Multiple Generators with Common Neutral

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shortcircuit2 said:
But Iwire...a neutral in a MWBC is from the same source...right?

So a hot in one MWBC would have potential to the neutral in another MWBC.
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Yes.



The 3 gen-sets are seperate sources so there would be no potential to the other thur a interconnected neutral.
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If the neutral of all three gensets were isolated there would be no potential between gensets.

But the neutral of all three (four really) are all connected so there is potential between the gensets (and utility) But that fact is not unsafe.
 
I am having a hard time following this thread, or I am missing something. You don't have a "common neutral" in the sense that under any condition, electrons (or current flow) from one generator and another generator are travelling along the same piece wire at the same time. It would take multiple faults to create a situation where this would occur, including at a short between two wires that should only share the same physical space inside the main panel. Draw it out.

Hint, the current doesn't have any way to flow along the neutral wire between the originating panel and the transfer switch. I tried to do a sketch, but it only makes sense to me, I can't figure how to draw it without crossing wires everywhere.
 
shortcircuit2 said:
All the neutrals are interconnected because the transfer switches don't break the neutral.
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I'm more interested in whether or not you will have problems if you don't group all of the CCCs of a circuit together. I don't know the code section that references it, but you can have issues if you don't keep them together.

BTW, you didn't answer my question.
 
hardworkingstiff said:
Are you running the neutrals from the generators to the 400-amp panel and the hots to the transfer switches? If so, how are you doing this? Where do the neutrals and phase conductors separate?
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My understanding it is all wired as normal feeders would be, each with their own neutral.

He called is a common neutral but it is separate neutrals connected to a common neutral bus at the 400 amp panelbaord.
 
iwire said:
My understanding it is all wired as normal feeders would be, each with their own neutral.

He called is a common neutral but it is separate neutrals connected to a common neutral bus at the 400 amp panelbaord.
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I understood he is not bringing the neutral from the generator to the transfer switch. I can't get a handle on what he's doing .... exactly.
 
shortcircuit2 said:
Take a look at the 1-line diagram below. The diagram shows a layout of 3 single phase 120/240volt separate generators feeding one building with simple 2-pole transfer switches.

Would there be trouble with the generator electronics and a common neutral between all 3 units?

Is there any concern if standard 2-pole transfer switches are used and 1 gen-set were to loose a neutral or hot leg?

Should the set-up use transfer switches that break the ungrounded conductor making them separately derived?
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I'll try going back to the start.

First question, no, because it isn't a common neutral in the sense that any one section of wire is common to the circuit of one generator and also to the circuit of the other generator under any normal or almost any fault condition.

Second question. No concern beyond freaks of nature. It would take some almost unimaginable fault combination to create a dangerous condition to personnel. You would be more likely to be walking under a power line and have it break and fall in to your hands, electrocuting you.

Third, you can, but then you would have to create a grounding electrode system, including building steel, for each of the three generators, and then think about it, the grounding system would actually have a similar interconnectivity as the neutrals you are concerned with.

Hope this helps given the confusion I have felt reading through our responses to you.
 
iwire said:
At the transfer-switch, but it is just a neutral bus, not a switched pole of the transfer switch.
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Yes this is correct. The neutrals from all 3 points connect at the non-switched bus in the transfer switch.

Look at the 1-line diagram in post #1...feeder in from the 400amp panel, feeder in from the generator, and feeder out to the subpanel.

All this x 3

I could use up to 6...right? (six disconnect rule)
 
shortcircuit2 said:
Yes this is correct. The neutrals from all 3 points connect at the non-switched bus in the transfer switch.

Look at the 1-line diagram in post #1...feeder in from the 400amp panel, feeder in from the generator, and feeder out to the subpanel.

All this x 3

I could use up to 6...right? (six disconnect rule)
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Depends, the way you drew your first picture, it implies that you have a 400 amp main disconnecting means. This is the one disconnect. any disconnects/breakers further down the line have nothing to do with the 6 disconnect rule. If on the other hand your drawing is slightly misleading and the individual breakers are in fact each a service disconnect, then yes you are limited to six. If I were the AHJ, I would not allow you to have space available in your switchboard to add more than 3 additional one pole breakers.
 
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