Overcurrent Coordination Setting Guidelines Generators

[rjmatthews8]

I am working on a project where I think that the breaker after the generator is undersized. SKM and other similar companies say to set the LTPU to 115-125% FLA of the generator. I have gone through the IEEE Buff Book, IEEE C37.102, and IEEE C37.13 for trying to reference this spec so that I can use it, but I have been unable to find where these numbers have come from. Is this a specific standard that I can find somewhere or is it a rule of thumb that is used industry wide?

My generator is 110kW, 1654 FLA, 480V, PF=0.8
The breaker in question has a 1600 amp plug.

Thanks for the help.

 

[davidbeach]

110kW, 1654 FLA, 480V, PF=0.8

At least one of those numbers is wrong. Probably should be 1.1MW (1100kW).

Could simply be that the cost difference between a 1600A breaker and a 2000A breaker wasn't worth it to somebody for the incremental generator capacity gained. It just means you can't fully load the generator.

 

[rjmatthews8]

Yes a 1100 kW generator. The breaker frame is already 2000 amps. The plug is 1600 amps in the MVT plus trip unit. Easy switchout of the plug can fix this. They are running at full load as we saw 2 phases about 1700 amps. I would just like to call out a spec from an engineering text of why I recommend sizing the breaker with a 2000 amp plug.

Matthews

 

[waross]

If this is a prime power set, the generator will be able to safely deliver 1819 Amps for one hour out of twelve.
If this is a standby set, you are close to the engine limiting the power output.
If this is a standby set you are already in overload. I regions where "winter diesel" fuel is supplied in the winter months, you may not be able to get 1700 Amps at full voltage on winter fuel grades.
Yes the breaker seems undersized, but so is the gen-set.

Bill
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"Why not the best?"
Jimmy Carter