utility main service ground and separately derived system grounding 2

[ladengr]

How do you interpret the following electrical installation?

1. 2000 Amps, 480/277 Volts Main service is grounded as per NEC 250B(2). Service has a GFP.
2. Four wire Generator neutral is bonded and grounded. Generator has two CBs and rated 400 Amps and has no GFP. 3 pole and 4 pole transfer switches are installed. 3 pole ATS serves a 3 phase motor load and no neutral wire is required. 4pole transfer switch serves 277 volts and 480 Volts loads.

Does this installation comply with NEC Article 250 - 20?

 

[rbulsara]

My vote is yes. Subject to the following:

If your generator rating is 1000A or more at 480V,you may want to review the NEC code requirement (assuming NEC is applicable) for having the GFP on all 'feeders' or services of such ratings.

Some insurance companies, such as FM GLobal, required a GFP on emergency side too. You may want to check with the insurance carriers of the facility, if in USA.

 

[ladengr]

Thanks for the response. Is the generator supply considered separately derived when we have 3 phase power supply to a motor through a 3 pole ATS without carrying a neutral to the utility main service. We have equipment ground from generator to main service through ATS.

 

[rbulsara]

ladengr:

Answer to your last post is also Yes. A system is seprately derived if their neutrals are not inter-connected.

The interconnection via ground conductor is not a problem as during a fault, all of the fault current on a source will eventuall return through the single N-G bonding jumper at either source.

When you interconncet the neutrals, there will be two parallel neutral paths returning the 'fault' current and the Neutral CT on the main bonding jumber will read only part of the 'fault' current.