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

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kreed

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Michigan City
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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
 
On another uncertainty, the generator is configured as a 4-wire delta 120/240v.

1.) Do I literally ground b-phase right off the generator? I'm not seeing a lot of info out there on supplying a grounded b-phase service from a generator. Also, it would be much easier for me to connect b-phase to ground at the transfer switch. Would that be acceptable?

2.) Do I just ignore any connection from the neutral (4th conductor) from the generator. The lift station has a 3-wire grounded b-phase from utility. So i have no need for the 4th conductor.

3.) The existing service from utility does not have a bond between b-phase and the gec at the first disconnect. Is that the correct wiring as well? Seems something is off. But I do not have a lot of experience with grounded phase either.
 
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If you have a corner grounded delta than you will not be able to get 120V from the center tap. You will need a transformer to do that.

When you say "grounded B", do you mean you measure 0V between B and ground or 208V between B and ground?
 
The utility service is a 3 wire 240volt grounded b phase. So, 240 volt phase to phase. a to ground = 240v, c to ground = 240v, b to ground = 0v
 
I do not need 120v from the generator. I only need 240v grounded b-phase. But the generator nameplate shows it is configured as a 120/240 volt 3 phase delta. so do i just ignore the 4th conductor and literally connect b-phase to ground right off the generator?
 
Elect117 said:
If you have a corner grounded delta than you will not be able to get 120V from the center tap. You will need a transformer to do that.
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Physics-wise, if you ground one of the corners of the center-tapped coil, you could still supply a 120V load with that corner as the "neutral" and the center tap as the live. You'd need a 3 busbar panel for distribution.

Depending on your interpretation of "one phase is used", that does or does not comply with NEC 250.26.

Anyway, sounds like the OP doesn't need the center tap on the generator, so they can just ignore it (after confirming that it is not internally bonded to anyway).

Cheers, Wayne
 
wwhitney said:
Physics-wise, if you ground one of the corners of the center-tapped coil, you could still supply a 120V load with that corner as the "neutral" and the center tap as the live. You'd need a 3 busbar panel for distribution.
Click to expand...

Ground both the center and a corner? Or just the corner and use the center?

Grounding both the center and corner makes for an issue in my mind. But grounding just the corner and using the center seems like it would work for some cases. Might be messy as a general service due to imbalance or circular currents.
 
kreed said:
I do not need 120v from the generator. I only need 240v grounded b-phase. But the generator nameplate shows it is configured as a 120/240 volt 3 phase delta. so do i just ignore the 4th conductor and literally connect b-phase to ground right off the generator?
Click to expand...

Yes. You would mirror the configuration of service. Same grounded leg.
 
kreed said:
I do not need 120v from the generator. I only need 240v grounded b-phase. But the generator nameplate shows it is configured as a 120/240 volt 3 phase delta. so do i just ignore the 4th conductor and literally connect b-phase to ground right off the generator?
Click to expand...
Yes, you would just ignore the 4th conductor. In fact it's prohibited to use the neutral as a circuit conductor if you are corner grounding the system.

BTW you still haven't clarified if the ATS is two-pole or three pole.
 
In my mind, ground the corner and ensure there are no bonds to ground from the center. Then just leave the center unconnected to anything downstream of the generator. If I bond the center, the grounded conductor would end at the transfer switch. There currently is not a 4th wire 'neutral' in the existing service. the grounded conductor is set up to be b-phase in the existing service. in my mind you would have to ground the corner to carry the grounded conductor throughout the entire system.
 
Elect117 said:
Ground both the center and a corner? Or just the corner and use the center?
Click to expand...
Obviously ground just the corner. Grounding both would be a dead short.

jaggedben said:
Yes, you would just ignore the 4th conductor. In fact it's prohibited to use the neutral as a circuit conductor if you are corner grounding the system.
Click to expand...
Is that your interpretation of 250.26, or do you have another reference in mind?

Cheers, Wayne
 
kreed said:
But i need to ensure the 4th conductor, the center tapped conductor, labeled neutral from the generator, is not bonded to the frame or ground in any way? correct?
Click to expand...
Correct. Because you have a grounded phase conductor. You will be bonding that accordance with 250.26 / 250.28.
 
kreed said:
But i need to ensure the 4th conductor, the center tapped conductor, labeled neutral from the generator, is not bonded to the frame or ground in any way? correct?
Click to expand...
Yes, absolutely. You need an unbonded generator, so one with 5 connections: 3 corners, the center tap, and the frame, with no continuity between the frame and any of the other 4. Then you will only use 4 of those connections, the 3 corners and the frame, and leave the center tap unconnected.

Then if your ATS is 2 pole, you don't have to do any grounding at the generator, it's not an SDS and the solidly connected B/grounded conductor will be grounded at the service.

If your ATS is 3 pole, you do have an SDS and have to ground it accordingly, at the generator or at the first means of disconnect (if that is separate). Of course, you could choose to use only 2 of the 3 poles and connect it like a non-SDS.

Cheers, Wayne
 
jaggedben said:
Yes, you would just ignore the 4th conductor. In fact it's prohibited to use the neutral as a circuit conductor if you are corner grounding the system.

BTW you still haven't clarified if the ATS is two-pole or three pole.
Click to expand...
the ATS is 3-pole. also the b-phase from the generator runs through a main breaker before I can ground it. In my mind this would cause a ground fault. But I feel like I missing something too as the ground is just being used as a reference at this point.
 
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