inspector141
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What is the maximum size OCPD (single breaker) allowed on a 120/208, 3 phase, 100kw/125kva natural gas generator?
Overcurrent protection for 100kw/125kva generator
- Thread starter inspector141
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IMO, in regard to the OCP on the generator itself, there is not a "breaker size" requirement on the NEC, but rather a manufacturer requirement.
OCP can be provided by inherient design or in some cases non-existant {see exception at 445.12(E)}.
Art 445 addresses this more specfifically.
Art 445 also addess sizing the conductors to the first OCP device.
Once an OCP device is involved, Art 240 would apply for the load side.
OCP can be provided by inherient design or in some cases non-existant {see exception at 445.12(E)}.
Art 445 addresses this more specfifically.
Art 445 also addess sizing the conductors to the first OCP device.
Once an OCP device is involved, Art 240 would apply for the load side.
inspector141
Senior Member
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- Westminster, MD
I have a job where the engineer is insisting on a 400 amp breaker on the generator which will serve the building via a transfer switch. I'm thinking the max allowed per 240 is 350 amp mcb. 125000/360=347 next standard size up is 350.
Does everyone agree? Or if the generator manufacture says 400 OCPD, is that OK?
Does everyone agree? Or if the generator manufacture says 400 OCPD, is that OK?
I think the E/Cs concern would only be sized the conductors from the generator OCP device based on Art 240.
The generator manufacturer can install a 400, 600, 800 etc. per his engineering specs or any listing requirements.
In the case of a generator feeding a motor load or for Fire Pumps, there may be a need for a higher amperage than the kw would indicate.
The generator manufacturer can install a 400, 600, 800 etc. per his engineering specs or any listing requirements.
In the case of a generator feeding a motor load or for Fire Pumps, there may be a need for a higher amperage than the kw would indicate.
I agree with Augie. As long as the generator instructions don't require a specific size breaker, you don't have to worry about sizing the breaker to protect the generator.
I would expect a 125KVA generator to be capable of delivering 125KVA continuously, so personally, I'd probably add an extra 25% onto the generators current rating. In this case, that puts us at 433 amps. So I might even spec a 450 Amp breaker, unless the budget was really tight.
Also, the larger main breaker sometimes makes it a little easier to coordinate with the downstream beakers.
Steve
I would expect a 125KVA generator to be capable of delivering 125KVA continuously, so personally, I'd probably add an extra 25% onto the generators current rating. In this case, that puts us at 433 amps. So I might even spec a 450 Amp breaker, unless the budget was really tight.
Also, the larger main breaker sometimes makes it a little easier to coordinate with the downstream beakers.
Steve
inspector141
Senior Member
- Location
- Westminster, MD
OK Thanks! I would be interested in learning how generator manufacturers and engineers derive their OCPD for generators. It seems to me that kva is kva and should be sized per article 240. Article 445 offers no specifics for overcurrent protection. Is their a standard for generator OCPD's?
Thanks for the response.
Thanks for the response.
For a generator, I start with whatever size breaker the manufacturer puts in. I wouldn't want to go any smaller than that. And there probably wouldn't be any need to go larger either.
Heres one 100KW generator that has an integral 400A breaker (see page 4):
http://www.olympianpower.com/cda/files/2478381/7/LEHF0014-00.pdf
Heres one 100KW generator that has an integral 400A breaker (see page 4):
http://www.olympianpower.com/cda/files/2478381/7/LEHF0014-00.pdf
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