HP and KVA
HP and KVA
(OP)
I work with several "old school" power engineers, all are very good utility engineers. Many use 1HP=1KVA for calculating ampacity. For instance; a 4160V plant bus has 6650HP worth of motors, they assume 6650Hp=6650KVA for estimating current. This omits motor eff, and power factor. Any thoughts on this issue? The results are typically 30% higher than actuals.






RE: HP and KVA
Systems are rated at multiples of 120 volts. (4 x 120V = 480V)
Using 480V as a multiplier rather than 460 gives an allowance for voltage drop between the transformer and the motor.
The 746 W per horse power plus motor losses, plus power factor adjustment, plus voltage drop comes close to 1 KVA per Hp.
From the motor current table in the CEC, and using 480 volts as a multiplier I get the following KVA values for 460 volt rated motors:
50 Hp. 5.4 KVA
100 Hp. 103 KVA
150 Hp. 150 KVA
200 Hp. 199 KVA
Close enough for me.
Years ago I was doing a design for a small shingle mill.
If I bought the transformer and used primary metering I would have to use about 1 KVA per Hp.
If the utility supplied the transformer and we used secondary metering, they would apply a diversity factor and use a factor of 0.5 KVA per Hp.
That was a long time ago, but perhaps some of the utility folk will comment on present practice.
Bill
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"Why not the best?"
Jimmy Carter
RE: HP and KVA
Example (from NEC 2005, for 3-phase AC motors):
1 HP - 1.6 kVA - error -37% (undersize)
10 HP - 11 kVA - error -9% (undersize)
50 HP - 51.48 kVA - error -2.8% (undersize)
200 HP - 190 kVA - error +5% (oversize)
RE: HP and KVA
Where are you finding 30% error?
As the motors get larger this approximation becomes more conservative. For medium-voltage motors 500 hp and above, it is admittedly very conservative.
For typical 480 V motors, it works quite well.
RE: HP and KVA
0.95 kVA/HP - Ind 100 < 999 Hp
0.90 kVA/HP - Ind > 1000 Hp
0.8 kVA/HP - Syn 1.0 PF