Generator breaker size

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Bjenks

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East Coast of FL
When sizing the feeder breaker for multiple motors you size it at the largest motor breaker plus the FLA of the other motors, then next size down. What if you have a generator feeding this through an ATS that shuts down half the motors upon power failure. Do you size the generator feeder breaker the same size as the utility service breaker since the largest breaker size is physically the same or do you size it hypothetically on what all the circuit breakers sizing would be if those motors were not there? I am trying to see if I can lower the breaker size and conductor for the generator feed.
 
Bjenks said:
When sizing the feeder breaker for multiple motors you size it at the largest motor breaker plus the FLA of the other motors, then next size down. What if you have a generator feeding this through an ATS that shuts down half the motors upon power failure. Do you size the generator feeder breaker the same size as the utility service breaker since the largest breaker size is physically the same or do you size it hypothetically on what all the circuit breakers sizing would be if those motors were not there? I am trying to see if I can lower the breaker size and conductor for the generator feed.
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The ATS needs to be sized for the service. The generator should be sized for the load. Just remember that with an ATS, you must size for what may come on at any given time. We have locked out certain equipment in the past that has capabilities under utility power, but not under standby power, which sounds like what you are talking about. This enables us to put smaller generators on larger services.
 
The size of generator and feeder depends on weather 700, 701, and 702. By that I mean if you have a 702 system there is no requirements for the size of generator or ATS. OP has not pionted out what type system so hard to say
 
tom baker said:
Can you cite the specific code rule for this?
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I do not know of one specificly, I will check. But you can also go with a smaller ATS that is sub-fed. I am doing one such tomorrow. But, using a house for example, if an HO wants to run everything in their house, with a 400amp service, but the actual load is only 275 amps, you can size the generator and OCP for the load, but the ATS needs to be sized the same as the service. A better example might if the HO wants to run the house but lock out the upstairs HVAC. The generator can be smaller, but the full house load runs through the ATS under both standy and normal operation, so the ATS must also be sized for the larger of the 2 loads.
 
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