neutrals to find the one with the current fluctuating by half an amp.

Joined: Jun 2001
Posts: 141
amp-man Offline OP

All excellent suggestions, thanks very much for offering them. Like that saying, "all of us are smarter than any one of us".

Yes, I also had the thought of using a drop light with a lamp flasher button in it. Or buying one of those cool Etcon load flashers.

Something simple that I'm going to try, is to do it by measuring voltage. I'll make sure every circuit has a load on it (a light, for instance), and then cut power at each breaker, one at a time, and measure voltage from the breaker terminal to ground. A lot easier than measuring current, eh?

For instance, breakers 1 and 2 supply an Edison circuit, and being properly wired, are on phases A and B respectively. Because both halves of the circuit have a load, if I open breaker 1, I should measure some voltage to ground at the load side of that breaker. The volatge originates from the half of the multiwire circuit fed by breaker 2, and is being backfed through the loads.

This is a variation of the suggestion to lift the ungrounded conductors and measure continuity. I suppose just opening the breaker and then measuring continuity would work, as well.

What made me think of this is the time I had a service call where the lights in the dining room of a house only worked (and only dimly) when the electric oven was on! The residents thought that the place was haunted; the lights would go on and off at random, too.

They'd lost one leg of the service, and when the oven was on, it backfed the dead pole so the the lights on that leg would light dimly. It was funny, they hadn't noticed that many of the receptacle outlets were dead...and the lights going on and off was when the oven reached temp and the element was cycling to maintain heat.

I'll report back after I've tried this.

Thanks again,


[This message has been edited by amp-man (edited 03-03-2005).]

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