HP and KVA 2

[tbyrne64]

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.

 

[waross]

Motors are rated at multiples of 115 volts. (4 x 115V = 460V)
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
--------------------
"Why not the best?"
Jimmy Carter

 

[tomad]

The HP rating indicates the output power ( real/mechanical at shaft) for a motor. the kVA rating refers to the input power of the motor (apparent power, drawn to deliver the mechanical output power. They refer to the full load situation. The output value is basically the input value x efficiency x power factor. Since the product of efficiency and power factor is on average 0.75 ( between 0.663 for 1 HP and 0.84 for 200 HP, per Energy Star qualification requirements)and the conversion factor between HP and kW ( 1 HP = 746 W)the error is within limits (for most motors, except small ones)
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)

 

[dpc]

They are not "omitting" anything. Their approximation of 1 hp = 1 kVA takes into account motor losses and power factor - 1 hp of shaft power takes about 1 kVA at the terminals of the motor.

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.

 

[tulum]

1.0 kVA/HP - Ind < 100 Hp and 0.8 PF Syn Motors
0.95 kVA/HP - Ind 100 < 999 Hp
0.90 kVA/HP - Ind > 1000 Hp
0.8 kVA/HP - Syn 1.0 PF