Stand-By Generator Load capability

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Sparky2791

Sparky2791

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Northeast, PA
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Electrical Design
Customer sent a cut sheet of an emergency generator they own and want to use to back up a new restaurant they are constructing. My question is regarding the actual capacity available from the generator. There is a circuit breaker installed directly on the unit rated at 800A. I know there are several factors to consider like motor in rush, starting load etc, etc that factor into this generator. Ultimately I am wondering why do they list on the cut asheet the ampacity based on KVA of the unit when the breaker is clearly set based on KW. 120/208V 3 phase. I would calculate this generator to be capable of 694 loaded to 100%. If KVA can be used we have much more capacity.


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If you were supplying a load that was 250kW and 0.8 PF, how much current would be coming out of the generator?
 
250kW with 0.8 PF meaning a load of 313kVA at 0.8 PF.. Same thing just said differently. I was saying it that way (referred to the real power part of the triangle) to demonstrate that at the rated 250kW with an assumed 0.8 PF (which IIRC is common for gens) the current out of the generator is the listed 867A.

250kW / 0.8PF = 313kVA

@ 313kVA with a 0.8 PF (therefore 250kW), the gen will be outputting around the 876A... this is NOT the same as saying the generator can handle 867A regardless of the power factor.

Am I understanding your question correctly?
 
You are correct. Ultimately looking for the expected max capacity this gen set could handle 250KW .8 PF. or 694A @ 208V 3 Phase.
 
LibertyEngineering said:
You are correct. Ultimately looking for the expected max capacity this gen set could handle 250KW .8 PF. or 694A @ 208V 3 Phase.
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I think you might still be confused? If you’re at 250kW and 0.8 PF your current will not be 694A.

If you’re at 250kW and 0.8 PF, then you’ll be at about 867A.
 
bwat said:
I think you might still be confused? If you’re at 250kW and 0.8 PF your current will not be 694A.

If you’re at 250kW and 0.8 PF, then you’ll be at about 867A.
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It seems the OP is not understanding real VS apparent power. Maybe this will jog him... if he has a 250 KW load at unity PF he can only have 694 amps.
 
I understand and listed incorrectly.

It lists the KVA as 312KVA (apparent power) with a power factor of .8 so the True Power or power this generator is capable of providing is 312 x .8 = 250 KW True power.
 
LibertyEngineering said:
I understand and listed incorrectly.

It lists the KVA as 312KVA (apparent power) with a power factor of .8 so the True Power or power this generator is capable of providing is 312 x .8 = 250 KW True power.
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Correct. Generators are rated differently than, say, a transformer. This because a genset has a prime mover and since very few industrial loads run at unity power factor they size the generator end to carry more current(KVA) and the engine is sized to the KW. Of course like everything there are exceptions to this general rule of machines being built for a .8 PF load but this is pretty common for most garden variety gensets.
 
If you could predict the load power factor, you would know which of the machine's limits you would hit first and that is as far as you would get.
You can go no farther than the kW limit of the engine (250kW), the kVA limit of the alternator (313kVA) or the circuit breakers trip characteristic which depending how much current for how long could be approximately 800A+
 
Thank Guys. Its been awhile sine I had to really think about this.... How about this one...If all of the loads calculated for the panels we are looking to protect with the generator, which is a slew of different loads , were calculated as VA (amps x volts) some having a power factor (motors and such) and some not would it be worth considering as a gut check multiplying the total KVA of all these loads added together by a PF of .9 to get the KW load? Realistically I am going to use the sizing program from the manufacturer and take the more accurate long approach to confirming gen set size but I am curious of your thoughts on this.
 
For OP:
KW is what is created at the shaft of the generator by its gas motor or whatever. KVA is the amount of electrical power that is can be generated by this KW at the shaft.

745 watts = 1 hp (at the shaft)

Conveniently for everyone, the motor receives this KVA and converts it to KW at the shaft of the motor. There is fundamentally not much difference between a generator and a motor.

So in this case, the 250KW gas motor or whatever turns your generator and at the most can create 313 KVA (electrical power). It doesn't make much sense to figure your amps from the 250KW because you have a power factor. However, it makes great sense to get your power from the 313 KVA.
P = V * I * 1.73
313,000 = 208 * I * 1.73
I = 869 amps.
Your generator at 208 volts could theoretically generate 869 amps at 208 Volts....or we could say it can generate at the most 313 KVA. You could also say that it could drive at the most a motor of 250KW.

And yes, there are some losses and start up stuff and so forth, but for the purpose of learning let's leave this out of the equation for now.
 
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