3-pole or 4-pole ATS

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Shujinko

Shujinko

Senior Member
I have a project in an existing building where a 14,000 square foot portion of the building is being renovated for a new tenant. The building has an existing main switchboard; 480Y/277V with a 4000A MCB with GFP protection. The existing building generator is 1500KW/1875KVA, 480Y/277V, Diesel Fuel. All the associated existing ATS's are 4-pole because of the GFP protection on the main switchboard.

The building owner is requiring the new tenant to provide their own separate generator, since the owner is reserving generator capacity for a different future tenant that needs alot of EM power. Since the new tenant for my project will only have a few small plug loads that need to be on generator (optional branch) we are specifying a 20KW/25KVA, 208Y/120V generator and a new ATS connected on the normal side to an existing building 208Y/120V distribution panel . My question is, does the ATS for the new tenant's 20KW generator need to be 3-pole or 4-pole? The way I see it I am considering the following 2 items:

1. The existing building switchboard has a 4000A MCB with GFP at 480Y/277V. And although we are connecting the ATS's normal side to a 208Y/120V existing distribution panel fed from distribution transformer (separately derived system). There still might be possibilities of nuisance trips on the 4000A MCB GFP at the Main Switchboard.

2. If another tenant comes and adds a generator/ATS to the 208Y/120V distribution panel we are connecting to, and both tenants have 3-pole ATS's, I believe we have more of a possibility of nuisance trips of breakers and electrical quality problems.

I am definitely leaning towards a 4-pole ATS but wanted to hear other people's opinion about my concerns and if they're warranted? Appreciate everyone's input in advance.
 
As you said, the three pole is sufficient if nobody else adds another one to the system, It will not affect the 480 volt ground fault main due to the 120/208 is a seperately derived system. A question though, is each tennant supplied by their own stepdown transformer? Or does the landlord have a large stepdown transformer for multiple tenents? If so, then a four pole may or may not be in your future. If the tennents are not sharing a generator, then three pole should be fine.
 
hillbilly1 said:
Just re-read your post, so the tennents do share the 120/208 transformer. As long as they are not sharing a generator, three pole will be fine.
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As long as the new generator does not have a neutral-ground bond. that cannot be removed. That is the critical determiner of whether the neutral needs to be switched.
 
GoldDigger said:
As long as the new generator does not have a neutral-ground bond. that cannot be removed. That is the critical determiner of whether the neutral needs to be switched.
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I've had Kohler's come with the neutral bonded to the ground, I just ordered the insulated neutral kit, and replaced the buss bar that was bolted to the frame. Easy enough fix.
 
I will make sure that there is no neutral to ground connection at the generator or At the ATS. And yes the tenants will be on the same distribution transformer and 208Y/120V distribution panel but different generators. I should probably check if the MCB at the 208Y/120V distribution panel has GFP protection because then maybe we would need a 4-pole transfer switch?
 
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