20KW generator overcurrent protection

[shortcircuit2]

The 20kw generator has a 100amp breaker in it. On natural gas its puts out 18kw.

18KW / 240 = 75 amps

Shouldn't there be a 80amp breaker to protect the gen-set?

 

[texie]

The 20kw generator has a 100amp breaker in it. On natural gas its puts out 18kw.

18KW / 240 = 75 amps

Shouldn't there be a 80amp breaker to protect the gen-set?

I'm assuming that this breaker is factory supplied. The NEC is silent on the size to protect the generator, but of course the the breaker has to protect the conductors on the load side. Also keep in mind that you can't size a breaker based on watts as that is not a true indicator of amps due to the PF. It is common for a generator to be rated at .8 PF, so the line current for 18 KW @.8 PF would be 94 amps. In other words, the engine can produce 18 KW of power, but there is enough generator to carry 22.5 KVA.

 

[shortcircuit2]

The generator has a PF of 1.0 and on NG is rated at 75 amps...

So if the gen-set is loaded at say 90amps with a 100amp breaker installed in the unit...won't the gen-set fail?

 

[jim dungar]

So if the gen-set is loaded at say 90amps with a 100amp breaker installed in the unit...won't the gen-set fail?

Probably not.
The output breaker rarely, if ever, actually protects the generator. The breaker protects the output wiring. The generator is usually protected by its own electronic circuitry (i.e. the governor controls). Add too much load to a generator and it usually just stops producing its output voltage.

 

[Besoeker]

The 20kw generator has a 100amp breaker in it. On natural gas its puts out 18kw.

18KW / 240 = 75 amps

18kW or 18kVA?

 

[templdl]

The question is if the UL489 breaker that is intended to protect wire have a TC curve that is coordinated with the TC curve that will cause damage to the generator. If the current draw on a generator be of such a magnetude to colapse the field on the generator how would the breaker be sized to prevent such a failure?
There must be an understanding as to where damage can occur and if a UL489 breaker has the ability to trip hat that point.
Remember that a generator is very seldom if ever oversized to a point where it has the capability of tripping a common breaker. Utilities have the ability to trip breakers because of theior ability to supply lots of current.

 

[jim dungar]

The question is if the UL489 breaker that is intended to protect wire have a TC curve that is coordinated with the TC curve that will cause damage to the generator.

We will probably never know if a standard UL489 breaker can be sized to coordinate with a small (kw rated) generator, as it is al but impossible to get actual damge curves from the manufacturers. However we are beginning to get some of their internal protection circuit TC curves.