Motor amp calculations

[MarkFVTC]

Why don't these two ways of finding motor current equal each other for a 2hp 120VAC motor?
Table 430.248 = 24FLC
2hp =1492W, 1492W / 120VAC = 12.43A

 

[david luchini]

Why don't these two ways of finding motor current equal each other for a 2hp 120VAC motor?
Table 430.248 = 24FLC
2hp =1492W, 1492W / 120VAC = 12.43A

Your calculation does not take motor efficiency or power factor into account.

 

[MarkFVTC]

If motor efficiency is 50% and power factor is 90%. It does come out.

 

[david luchini]

If motor efficiency is 50% and power factor is 90%. It does come out.

What if the efficiency is 62% and the power factor is 85%?

 

[MarkFVTC]

Yes, you can fudge both numbers for it to calculate out.
Thanks for the help.

 

[Jraef]

HP is a mechanical value, not electrical. To get (single phase) Amps from HP, it is Watts / Volts x PF x Eff. Also, for the NAMEPLATE values, make sure you read the NAMEPLATE volts. For 120V, it is usually 115V on the nameplate. So for a 2HP motor, it is 1492 / 115 x .7PF x .8Eff = 23.16A. a lot closer to the NEC tables. The NEC tables are based on "industry typical" 4 pole motors, decidedly slanted to be able to cover the worst case, and, because the tables go a long way back before energy efficiency was a thing, the values are higher than we see on motors today. They are based on roughly 1970s motor technology (the oldest NEC I have is 1978, the chart is the same).