Sharing neutral from separate panel

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George Stolz

George Stolz

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Windsor, CO NEC: 2017
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Hospital Master Electrician
Finally got to do some commercial! It's an office building with a 3 phase 120/208 1200-ish amp service (didn't look, that's a guess) with four meters. Apparently on the rough, some misguided soul pulled a 12-3 run to two furnaces in the attic from panel D, and a 12-3 run to two furnaces on the main floor from panel B. These were supposed to be 4 separate 12-2 runs from each of the four separate panels.

My question (which will focus on one pair): Would it be code-legal (first) and in tune with the laws of physics (secondly) to pull a 12-2 run between panel A and B? In essence, taking power from the A phase of Panel A, and the B phase of panel B, attach it to the existing 12-3 home run, using the neutral from panel B? (Provided I were to test and verify that the phases did match up panel to panel.) The reason being that Tenant "B" doesn't want to pay for the heat for Tenant "A".

Bear in mind: one transformer, one service, 30 minutes labor.

As it was, my idea was controversial, so we spent 8 hours labor getting a new home run to the attic from the main floor. And we ain't done yet. So was I right in my theory, or right in keeping my mouth shut?

--George
 
Re: Sharing neutral from separate panel

I?ll address the physics first. Once you separate the power leaving any given point (i.e., 4 separate meters, leading to four separate panels), you cannot safely bring them back together again. The load on any one panel will differ from the load on a second panel. That gives a difference voltage drop, and more importantly, a different phase shift. That is, the current in one panel will lag the voltage in that panel by some amount, the current in the second panel will lag the voltage in that panel by some amount, and the two amounts will not be the same. You would risk a non-synchronized closure (with possibly explosive consequences) every time you closed one of the breakers.

Next, let?s talk about safety from another viewpoint. Your concept would have set up a situation in which a worker could turn off the breaker on panel B, go to work on the B furnace, and discover (perhaps too late) that there is a voltage coming in from panel A. In other words, you would need to turn off two breakers to de-energize one load.

Finally, as to code, you would have had a violation of 210.4: ?All conductors shall originate from the same panelboard.?
 
Re: Sharing neutral from separate panel

210.4 would not allow what you wanted. as well as other problems


210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.
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Re: Sharing neutral from separate panel

Let me expand on this comment:
Once you separate the power leaving any given point. . . , you cannot safely bring them back together again.
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It occurs to me that running two or more conductors in parallel raceways, and connecting them at both ends of the run, fits this description. This is safe, however, because (and only because) we are required to keep the same physical characteristics (e.g., length, cable type, cable size) in each of the parallel raceways.
 
Re: Sharing neutral from separate panel

It looks like we all jumped onto the same code article at about the same time. Great minds think alike! :D :D
 
Re: Sharing neutral from separate panel

I'm impressed. :)

Roger
 
Re: Sharing neutral from separate panel

I am not, thats Too easy for you guys. Your the best in the country.
 
Re: Sharing neutral from separate panel

Well, schucks. Guess I'll stick to doing tricky things in houses... :)

It's pretty wild to walk away for an hour and get thirteen responses, that all agree! :D

Thanks, guys.
 
Re: Sharing neutral from separate panel

Okay, I think this might be the answer to something that Matt and I had a discussion about today...but I'm not sure. Here's the deal...say you pulled a neutral one panel, and a hot from another panel? This would still function as a circuit, correct? The heading of 210.4 is Multiwire Branch Circuits, and since the neutral would not be shared, and only used for one circuit, this would be a multiwire BC...so...wouold it be a violation of 210.4? If not, is it legal? I don't really know why you'd want to do something like I described...just curious. :D

At first, when I read that, I thought it might be, but then I read it agin and realized it talked about multiwire...so...educate me, please. :)

[ December 16, 2004, 02:49 AM: Message edited by: midget ]
 
Re: Sharing neutral from separate panel

Originally posted by hurk27:
George the funny thing is: look at the time of each post. :)

Midget, As I understand it, multiwire branch circuits are when you use pre-made cable such as MC, NM-B, etc, to make a circuit. Might not be 100% correct on that. If you look in "Definitions" you will find the 100% correct answer to that.

In the scenario I described, I would be sharing a neutral for two circuits between panels. This would be violating 210.4. Now, my next question for the guys would be, "If I was using MC cable with an insulated ground, re-identified the ground as a neutral on both ends, and grounded my furnaces to a nearby water pipe, that would be legal?" This actually isn't an option for me, as it happens that this circuit was pulled in NM.

My desire to do this highly irregular and ill-advised stunt was to avoid the hole I have in my head from roofing nails, and having to get into the attic in the first place. Plus, it would show off my unconventional way of thinking to my co-workers. It literally consumed at least six hours labor for the attic portion yesterday, leaving the first floor repull for today, and the illegal and dangerous fix I was contemplating would be done in under an hour. Which just goes to show that if you're not sure, it's probably safer to stick with what you know or get superior advice. Better to spend the extra time and material than to jeopardize people and equipment for "being clever." :) I understand now that very bad things could have happened if I'd had my way. :)
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Re: Sharing neutral from separate panel

As I understand it, from the definitions, it doesn't say anything about cable type...as I interpret it, you could pull single conductor, and share the netural, and have a multiwire branch circuit? Am I wrong in my thinking?

Plus, did you read my question? Would what I asked be a violation? :p I think so, but I wanna know for sure.
 
Re: Sharing neutral from separate panel

You mean, is it legal to use one neutral for several circuits? Yes, you just need to be sure the circuits are on different phases so you don't overload the neutral.

Write out the definition, I'm too lazy to get my book... :D
 
Re: Sharing neutral from separate panel

Midget yes a multiwire circuit can be run using any chapter 3 wiring method.

One thing that wasn't mention on this is all circuit conductors are required to be ran together when a metal raceway is use or where a circuit conductor passes through any metal openings (Panel cabinet). This is to prevent inductive heating of the metal. See 300.20

The other thing is the magnetic field this would produce would be high.

And what Charlie touched on is if someone were to shut off the power to work on one of these furnaces and had to take apart the neutrals there would be a shock hazard because the other circuit would still be energized. this would put the return neutral at a 120 volt potential to anything grounded.

[ December 17, 2004, 01:10 AM: Message edited by: hurk27 ]
 
Re: Sharing neutral from separate panel

What if you were to identify the piece of equipment as being fed from multiple locations? Or is that not what those signs that say that are to be used for?
 
Re: Sharing neutral from separate panel

Originally posted by midget:
What if you were to identify the piece of equipment as being fed from multiple locations? Or is that not what those signs that say that are to be used for?
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:eek: You'd still be in Violation of 210.4. All must originate from same panelboard, loadcenter, etc...

Using a common Neutral to carry the unballanced portion of 2 or more current-carrying conductors from different panels is not only illegal, but, extremely dangerous! Signage may alert someone of the danger - but - this won't make it legal.
 
Re: Sharing neutral from separate panel

And what Charlie touched on is if someone were to shut off the power to work on one of these furnaces and had to take apart the neutrals there would be a shock hazard because the other circuit would still be energized. this would put the return neutral at a 120 volt potential to anything grounded.
Click to expand...
Supposing my original layout with two furnaces fed from the same panel were acceptable to the customer, needing no change, would you have put the furnaces on a two-pole breaker? Using two single pole breakers would not be in violation of 210.4, but would pose the same risk you describe, which begs the question: Does a competent electrician who opens up a disconnecting means bearing a three-wire home run need this clarified?

Not trying to pick ya, Wayne, it just came across as a little extreme. :)

[ December 18, 2004, 12:08 PM: Message edited by: georgestolz ]
 
Re: Sharing neutral from separate panel

Originally posted by Midget:
Here's the deal...say you pulled a neutral one panel, and a hot from another panel?
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Hey, I finally figured out what your original question was! :D

While it might work, it definutely is not a good idea. But, no worries, I seriously doubt you'd have occasion to.
 
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