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Old 06-29-2009, 10:58 AM   #1
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Default Same phase sharing neutral

Hello,

I was wondering if someone could give me a good idea of what would happen if two of the same phases were to share a neutral.

What would the outcome be? Would the breakers merely overlead and trip? The neutral, as I see it, would be overloaded. Would the neutral blow up at the splices?

Now this leads me to my second question, if two of the same phases share the same neutral, the neutral is, as I stated earlier, overloaded. So theoretically if you have 120V 15A on each circuit, what is the return on the neutral? 30A, 240V? If you were to up the size of the neutral wire to #10 AWG or larger (sufficient size to carry 30A), would this carry the load back to the panel? Perhaps the problem is not with the magnitude of the current and/or voltage, perhaps it is that both phases reach the peak of their sin wave at the same time? Clueless here

All of this is merely theoretical, things I have pondered on the job. Please, don't only let me know the answers to my questions, but let me know if some of my questions are worded incorrectly as well. I am all ears

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Old 06-29-2009, 11:22 AM   #2
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You have pretty much nailed it. Except for the voltage on the neutral. If two circuits on the same phase share a neutral, the current in the neutral will add. The voltage across the neutral is still zero, or very close.

I don't know about Canada, but the NEC does allow this if the neutral is sized appropriately. However, I have never seen anyone do this, and would never do it myself.

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Old 06-29-2009, 11:56 AM   #3
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Awesome! Thanks for the prompt reply.

Well, hearing my assumptions are almost all correct is a good thing I just want to work out a few more things though if you don't mind!

The voltage across the neutral is still zero... hmm. By this do you mean that the change in voltage is zero/minimal? Or do you quite literally mean, the measure of voltage on the neutral is zero, due to voltage drop at the load?

Also to clear up my earlier question: What would happen if two of the same phase was to share the same neutral.

A. Blow up at the neutral splices?
B. Trip the breaker?

Looking forward to see what you have to say

Wow, so the NEC would allow this... interesting. So theoretically, one could return two live wires of the same phase so long as the neutral was of sufficient size.

Thanks again
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Old 06-29-2009, 12:18 PM   #4
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The voltage on the neutral is very close to zero because it s connected to ground at the service equipment. When a current flows, that changes slightly, but not enough to be significant.

An overloaded neutral would simply overheat if it wasn't sized properly. This would be especially evident at splices. I have seen wire nuts melted from this very thing. Nothing as dramatic as an explosion. And no breakers would trip unless the hot wire feeding the circuit was overloaded.

And while the NEC allows it, it is more or less a hold over from days gone by. We now require that circuits that share a neutral to be disconnected simultaneously, but with the way panels are laid out, it would be hard to connect the handles of breakers together that are on the same phase.
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Old 06-29-2009, 01:28 PM   #5
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Thanks alot, very informative You have answered all of my questions.
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Old 06-29-2009, 06:05 PM   #6
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I don't know about Canada, but the NEC does allow this if the neutral is sized appropriately. However, I have never seen anyone do this, and would never do it myself.
I might disagree with this only in practical sense. The NEC 2008 now requires simultaneously disconnecting all ungrounded circuits of a MWBC. Art. 210.4.

Technically this would not be considered a MWBC but I would think the effect would be the same. I wonder if the 2011 would have an amendment to this.
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Old 06-29-2009, 08:06 PM   #7
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When you open a panel and see a neutral that is remarkably brown in color, that's your clue that it might be a multiwire branch circuit that someone fed both hots with the same leg.
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Old 06-29-2009, 08:11 PM   #8
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Two circuits on the same phase sharing a neutral is permitted by the NEC, AFAIK, but I've never heard of it done.
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Old 06-29-2009, 08:35 PM   #9
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My take on this scenario is, if the "neutral" is shard by 2 hots on the same phase or leg, it is technically a grounded conductor. The voltage seen on the grounded conductor is still 120vac only the current is additive. Thus meaning 15A and 15A on L1 or "A" phase is 30A, which would mean you would need a grounded conductor capable of handling 30A. It is only considered a "neutral" on a MWBC and then it only carries the unbalanced portion of the load.
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Old 06-29-2009, 08:45 PM   #10
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My take on this scenario is, if the "neutral" is shard by 2 hots on the same phase or leg, it is technically a grounded conductor. The voltage seen on the grounded conductor is still 120vac only the current is additive. Thus meaning 15A and 15A on L1 or "A" phase is 30A, which would mean you would need a grounded conductor capable of handling 30A. It is only considered a "neutral" on a MWBC and then it only carries the unbalanced portion of the load.
Yes, most of us know it is a "grounded conductor", but it is just easier to call it a neutral. When I wire a receptacle, I land the hot and neutral wires, not the hot and grounded conductor. I don't think anyone has ever been confused by it.
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Old 06-29-2009, 10:45 PM   #11
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Never seen it done... never thought about it and never looked it up. Oh well now I am woundering why you ask that question, what brought it to your mind? I know you said stuff came to your mind when you were at work.
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Old 06-30-2009, 08:30 AM   #12
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I was pulling a new phase wire for dual switching T8's in a Canadian tire and a co-worker asked me if it would be ok to share the neutral of another circuit. I gave a reasonable answer but I was incurred with curiosity. I just had to find out a little more.
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Old 06-30-2009, 03:23 PM   #13
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Ive seen it done several times but usually on a service call where a neutral has been lost. Usually find a melted wire nut or wire burned into inside the panel at the neutral bar.
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Old 06-30-2009, 05:13 PM   #14
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It is only considered a "neutral" on a MWBC and then it only carries the unbalanced portion of the load.

What!? So you are saying that if it's not a MWBC then there is no neutral? So this means that my dishwasher which has a 12-2 romex homerun which is 120 volts and the black wire is connected to the 20 amp breaker and the white wire is landed to the neutral/ground bus bar, that the white wire isn't a neutral but instead is a grounded conductor? Are you aware that they are the same thing? Jesus christ man. Wake the "F" up for crying out loud!
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Old 06-30-2009, 05:35 PM   #15
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What!? So you are saying that if it's not a MWBC then there is no neutral? So this means that my dishwasher which has a 12-2 romex homerun which is 120 volts and the black wire is connected to the 20 amp breaker and the white wire is landed to the neutral/ground bus bar, that the white wire isn't a neutral but instead is a grounded conductor? Are you aware that they are the same thing? Jesus christ man. Wake the "F" up for crying out loud!

Take a breath, step back.......then look for 'neutral' in the '05 or earlier.

Up through the '05, there was a neutral only in MWBCs. Otherwise, it was technically the grounded conductor.

I think the '08 is a step towards bringing the vulgar terms used by field electricians into acceptance.
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Old 06-30-2009, 05:40 PM   #16
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Take a breath, step back.......then look for 'neutral' in the '05 or earlier.

Up through the '05, there was a neutral only in MWBCs. Otherwise, it was technically the grounded conductor.

I think the '08 is a step towards bringing the vulgar terms used by field electricians into acceptance.
oh ok that's better.
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Old 07-04-2009, 04:46 AM   #17
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So this means that my dishwasher which has a 12-2 romex homerun which is 120 volts and the black wire is connected to the 20 amp breaker and the white wire is landed to the neutral/ground bus bar, that the white wire isn't a neutral but instead is a grounded conductor?
Of course, we all know they're the same thing, but technically speaking, what you just said is exactly correct.

I use 'neutral' for grounded, and even 'outlet' for receptacle, and haven't been hit by lightning yet.
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Old 07-26-2009, 08:40 PM   #18
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well you wouldnt get 240 volts because even though your sharing the neutral your only going through half of the wound coil back at the transformer. There isn't enough difference in potential. Good insight though nonetheless.
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Old 10-23-2009, 09:17 AM   #19
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I assume what you're talking about is having the same phases from 2 different panels (supplied by different transformers). If this is the case, you'll be creating a ground loop which is to be avoided.

If instead, you're referring to 2 panels fed from the same transformer, then there would be no problem as long as the neutral is sufficiently large enough to handle the load.
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Old 04-05-2010, 02:20 PM   #20
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You should'nt use a common neutral for branch circuits supplying equipment.If for whatever reason you lose that neutral a series circuit is created which can reak havoc with the voltages supplying that equipment.Ask me how i know.Our code does'nt allow it for that reason.

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