apj

Hello everyone, I am giving a customer a quote on a standby system for a building with two services, one three phase and one single phase, both 120/240.  My question is about the ATS and grounding.  The single phase service supplies the building and the three phase service supplies condensers outside.  The single phase service is connected to a grounding electrode system, the 3 phase is not.  If I am correct the code requires that both systems be connected to the same GES at the building.  I will be utilizing two transfer switches, but I am thinking that if I connect the 3 phase system to the GES currently at the building then I can use standard switches, but if they are left as they are, I may have to utilize ATS switches that switch the neutral as to not have two ground return paths to the generator.  At first I thought I would have to install two generators, but an industrial dealer I have been working with says I can do it with one industrial upsized generator with two breakers installed, one for each service. I was worried about the imbalance in the alternator since the condensers won't run year round.  I know that if I switch a neutral then I, in a sense, have a separately derived system and must ground and bond at the generator.  I am not sure when an application requires switching a neutral, and what my specific situation requires here.  Just need some understanding. Thanks in advance.

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Brian Baughman
Why?  Forget about the ATS's for a second, how do you plan to balance the load across all 3 phases of the generator so that the load on any phase is not more than 25% of the load on the other phases?  Good luck with that on a high leg delta generator.

In your proposed application, the only NEC compliant method is to install the generator as a separately derived system, and switch the utility neutral at each service.  So you will need a 4 pole ATS for the 3 phase service and a 3 pole ATS for the single phase service. 

Trust me, the additional cost for those ATS's is more than the cost of a 22 kW air-cooled generator with a 200A service rated ATS, with the labor included.
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apj
I originally was going to install two generators because I was worried about the 25% imbalance on the alternator, and I can't get a 'D' code alternator on a commercial unit.  However, when I ran it by my industrial rep he said it could be done by upsizing the alternator in the generator and putting two breakers in the generator.  His engineer said it is okay with large enough unit.  I would need to install a 30kw for the building and a 60 kw for the condensers, or one 70kw with a 100kw alternator according to the engineer.  So apparently the imbalance is not a problem with a large enough unit.  However, I am thinking if I connect both services to the GES like they should be than I would not have to use neutral switching ATS switches, but if the cost difference is minimal then why bother.
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Geoff Z
I agree. It is a slippery slope to try and balance the load on a delta generator. Especially when you add a single phase and 3 phase service into the mix. The generator isn't going to be happy if not done right. I opt out of being involved simply because of the liability as a dealer. Doesn't mean it can't be done just that I'm uncomfortable with it.
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78buckshot
apj, how old are the condensing units? and what are the tonnage ratings. I would think long and hard on the back-up power if the current cooling equipment has much age on it. If the only thing in the building on 3 phase is the cooling and depending on how vital the cooling needs are you might be able to put off installing a 3 phase generator. When and if the cooling is up-dated it could be single phase and re-configured to provide a more positive result. I'm just thinking outside the box knowing how easy it is for me to get stuck on one issue and can't see past the forest 'cause the tree's are in the way.
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apj
Not sure of the age, but they are 5 ton units, four of them.  I agree, I don't know who installed the three phase units on a single phase building, but it might be cheaper for them to change out the a/c units, but they insisted on having the air conditioning on backup power as well.
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78buckshot
Well that's good to know for future considerations, 5 tons are in the residential realm so could be single phase when updated.
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Birken Vogt
You didn't exactly state, but are both services derived from the same utility transformer bank?  Everybody assuming delta on here but is it actually a delta service?  What is the three phase service grounded to at the moment?  Are both services adjacent to each other?
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apj
Each service has its own transformer bank on the pole, they are about 30 feet apart.  The 3 phase service has a neutral tap coming into the disconnect that the egc's are connected to, but it is not tied into a GEC like the single phase service. The single phase service is connected to a ground rod at the building.  I could run down the wall of the building and bring the GEC over to the 3 phase service and tie it in.
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Birken Vogt
Is the three phase service delta or wye?
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apj
120/240 delta
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Birken Vogt
Well at least the voltages [mostly] match.  Has it been considered to eliminate the single phase service and run the whole building off the delta service?  That is how it would be done around here.

Anyway, imbalance is only a big deal when you are operating at or near full load capacity also.  It has to do with oddball magnetic things happening to the rotor.  If the majority of the load is condensers it should be a non-issue.  Or you can oversize the alternator as much as needed to keep it under control.

The way this would be done here is to tie the ground/bond system at the service points very solidly together and keep both transfer switches close to or integrated with the actual point of service so there would be no current flow on the ground/bond conductors in any mode of operation.
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