Full Load Amps

[mikeybmdb32]

Can anyone tell me why the full load amps listed in the NEC do not equal the motor power rating divided by the motor voltage rating? For instance, a 10hp 230V single phase motor is rated at 50Amps according to NEC, but using I=V/P, you get 32Amps.

 

[ray cyr]

At 32 amps and 230 volts, to have 10 hp of output would be almost perfect conversion of electrical energy to mechanical energy. That's not gonna happen. The electrical energy is also converted to heat in the windings and through the bearings etc. (along with some other inefficiencies). So to get 10 hp of output, the amperage must be increased. Also, the NEC seems to always list much higher amperage ratings than the motor nameplates do.

 

[infinity]

Here's the note for T430.239. As you can see the amperage represents a range of voltages. It's likely that those table values would represent the worst case scenario. Also the table values may reflect mechanical factors as ray mentioned.

The following values of full-load current are for motors running at speeds usual for belted motors and motors with normal torque characteristics. Current in the common conductor of a 2-phase, 3-wire system will be 1.41 times the value given. The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.

 

[suemarkp]

There is also Power Factor to consider. Each of these percentages multiplies, so it is easy to get a 50% increase in current from what a perfect world would indicate.