Generator set calculation

[Rusty lum]

what is the calculation: 2no 22KW water pumps how much KVA the gen set required.Thanks for the help.

 

[iceworm]

what is the calculation: 2no 22KW water pumps how much KVA the gen set required.

165kva

 

[charlie b]

I am not sure I understand the question, but I don't think Iceworm's answer is right. Are you saying you have two water pumps, and that each draws 22 kw? So you have a total of 44 kw. All you need to know is the power factor of each pump's motor. Not knowing anything else, I would assign a power factor of 0.8. That will give you a KVA value of (44 / 0.8) or 55 KVA, just for the two pumps. But you didn't say whether any other load would be powered from the same generator. So the total KVA cannot be determined from the information you did give.

 

[Smart $]

I am not sure I understand the question, but I don't think Iceworm's answer is right. Are you saying you have two water pumps, and that each draws 22 kw? So you have a total of 44 kw. All you need to know is the power factor of each pump's motor. Not knowing anything else, I would assign a power factor of 0.8. That will give you a KVA value of (44 / 0.8) or 55 KVA, just for the two pumps. But you didn't say whether any other load would be powered from the same generator. So the total KVA cannot be determined from the information you did give.

If the pump motor is rated in kW, that's usually output kW, so it can be directly converted to HP... then 430 Tables come into play for determining kVA.

And don't forget 125% factoring for largest motor.

 

[GoldDigger]

If the pump motor is rated in kW, that's usually output kW, so it can be directly converted to HP... then 430 Tables come into play for determining kVA.

And don't forget 125% factoring for largest motor.

Or more than just largest motor if you expect all of them to try to start the moment the genny comes online.

 

[iceworm]

what is the calculation: 2no 22KW water pumps how much KVA the gen set required. ...

165kva

I am not sure I understand the question, but I don't think Iceworm's answer is right. ...

... So the total KVA cannot be determined from the information you did give.

Rusty -
Charlie is correct - you did not give enough information. And my response was frivolous - I didn't give any information either.

This time with some explanation:
This is not a code question. The answers are not in the NEC. It is a design issue. And it is preliminary design - you are trying to get an idea of the size of equipment needed. How big must the gen be to start the motors and have an acceptable voltage drop?

Following comments are based of 480V, 60Hz - which you likely aren't. But the concepts still apply.

Couple of things to keep in mind:
Generators this size have fairly low short circuit current - possibly less than 1000A. The two motors, starting together, locked rotor current is ~500A

So, simultaneous starting 2 x 22kw motors, keep the voltage drop under 25%. To keep the vd down, the gen size will be in the 3X range.

44kw x 3 = 132kw

I would expect the gen to be rated at .8pf. Gen (kva rating) = 132/.8 = 165kva.

If that is more gen than you want to get, consider:
Stagger start the motors
Soft starts
Allow more voltage drop

Hope this helps

ice