Dual generator sizing

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a.dramit78

Member
Location
Vinton virginia
Occupation
Business
Hello experts,
I have a retail business with a 3 phase 120/208 service I assume 400 amps. Inside O have 3 breaker panels, two single phase and one three phase. I have a single phase 24 kw generator which supplies to 1st single phase box with a 200 amp transfer switch.
I want to power 3 phase box with another generator. I have found a 80kw diesel 3 phase generator for that. Some electricians suggested that it can directly tap into 3 phase box through a transfer switch without any issues and both generators can be run simultaneously since they are supplying to different boxes. But one electrician put me in doubt that since one phase generator is supplying to one hot wire, and if 3 phase generator is also fired it would supply to all 3 hot wires and one wire shall get supplied by both generators and may lead to explosion. Please suggest how should this be done. I don't want to get rid of smaller generator since I plan to use bigger generator only if we lose power for more than 6 hrs, cause I have 3 phase coolers and HVAC and may not need them for small power outage. Please advise.
 
(approved as it is not a "how to" question but providing general knowledge)



Using transfer switches on each panel and a competent electrician you should have no problem using both generators.
 
augie47 said:
(approved as it is not a "how to" question but providing general knowledge)



Using transfer switches on each panel and a competent electrician you should have no problem using both generators.
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Thanks a lot. Can you have a discussion with LarryFine as to why he thinks it can't be done but you think it shouldn't be a problem.
 
It’s unusual that you have two single phase panels, and one three phase panel. What you want to do will work, but there may be a problem, with the panel configuration you have, it is highly likely it is a delta service, which means it is 120/240 with a high leg. If the 80 Kw generator is 120/208, it may not be compatible with your present loads. If there are only two transformers hanging on the pole, it definitely is a high leg service. If your electrician doesn’t know if it’s a delta service or not, move on to another contractor, because it isn’t going to end well if it is a delta service.
 
hillbilly1 said:
It’s unusual that you have two single phase panels, and one three phase panel. What you want to do will work, but there may be a problem, with the panel configuration you have, it is highly likely it is a delta service, which means it is 120/240 with a high leg. If the 80 Kw generator is 120/208, it may not be compatible with your present loads. If there are only two transformers hanging on the pole, it definitely is a high leg service. If your electrician doesn’t know if it’s a delta service or not, move on to another contractor, because it isn’t going to end well if it is a delta service.
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Thanks, the 3 phase panel does have some empty slots marked high leg. As far as the generator is concerned the supplier said they can wire it to 120/208 or 120/240 without any problem. I am just looking for an electrician who can figure this out. I shall try to attach the pictures of supply and panels, so far I have not figured a way to attach pictures.
 
a.dramit78 said:
Thanks, the 3 phase panel does have some empty slots marked high leg. As far as the generator is concerned the supplier said they can wire it to 120/208 or 120/240 without any problem. I am just looking for an electrician who can figure this out. I shall try to attach the pictures of supply and panels, so far I have not figured a way to attach pictures.
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You definitely want an electrician that’s familiar with delta services. One that is not, can let the magic smoke out! LOL!
 
a.dramit78 said:
Thanks, the 3 phase panel does have some empty slots marked high leg.
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You have a high-leg delta.

The two "regular" legs and the neutral are identical to 120/240v 1ph, plus a third phase.

If the generator can be supplied as a high-leg delta, then you can do what you want to.

As suggested, make sure your electrician is extremely familiar with high-leg 3ph systems.

If not, several of us aren't far from you. ;)
 
LarryFine said:
You have a high-leg delta.

The two "regular" legs and the neutral are identical to 120/240v 1ph, plus a third phase.

If the generator can be supplied as a high-leg delta, then you can do what you want to.

As suggested, make sure your electrician is extremely familiar with high-leg 3ph systems.

If not, several of us aren't far from you. ;)
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a.dramit78 said:
And how should the generator be wired as 120/208 or
120/240
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Neither. The orange-taped wires confirm the high leg, by the way.

"120/208" is 3ph wye, with three 120v lines to the neutral, and 208v between lines.

"120/240" is 1ph, just like in your house, minus the third, high-leg line.

You need to ask the generator supplier whether he can accommodate a high-leg delta.

He will know what you mean.
 
By the way, in case I wasn't clear earlier, the 120/240v portion (i/e., everything except the third line) of a high-leg delta is EXACTLY THE SAME as a 120/240v service, such as in your home.

In fact, the high-leg open delta began as a modification to existing 1ph services. Add one wire and now you can power that new-fangled three-phase refrigeration unit.
 
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