Generator Question

[Jimmy7]

I’m looking at drawings for a small commercial building. The drawings call for an optional standby generator. According to the drawings, they are proposing a Service Rated 400 Amp 277/480V 3 pole Automatic Transfer Switch. From the utility transformer they are specifying 4 #600 kcmil, on the load side there is a 400 amp panel which will be fed with 4 #600 kcmil & 1 #3G, from the 225 amp generator breaker they are proposing 4 #4/0 and 1 #4 ground. I’m questioning the generator size, should it be larger, or is the size of the generator just determined on the load that needs to be backed up? If it is just the load that currently needs to be backed up, do you need to consider future expansion when sizing the generator?

 

[augie47]

The feeder from the generator should be based on the overcurrent device at the generator.
You will need to address load shedding to assure the generator breaker does not trip.
(See 702.5)

 

[ron]

I’m looking at drawings for a small commercial building. The drawings call for an optional standby generator. According to the drawings, they are proposing a Service Rated 400 Amp 277/480V 3 pole Automatic Transfer Switch. From the utility transformer they are specifying 4 #600 kcmil, on the load side there is a 400 amp panel which will be fed with 4 #600 kcmil & 1 #3G, from the 225 amp generator breaker they are proposing 4 #4/0 and 1 #4 ground. I’m questioning the generator size, should it be larger, or is the size of the generator just determined on the load that needs to be backed up? If it is just the load that currently needs to be backed up, do you need to consider future expansion when sizing the generator?

No consideration for future generator load is required when sizing the generator, but as pointed out, the feeder gets sized based on the gen output breaker and if it is less than the calculated load, you will need to load shed.

 

[don_resqcapt19]

Does the load calculation for the total load that could be connected to the generator exceed 187kVA? If so, there may be a 702.4(A)(2) issue if the transfer switch is automatic.