3 Phase Motor Amperage 2

[Reid2006]

Quick (and probably painfully simple) question:

I have a 3phase, 60Hz, 3HP motor wired up to 460V.

Based on this information alone, is it possible for me to calculate the running aperage? Do I need to assume a power factor to accomplish this? What should that assumption be?
Thanks

 

[dbaird]

Use NEC Table 430.250....
3HP Full-Load Current @ 460 Volts = 4.8 Amps

Obviously when running below full load you will be under this value.

If you can get all the information off the nameplate work the following EQ:

Amps=HPx746/VOLTSx%EFFxPFx1.73
or you could estimate efficiency 92% and power factor 80%

This would give you:
Amps=3x746/460x.92x.80x1.73
=3.8

So you would be somewhere between 3.8-4.8 Amps

Obviously the best thing would be to actually take an Ammeter and measure the current.


David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[CJCPE]

See also:

thread237-142080

 

[aolalde]

Exactly as dbaird said. The line current is given by; Il = HP*746/(1.732*V*EFF*PF)

Yes you need to know or assume the power factor (PF) and efficiency (EFF) too.

For this size motor assuming it is 4 poles I will give ; PF~0.80 and EFF~0.85

Il = 3*746/(1.732*460*0.85*0.8)= 4.13 amperes

 

[Reid2006]

Excellent! Thanks guys!