Genset Feeds 3 & 4-pole Switches

[msimms]

I have a very strange application that a project manager brought to my attention at an industrial plant. Recently, the project manager installed a 100kW Generator on a 120/208V service with two feeds; one that feeds UPS power for server equipment, and the second that powers HVAC for the server room. The problem is what the Generator OEM provided him. For some reason, they gave him a 4-pole, 225A ATS and a 3-pole, 125A ATS, so now, I have 4-pole switch for one load, and a 3-pole for the other load. I have never seen such an installation before, and wanted to know for sure that there isn't a code issue.

I did confirm with the project manager that they did not install a grouding electrode conductor at the generator which you would normally be for separately derived system, but he only bonded the equipment grounding conductor to the grounded electrode system due to the existence of the 3-pole ATS which still inter-connects the neutral back to the distribution panel, to make it non-separately derived.

I've reviewed the NEC, and the only thing I can see us doing is to follow NEC 700.7(B) for signs to make sure someone knows about the potential shock hazard if the GEC is removed at the MDP.

See attached for reference to what I'm seeing here. Again, the generator OEM doesn't see any issue, but it sure is funky to me. Feedback is appreciated.

 

[ceb58]

I have a very strange application that a project manager brought to my attention at an industrial plant. Recently, the project manager installed a 100kW Generator on a 120/208V service with two feeds; one that feeds UPS power for server equipment, and the second that powers HVAC for the server room. The problem is what the Generator OEM provided him. For some reason, they gave him a 4-pole, 225A ATS and a 3-pole, 125A ATS, so now, I have 4-pole switch for one load, and a 3-pole for the other load. I have never seen such an installation before, and wanted to know for sure that there isn't a code issue.

I did confirm with the project manager that they did not install a grouding electrode conductor at the generator which you would normally be for separately derived system, but he only bonded the equipment grounding conductor to the grounded electrode system due to the existence of the 3-pole ATS which still inter-connects the neutral back to the distribution panel, to make it non-separately derived.

I've reviewed the NEC, and the only thing I can see us doing is to follow NEC 700.7(B) for signs to make sure someone knows about the potential shock hazard if the GEC is removed at the MDP.

See attached for reference to what I'm seeing here. Again, the generator OEM doesn't see any issue, but it sure is funky to me. Feedback is appreciated.

View attachment 6065

The thing to do is make sure the 4 pole is not switching the neutral. If it is and the 3 pole is not then you must remove the neutral from the switch and tie the 3 neutrals together isolated from the can.. Also at the gen make sure the EGC is not bonded to the neutral

 

[msimms]

okay, so there is nothing code related, but what engineering practice? it seems ridiculous to think that someone would have two differing types of switches of a genset.

 

[kwired]

Could be possible that manufacturer does not make a 3 pole, although it seems unlikely for 225 amp.

I am not real familiar with ATS switches that are out there but do know that sometimes with manual transfer switches and disconnects you may be limited on selection when you get in the 400 or 600 amp sizes or larger, and may have to purchase a three pole where a two pole would work or something similar to that scenario.

 

[steve66]

I'm not sure if your setup is a code violation, or any kind of real problem.

I have seen xfer switch setups like that before, but when I've seen it, its usually been a 4 pole ATS added to a installation that already has a 3 pole ATS.

Under utility power, the 4 pole switch gets its N-G bond at both the utility transformer, and the 3 pole ATS. (You have 2 N-G bonds, but its not really any different than any other utility service where the utility bonds N-G at the xformer, and the contractor bonds N-G at the 1st disconnect.)

Under generator power, the 4 pole ATS gets its N-G bond from the 3 pole ATS. My biggest concern is what sizes are the ATS's? Is the 3 pole larger than the 4 pole? If not, the neutrals ran to the 3 pole ATS might be undersized for the 4 pole. In effect, the N-G bond would be undersized.

All in all, I would much prefer to see two 4 pole ATS's, with a N-G bond at the generator.

 

[paul]

Don't use the 4th pole at the one transfer switch. Put in an isolated neutral bar, instead.