Grounding a Genset/Main means fo disconnect

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Guyute

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Ft Collins, CO
I have a 400 amp service with a 400amp em disconnect for the genset with an ATS. There is no breaker on the genset, and the neutral is bonded at the genset. Do I bond and ground the 400amp em disconnect to the ufer, water, etc or is the ground rod and ufer at the genset suffiecient for my main bond. My big confusion is if in a back up power situation doesn't the 400 amp em disconnect become the main means of disconnect and a seperately derived system. If this is the case then I would need to have my main bonding electrode at the em disconnect sized according to 250.66. I also have to have an equipment bond between the generator and the disconnect. Any thoughts.
 
Does the ATS have a switched neutral? If not, then the generator neutral needs to be connected to the service neutral, and not at the generator.
 
Yes the ATS has a switched neutral. Once in back up mode the genset is creating the neutral, just like deriving a neutral from a transformer. This being the case wouldn't the neutral need to be bonded at the first means of disconnect, in this case the 400amp emergency disconnect not at the generator.
 
Where is the utility power bonding jumper? Is there a disconnect between the generator and the ATS?

Absent other info, it sounds like the generator's bond should be either at the generator itself or on the generator side of the ATS. If there's a disconnect between them, put the bond there. The generator will also need a proper grounding system, most likely a GEC to the existing system. (I find that it helps to think of the generator as another utility service for something like this.)
 
There is a disconnect on the utilty side that load sides to the ATS, and then there is another disconnect between the gen and ATS. I have a GEC at the generator and an equipment bond from the gen to the disconnect. If I bond at the disconnect between the ATS and gen can I use the same ufer to ground at the disconnect or do they need to be seperate. It shouldn't matter given that it is not the grounded conductor that is being shared.
 
Have you checked out 250.30? Is the generator inside or outside the building?

Since you have a seperately derived system 250.30 (A) will cover your install.

If the genset is outside you will have to comply with 250.30 (C) also.
 
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Thats the ticket. I just wasn't sure if a genset was considered a seperately derived system. After reading through it, including 250.35 as long as the neutral is not a solid neutral in the ATS then it is a seperately derived system. Thus, the same rules apply as a transformer, meaning I can bond and ground the system either at the generator or the first means of disconnect and size according to 250.66. Thanks for helping me work threw it.
 
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