Generator Sizing.

[ACY]

i just read through FAQ237-766, which related to generator sizing. I was hoping there is a formula for working out generator sizing, (kVA) in relation to either Amps, and/or Watts of attached load.

 

[rbulsara]

Yes, you can.

Incidently, I posted that FAQ.

You can total up your connected watts or amps and come up with a kW/kVA rating. However, that will be your 'base' load. If any of those loads include large motors, you should allow for starting kVA of that motor(s) as implied in the FAQ.

Also other considerations, such as derating for harmonic loads etc. need to to be considered, specially if you have significant UPS and/or VFD loads.

For for the formula for KVA is not different than the basic formulas. kVA=kW/pf

KVA = sqrt 3* kV* A.

Always allow for some growth and cushion.

I am not sure what your application is.

Sizing smaller generators becomes trickier than larger ones as the relative impact of a large motor is more severe on smaller generators.

 

[ACY]

Thanks Rbulsara, I was after a basic formula that i was hoping may be constant through all generator sizing. I have taken note of variations that may occur, such as starting currents. I some times am asked this question, but not having had much to do with generators, dont know any formula as such. I shall also continue looking some sites on this. Thanks again.

 

[fennell]

ACY - Presumably you are sizing a generator because you want to buy one. If you are going to buy a new one from a reputable company the best thing to do is to ask send them all the details and get them to size it for you. Go to more than one supplier, (let each of them know that too) and see what you get back.

For starting one large motor, the rule of thumb I follow is kVA (of genset required)= 2.5 x motor kW. Hasn't let me down yet!