HELP! Generator Ampacity

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xguard

Senior Member
Location
Baton Rouge, LA
We have a 3 phase generator - 400 kw, 500 kva, (601.4 Amps) 480/277 VAC. It has two circuit breakers, both 100 percent rated. The one on the left is 600 Amps, and the right 400 amps. Both are adjustable and currently set to the maximum 'long time' position. The 600 amp breaker (left) has two conductors per phase on the line side. The 400 amp breaker has one conductor per phase on the line side.

One breaker I want to use to connect to a transfer switch for a building. The other to a transfer switch for a chiller.

If the total available output of the generator is 601.4 amps, I'm I must divided this 601.4 amps between the two breakers. Anyone have an idea of how to determine how much each breaker (and thus the generator conductors) can be loaded? My original assumption was 200 amps on the 400 amp breaker and 400 amps on the 600 amp breaker. But I don't really know. I have a call in to the generator supplier but in the mean time any information you could provide would be fabulous.
 

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mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
445.12

445.12

I would revert to 445.12 (A) which gives allowance for protection by design. Perform load calculations to assure the generator is protected from overload and that the cables meet the load requirements.
 

ron

Senior Member
There is likely protection built into the controller for a gen of that size for the 445.12(A) compliance and the line side conductors for those breakers are sized by the gen vendor / manufacturer.

Your sizing is 601.4A total load in any combination that wouldn't cause the breakers to trip.
 
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