MVA Method for Short Circuit Calc's - question

[angrymonkey]

Is there a generic rule of thumb to apply to motor loads?

I know most will list an MVA and per unit impedance which is obviously all you need.


However, what about when you know nothing about the motor loads other than a brief estimation?

-Like a couple 4000A switchboards feeding multiple MCC's with 1600A mains.


Are there any generalities to apply so you can figure out your short circuit available at certain points in the system in a nice estimation?

 

[mayanees]

motor contribution

motor contribution

I use a worst-case short-circuit increase at any switchboard of 25-30% of the value calculated without motor contributions.
JM

 

[kingpb]

Assuming we are talking about the same MVA Method: for motors on buses rated smaller than say 50-75Hp, what you can do is add all the Hp's together (if you know them) or simply estimate the total Hp on the bus; to add up the Hp's, assume 1Hp = 1 KVA, then divide by impedance Z" = 0.25 pu.

If you don't know the motor sizes but can estimate the load in amps, say the MCC is rated 1600A, then maybe assume 80% of 1600A, which would be 1600 x 0.8 x 480 x 1.732 = 1.064MVA worth of motor load(a little over 1000Hp)

Divide by 0.25 impedance = 1.064MVA/0.25 = 4.25MVAsc contribution

Then combine per MVA Method rules.