Generator Load Capacity

[jonn46]

I Have A 180kw Gen. @ 208 3ph.
When I Calculate Load Capacity I Apply P.f. Of 0.8 To The Calculation.
The Plan Reviewer Apparently Did Not.

Isn't This The Way Is Typically Done?
My Load = 625 Amps ( 180kw/360/0.8 )
His Load = 500 Amps ( 180kw/360 )

Who Is Right?

Thanks For Input

(Edited to correct the display, in that the "0.8 pf" was not visible in the display.)

 

[kingpb]

Current is calcualted on KVA, and therefore the rated capacity ( in amps) of the generator is KW divided by pf, line-to-line voltage, and sqrt 3.

This is the 625A you refer too. It is unclear as to how the 500A was obtained, but it appears incorrect.

 

[dsteves]

Generator sizing

Generator sizing

Generators are typically designed and built with an internal 0.8 power factor. If your generator is in fact capable of delivering 180 kW, its KVA capacity is 180/0.8 = 225 kVA.

It is important to know whether your generator's rating is 225 kVA or 180 kW or 180 kVA.

If the 180 kW rating is correct, then the generator is capable of delivering 625A into a load, but only at a 0.8 PF. Therefore, you must resolve your load into real kW (kVA * PF) to know whether you have enough capacity.

As a real-world example, let's say I have a 31.3 kVA 120/208Y generator and a 30.0 kVA UPS connected downstream from it. The UPS has a 0.96 PF and draws 30 kVA when charging batteries, making its draw 28.8 kW real power (neglecting any load). The generator makes 31.3*0.8 = 25.0 kW. Oops! The generator won't charge the UPS batteries!

Dan

 

[winnie]

Generator capacity is different from load.

Generator capacity is limited by the alternator KVA, and the prime mover mechanical output (and the over-all efficiency of the system).

The load is determined by the supply and what the load draws from that supply.

If you have a 180KW generator, feeding a 180KW 208V three phase load with unity power factor, then the load current is 500A. The generator may be capable of _supplying_ 625A to a 0.8PF load, but the load is only drawing the 500A.

-Jon

 

[dsteves]

Right. the generator will supply a 180 kW load at unity power factor. It will even supply a 180 kW load at a 0.8 power factor, although that's 100% load, and hopefully the prime mover is sized sufficiently to overcome generator system component losses. The generator will probably not run a 180 kW load with a 0.7 pf, as that would require 257 kVA from a device which can only muster 225 kVA.

Dan

 

[W6SJK]

This is the wrong way to check generator size anyway! If the plan review was so worried about confirming genset size, he should be asking for starting and running load calcs, motor starting sequence etc. Have you checked that?

 

[bsh]

If there are large motors that will run with generator power be sure to include the motor starting kva in your generator load calc. Motor starting may require a larger generator in order to start the motors.