Generator Breaker Sizing

[tkb]

I have a 100kW generator that I am going to order and have a question on the sizing of the output breaker.

What does the generator manufacturer go by to size the output breaker?
I see in 2008 NEC 445.13 the wire from the generator terminals to the breaker need to be sized at 115% of the rated output.

I don't see how they size the output breaker though.

This particular generator is 100kW at 480 volts. The output is 150 amps and the breaker is going to be 175 amp.

If it was 125% of the output then it would be 187.5 amp.
I would have rounded up to 200 amp per 240.4 (B).

How does the generator supplier size this breaker.
Is it specified by the NEC or their own criteria?

 

[Buck Parrish]

I have a 100kW generator that I am going to order and have a question on the sizing of the output breaker.

What does the generator manufacturer go by to size the output breaker?
I see in 2008 NEC 445.13 the wire from the generator terminals to the breaker need to be sized at 115% of the rated output.

I don't see how they size the output breaker though.

This particular generator is 100kW at 480 volts. The output is 150 amps and the breaker is going to be 175 amp.

If it was 125% of the output then it would be 187.5 amp.
I would have rounded up to 200 amp per 240.4 (B).

How does the generator supplier size this breaker.
Is it specified by the NEC or their own criteria?

May be it would help to protect the motor if they "round down" instead of up.

 

[tkb]

But you loose out on a continuous output based on 125%.

 

[Buck Parrish]

That would be like running your truck at 90 mph all night or even several days. IMHO

 

[tkb]

Yea.. your right about that.
I guess if you need that much, you would up-size the generator.

I only recommended load to 80% max kW anyway.

 

[ceb58]

The power factor is involved in the sizing.

480v x .8pf= 384amp
100,000/ 384= 260.41amp
260.41/ 1.73 = 150amp
150amp x 115%= 173 amp round up to 175amp