Generator breaker size

[User Name]

Hey guys, just a curious question. A lot, if not all, of the standby generators I hook up, the breaker installed on the unit is usually 20-30 amps larger than the rated amps of the generator. I asked my salesman why, and he said it's usually just what's available in the next size up is what they put in.

But I've done generators rated for 80, with 100 amp breakers in them, and 80 amp breakers exist. Same with 100 amp generators with 125 amp breakers in them. I'm going to be doing a 40KW, rated at 167, with a 200A breaker in it, so 33 amps larger, is what caught my attention.

I always wire to the size of that breaker to stay on the safe side. But I wonder are there other fail safes in the generator to keep it from overloading? Seems like it would be real easy to size a generator, and a couple years later someone adds a big load, like a hot tub or pool, or switches from gas appliances to electric, and then it's overloaded if it kicks on.

Like I said, just curious.

 

[LarryFine]

I look at it as the same as a service; the 83% allowance applies for the generator feeder.

 

[CoolWill]

The code requires the line side conductors from the generator to the breaker to be sized 115% of the rated output current, so it seems reasonable that the breaker would be sized the same.

 

[User Name]

The code requires the line side conductors from the generator to the breaker to be sized 115% of the rated output current, so it seems reasonable that the breaker would be sized the same.

That makes sense, but it still seems odd to me you could have a generator rated at 167 amp, put a 190 amp load on it, a 200 amp breaker isn't going to give up. They've got to have some kind of over current relay built in that would give up before the breaker don't they? Or maybe the engine might not be able to crank against a load that big and stall?

 

[Seven-Delta-FortyOne]

Generators constantly monitor the output frequency, and during an overload, the frequency drops and the generator will shut down.

When I took the Generac training, the instructor overloaded a unit until it shutdown, and shutdown was around 57 hertz if I remember correctly.