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Old 09-02-2011, 04:47 PM   #1
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Neutral path between two main panels


I just came across DIY Chat - it looks like a great place. I've searched around but cannot seem to identify what the rules are regarding a neutral path between two primary panels so I thought I would post my question. Here is my situation:

I have 400 amp service (1 meter) (maybe 320 amp - I don't know for sure) feeding two 200 amp main panels. One panel is not a sub to the other, they both have main power feeders and each panel has a single ground/neutral bar. I assume the main feed neutrals are connected at the meter base but I don't know that for sure.

I am considering installing a backup generator and I am only interested in doing this in a safe, code approved way. One option may be to install a 400 amp transfer switch before the main panels but that would get very, very expensive and require a tremendous amount of rework.

If I use two transfer switches or two backfeed breakers *with* interlocks (one for each panel) so that I can energize both panels from the generator it would seem that I would be creating permanent neutral path between the panels because the transfer switches or interlock breakers do not switch the neutral . I've read that this secondary neutral path is a no-no.

I am confused, however, because my water pipes are currently bonded to both panels (it was that way when I bought the house). So it seems, the water pipe bonding has already created a secondary neutral path between both panels, no?

Is it OK if I create another neutral path between my panels with my generator wiring? If not, should my water pipes only be bonded to one panel?

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Old 09-02-2011, 05:12 PM   #2
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Neutral path between two main panels


Are these two panels in the same building?

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Old 09-02-2011, 05:18 PM   #3
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Neutral path between two main panels


Interlocks on each panel will work. You will not be creating parallel neutrals if properly installed.
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Old 09-02-2011, 05:48 PM   #4
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Neutral path between two main panels


The panels are located in the same building. They sit side-by-side.

What would be the proper method to do the interlocks and not create a neutral path? Interlock breaker in panel 1 connected to generator and interlock breaker in panel 2 connected to normal breaker in panel 1?
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Old 09-02-2011, 08:17 PM   #5
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Neutral path between two main panels


Wiring from interlock breaker 1 goes to j box. Wiring from interlock breaker 2 goes to the same j box. Wiring from panels to j box is spliced to the wiring or cable going to the generator inlet.
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Old 09-02-2011, 09:02 PM   #6
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Neutral path between two main panels


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Wiring from interlock breaker 1 goes to j box. Wiring from interlock breaker 2 goes to the same j box. Wiring from panels to j box is spliced to the wiring or cable going to the generator inlet.
Don't you need a disconnect also at the inlet? 225.31 and 225.32
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Old 09-02-2011, 11:45 PM   #7
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Neutral path between two main panels


brric, thanks for trying to help me understand this but I'm missing something. I've attached a drawing of my interpretation of your description but I don't understand how this does not create the secondary neutral path.

Code05, I don't think another disconnect would be required at the inlet but it may be. I'll read up on that once I understand the shared neutral problem.
Attached Thumbnails
Neutral path between two main panels-sharedneutralgeninterlock.jpg  
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Old 11-02-2011, 08:41 PM   #8
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Neutral path between two main panels


emiller,
Did you ever figure this out? Your diagram is exactly what I was planning to do on my two panel 300 amp service, until I heard about the concern with the second neutral path. Did you find out if this is safe and legal?

If its not, it looks like I'm stuck with running eack interlock to a seperate generator plug box on the outside. Then I would have to buy a genny with both a 30 amp and 50 amp plug output and run 30 amps to one panel and 50 amps to the other....might actually be better that way.

Last edited by reb162; 11-02-2011 at 09:51 PM.
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Old 11-02-2011, 09:37 PM   #9
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Neutral path between two main panels


Use separate inlets (male receptacles) on the side of the house, one for each panel to generator feed, and two cord and (female) plug sets coming out of the generator. No junction box down below as shown in the diagram.

The "secondary neutral path" will then exist only when the generator is connecte dup.

In which case the primary neutral path will be down below (shown on the diagram as secondary) and the secondary neutral path will be up above (shown as the fat gray lines).
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Last edited by AllanJ; 11-02-2011 at 09:39 PM.
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Old 11-02-2011, 09:54 PM   #10
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Neutral path between two main panels


So essentially, the exposure for the secondary neutral path is limited to only when the generator is hooked up and when the interlocks are on the utility, there is no secondary neutral path. Is it legal to have the secondary neutral path, even for the limited time on generator?
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Old 11-02-2011, 11:07 PM   #11
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Neutral path between two main panels


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Originally Posted by AllanJ View Post
Use separate inlets (male receptacles) on the side of the house, one for each panel to generator feed, and two cord and (female) plug sets coming out of the generator. No junction box down below as shown in the diagram.

The "secondary neutral path" will then exist only when the generator is connecte dup.

In which case the primary neutral path will be down below (shown on the diagram as secondary) and the secondary neutral path will be up above (shown as the fat gray lines).





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Old 11-03-2011, 07:19 AM   #12
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Neutral path between two main panels


If one panel has a 30 amp backfed interlocked breaker and the other has a 50 amp one then nothing prevents you from using a 50 or so amp generator with one receptacle and using a Y cord with one male and (to plug into the generator) and two female ends for the inlets on the side of the house.

(The inlets and the cables going to the panels and the Y cord must be sized for the current source, here, the generator's breakers, such as being 6 gauge copper wire for 40 to 60 amps.)
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Last edited by AllanJ; 11-03-2011 at 07:31 AM.
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Old 03-17-2014, 07:35 PM   #13
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Neutral path between two main panels


A search revealed this thread which I would rather revive than rehash.

I have the same setup as EMiller. Can anyone answer reb162's question, "Is it legal to have the secondary neutral path, even for the limited time on generator?"

Regardless, since the primary neutral path already exists between the two panels, why not use it by connecting both hots, the neutral and the ground from the generator inlet to one panel and only both hots to the other panel (capping off the neutral and ground)?

Otherwise, the only affordable option seems to be moving all essential loads to one of the two panels...

Last edited by bjmsam; 03-17-2014 at 07:41 PM.
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Old 03-17-2014, 07:48 PM   #14
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Neutral path between two main panels


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Originally Posted by bjmsam View Post
Otherwise, the only affordable option seems to be moving all essential loads to one of the two panels...

Or add another panel off one of the main panels.
Bring all essential loads to it and separate neutral and grounds.
Feed with generator.
Use lock out on that panel for main breaker and generator breaker.

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bonding, generator, interlock, neutral wire, transfer switch


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