Generator Output Breaker and Feeder Size

[faresos]

Hello everyone:

The client is planning on installing 1000KW, 480Y/277V, natural gas, standby generator that will be backing up a 1200A building main service switchboard via 1200A ATS. The feeder between the generator and the switchboard is (3) sets of 600KCmil which has 1260A ampacity). I assume the generator output breaker will be set around 1503A. Now my questions:

1. Can we still use the (3) sets of 600Kcmil since the main switchboard is protected by 1200A main breaker or we need to dial down the generator output breaker to 1200A?
2. If we dial down the generator output to 1200A, this will limit the generator output to 800KW. Will this has impact on the generator performance beside limiting the capacity?

I don't see an issue with the feeder sizes since it's protected by 1200A breaker at the main service but I think we will have an issue with the ATS since it's rated at 1200A and we don't have an option but to dial down the generator breaker to 1200A.

Any thoughts?

Thanks,

 

[petersonra]

Hello everyone:

The client is planning on installing 1000KW, 480Y/277V, natural gas, standby generator that will be backing up a 1200A building main service switchboard via 1200A ATS. The feeder between the generator and the switchboard is (3) sets of 600KCmil which has 1260A ampacity). I assume the generator output breaker will be set around 1503A. Now my questions:

1. Can we still use the (3) sets of 600Kcmil since the main switchboard is protected by 1200A main breaker or we need to dial down the generator output breaker to 1200A?
2. If we dial down the generator output to 1200A, this will limit the generator output to 800KW. Will this has impact on the generator performance beside limiting the capacity?

I don't see an issue with the feeder sizes since it's protected by 1200A breaker at the main service but I think we will have an issue with the ATS since it's rated at 1200A and we don't have an option but to dial down the generator breaker to 1200A.

Any thoughts?

Thanks,

I don't understand your numbers. The generator maxes out at 1200 Amps. How does putting a 1200A CB on it limit it to 800A?

I would not assume anything about the CB included with the generator. It should not be real hard to find out what it really is.

Personally, I don't see how you can protect wires with an ampacity of 1260A with a 1500A CB, so I agree you would need to dial it down. But the generator does not know you did this. It will still provide as much current as is requested by the loads.

Depending on your calculated load the 1200A CB at the generator might need to be 100% rated, but it seems unlikely unless the switchboard main is also 100% rated.

 

[Jraef]

...I assume the generator output breaker will be set around 1503A.

Yeah, this part makes no sense. Even if you meant 1500A (not 1503), breakers that size are almost never unable to be "adjusted", although sometimes the trip settings are changed by changing a "rating plug".

As Bob said, don't ASSume anything...

As to the breaker "limiting the generator to 800kW", that's not exactly true. The generator is still CAPABLE of 1,000kW, the 1200A breaker is just going to protect your 1200A service, which is already limited to... 1200A! The fact that the generator is bigger is nice, especially if needed to start large loads, but it irrelevant as far as the protection circuit.

 

[petersonra]

Yeah, this part makes no sense. Even if you meant 1500A (not 1503), breakers that size are almost never unable to be "adjusted", although sometimes the trip settings are changed by changing a "rating plug".

As Bob said, don't ASSume anything...

As to the breaker "limiting the generator to 800kW", that's not exactly true. The generator is still CAPABLE of 1,000kW, the 1200A breaker is just going to protect your 1200A service, which is already limited to... 1200A! The fact that the generator is bigger is nice, especially if needed to start large loads, but it irrelevant as far as the protection circuit.

I think his 1503 A number came from using 80%.

1000 kW @ 480 V is about 1202.8 Amps.

1202.8 / 80% = 1503.5.

I have no idea where the 800 A number came from.

Most of the electronic breakers I see these days have adjustment screws on them for all three (or four) settings, and a lockable cover.

 

[faresos]

I don't understand your numbers. The generator maxes out at 1200 Amps. How does putting a 1200A CB on it limit it to 800A?
It limits to 800KW (1200 X 0.831 =997KVA =798KW @ 0.8PF)

I would not assume anything about the CB included with the generator. It should not be real hard to find out what it really is.

Personally, I don't see how you can protect wires with an ampacity of 1260A with a 1500A CB, so I agree you would need to dial it down. But the generator does not know you did this. It will still provide as much current as is requested by the loads.
wouldn't the downstream breaker (1200A at main switchboard) protect the wires even though the upstream breaker is 1600A? however, the issue is with the ATS, the upstream breaker (generator) needs to be 1200A since the ATS is rated at 1200A.


Depending on your calculated load the 1200A CB at the generator might need to be 100% rated, but it seems unlikely unless the switchboard main is also 100% rated.
The plan is to have the generator breaker 1200A to be rated at 100% because the switchboard 1200A breaker is rated at 100%.

 

[faresos]

I think his 1503 A number came from using 80%.

1000 kW @ 480 V is about 1202.8 Amps.

1202.8 / 80% = 1503.5.

I have no idea where the 800 A number came from.

Most of the electronic breakers I see these days have adjustment screws on them for all three (or four) settings, and a lockable cover.

The 1503A number came from:
1000KW = 1250KVA @ 0.8PF = 1503A @ 480V

its 800KW not 800A (1200A X 0.831 =997KVA =798KW @ 0.8PF)

 

[faresos]

Yeah, this part makes no sense. Even if you meant 1500A (not 1503), breakers that size are almost never unable to be "adjusted", although sometimes the trip settings are changed by changing a "rating plug".

As Bob said, don't ASSume anything...

As to the breaker "limiting the generator to 800kW", that's not exactly true. The generator is still CAPABLE of 1,000kW, the 1200A breaker is just going to protect your 1200A service, which is already limited to... 1200A! The fact that the generator is bigger is nice, especially if needed to start large loads, but it irrelevant as far as the protection circuit.

I only need 1200A service from the generator but I was concerned if I use 1200A in lieu of the max FLA of the generator (1503A) I might have issues with generator output.

Thank you all for your replies!

 

[ron]

I'm guessing the 1200A utility main is a regular fused switch or breaker, so it is not 100% rated. That only means that you can draw 1200A non-continuous (less than 3 hours) load, but you still want the generator to support the load, so your generator output protection should match the 1200A utility disconnect (100% rated or not).

Depending on the load power factor, you may end up with more than 1204A able to be provided by the gen, since it is typically rated at 1250kVA with a 1000kW rating.