Correct grounding for a standby generator on 240v grounded b phase

K

kreed

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Location
Michigan City
Occupation
Electrician
I am installing a standby automatic generator on on a 240v 3phase delta grounded b phase service. This is the first time I have installed a generator on a system with this configuration. I have a couple questions and concerns. Looking for some confirmation and/or direction. First off, as all of the service conductors will not be connected at any point between the utility and generator, I see the generator as a separately derived service. This would require me to install a grounding electrode and a GEC. I am told I will just need to literally connect a conductor from b phase to a grounding electrode(ground rod). One concern is the bonding between the grounded b phase and the GEC. Can the conductor that grounds b phase also serve as the GEC? If so, how does the bonding work in the transfer switch? I feel like the b phase to ground and the GEC need to be separated with the GEC having an independent conductor from its own electrode to the transfer switch. And bonding the frame of the generator as well. No where in the utility service is b phase bonded to the grounding system of the service. B phase is grounded at the transformer and the service is grounded at the service. I believe this is the correct way to install the grounding. No bond to b phase at the service disconnect. If all this is correct, I don't believe there is a required distance between the two different electrodes. So it should be the same for the generator. Am I making this more complicated than it needs to be? Grounded b phase always gets the brain smoking!
 
This is called a corner grounded delta. That might help your search online for how to install the generator. You might also want to make sure the generator is suitable for this type of set up.
 
I am assured by the manufacturer the generator is configured for the 240v grounded B phase. My concerns revolve around grounding the B phase and the service itself....
 
250.26(4). Yes you will have a GEC connected to the grounded phase conductor.

Your B phase is considered a grounded conductor.

See also see 200.6, 240.22, 230.75,

To get a better understanding, you might want to watch some videos on youtube explaining it.
 
The grounding of such a system is equivalent to the grounding of a normal 120/240V single phase system. Both systems have 3 circuit conductors, one of which is grounded.

So if you use a 2 pole transfer switch, and solidly connect the generator B phase to the service B phase, this is not an SDS, and no GECs are required for the transformer. The generator's B phase is grounded via the B phase grounding at the service.

If you choose to use a 3 pole transfer switch, then you have an SDS, and you must provide a GEC connection to the generator B phase. You'd have 5 conductors going to the generator: the 3 circuit conductors, the EGC/bonding jumper, and the GEC. Per 250.121, if you meet the terms of the exception, one conductor could play the role of both EGC and GEC. If you choose to use 5 separate conductors, the GEC does not need to be routed to the transfer switch, it can go straight to the GES.

Cheers, Wayne
 
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