Generators in Parallel rated more than ATS

[mallard duck]

We have two (2) 300 kVA, 120/208 V Genset each paralleled into Asco 7000 series transfer switch. Transfer switch is rated 1200 A. The load side of transfer switch is connected to 1200A panel board bus. Normal power from utility is from secondary side of 300 kVA, 120/208 V transformer. In the event of normal power failure both the generators paralleled could dump 1666 A into 1200 A rated transfer switch.

The contractor sayes that because the load will be less than what the generators can put out and the load should never exceed 1200 amps it meets code.

My problem is there is a small possiblity it could and the generators would then exceed the listed lable on the transfer switch and the main distribution panel. Does this meet NEC?

 

[ceb58]

We have two (2) 300 kVA, 120/208 V Genset each paralleled into Asco 7000 series transfer switch. Transfer switch is rated 1200 A. The load side of transfer switch is connected to 1200A panel board bus. Normal power from utility is from secondary side of 300 kVA, 120/208 V transformer. In the event of normal power failure both the generators paralleled could dump 1666 A into 1200 A rated transfer switch.

The contractor sayes that because the load will be less than what the generators can put out and the load should never exceed 1200 amps it meets code.

My problem is there is a small possiblity it could and the generators would then exceed the listed lable on the transfer switch and the main distribution panel. Does this meet NEC?

Looking at 705.30 which takes you to 445.12. There would need to be a an overcurrent device/disconnecting means before the transfer switch. If it had a rating of 1200amps then it would comply.

 

[charlie b]

Another thing to consider is that placing generators in parallel will increase the amount of fault current that would be supplied to a fault downstream of the ATS. So you would need to calculate the fault current, and compare it to the fault rating of the ATS (and other downstream panels).

 

[brian john]

One way to think about this IF proper OCP is installed ahead of the ATS and the AIC does not exceed the ratings of the equipment, you are operating no different than you do when connected to the utility, except the utility has a lot more than two generators operating in parallel.