looking at some drawings for a project there is a generator 350kw at 208 volt 3phase the drawings indicated a circuit breaker on the generator at 1000a and indicated 100% rated? but the generator is rated 438kva about 1216a, would the 1000a breaker still work? Just curious, would you normally size the breaker on kw or kva?
Generator OCPD
- Thread starter nuibe
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I have often taken this into consideration because a generator is quite different than a utility.
Are you trying to protect the generator or cable?
You will have to select a breaker based upon the generators ability to generate enough over current for a long enough period of time to trip the breaker thermally and also can generate a fault current high enough in magnitude to trip the breaker instantaneously.
As such find out what the generator's rating capacity is. Remember that this is not an issue when applying a breaker on a utility system because the utility has a much greater capacity.
Thus there is a possibility that the generator may not be capable of tripping a common breaker.
Are you trying to protect the generator or cable?
You will have to select a breaker based upon the generators ability to generate enough over current for a long enough period of time to trip the breaker thermally and also can generate a fault current high enough in magnitude to trip the breaker instantaneously.
As such find out what the generator's rating capacity is. Remember that this is not an issue when applying a breaker on a utility system because the utility has a much greater capacity.
Thus there is a possibility that the generator may not be capable of tripping a common breaker.
Rick Christopherson
Senior Member
On a diesel/electric generator, the circuit breaker is performing double-duty. It not only protects the windings in the generator, but also the engine itself.
The windings in the generator are capable of sustaining 1216 amps regardless what the powerfactor is. The diesel engine is capable of delivering 350 kw (which is equal to 470 horsepower). The circuit breaker is sized to protect the whole system. With a purely resistive load, the circuit breaker will trip when the engine experiences 360 kw, even though the windings in the generator are still capable of taking another 200 amps. It is designed to protect the system in a worse-case situation.
Edit to Add: Sorry templdl. We cross posted. Please note in the OP's question that he is referring to the on-board factory circuit breaker of the generator. Not a subsequent downstream breaker.
Second Edit to Add: By the way, the fact that the breaker is rated for 100% it means you are not required to derate it for the typical 80% loading in the NEC. You can pull the full 1000 rated amps out of the system continuously.
The windings in the generator are capable of sustaining 1216 amps regardless what the powerfactor is. The diesel engine is capable of delivering 350 kw (which is equal to 470 horsepower). The circuit breaker is sized to protect the whole system. With a purely resistive load, the circuit breaker will trip when the engine experiences 360 kw, even though the windings in the generator are still capable of taking another 200 amps. It is designed to protect the system in a worse-case situation.
Edit to Add: Sorry templdl. We cross posted. Please note in the OP's question that he is referring to the on-board factory circuit breaker of the generator. Not a subsequent downstream breaker.
Second Edit to Add: By the way, the fact that the breaker is rated for 100% it means you are not required to derate it for the typical 80% loading in the NEC. You can pull the full 1000 rated amps out of the system continuously.
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Leestarkweather
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Genset could be oversized for Non-Linear Loads
Genset could be oversized for Non-Linear Loads
We install a lot of gensets for backup of customer's systems (Sec. 702), and often oversize the alternator head due to very high UPS and rectifier loads. We order a 250KW genset with oversized 300 or 350KW alternators, which is more fuel efficient than upsizing the engine also. The inertia/carry thru or what we call "stiffness" is very important on that type of load profile. We just installed twin (redundant) 250's with 3P 800A Output CB's for a 240V 3-phase 3-wire system (the alternators were sized at 350KW). So there's lot's of reasons that the output CB on the genset is smaller than the output capacity x 115% of the alternator (you can use KVA, not the KW rating, and be code compliant).
Genset could be oversized for Non-Linear Loads
We install a lot of gensets for backup of customer's systems (Sec. 702), and often oversize the alternator head due to very high UPS and rectifier loads. We order a 250KW genset with oversized 300 or 350KW alternators, which is more fuel efficient than upsizing the engine also. The inertia/carry thru or what we call "stiffness" is very important on that type of load profile. We just installed twin (redundant) 250's with 3P 800A Output CB's for a 240V 3-phase 3-wire system (the alternators were sized at 350KW). So there's lot's of reasons that the output CB on the genset is smaller than the output capacity x 115% of the alternator (you can use KVA, not the KW rating, and be code compliant).
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