explanation of table 430.248

[Steve89]

can anyone explain the values of 430.248,
let's take a 1/2 hp motor for example, according to the table it draws 9.8A at 115v.

If 1hp=746w, then 1/2hp=373w

using ohm's law I=P/E
373w/115v=3.24A

can anyone explain this to me...any help would be greatly appreciated thanks

 

[jim dungar]

can anyone explain the values of 430.248,
let's take a 1/2 hp motor for example, according to the table it draws 9.8A at 115v.

If 1hp=746w, then 1/2hp=373w

using ohm's law I=P/E
373w/115v=3.24A

can anyone explain this to me...any help would be greatly appreciated thanks

The HP rating of the motor is its mechanical output capacity. The 746W/HP simply converts output work to an electrical value.

What you need is to convert the output of the motor to its input requirements. The difference between input and output is Efficiency. Efficiency need to include the electrical (winding losses and capacitors) of the motor as well as mechanical losses (things like windage).

Small horsepower motors are usually relatively inefficient. And, the FLA in the NEC have been purposely chosen to represent a motor that is unlikely to ever exist. This way we don't have to re do the circuitry if we swap the motor with that of a different manufacturer.

 

[glene77is]

Jim,
Is the 1/2 horse motor running at 33 percent efficiency?

 

[jim dungar]

Jim,
Is the 1/2 horse motor running at 33 percent efficiency?

I don't know, and it doesn't make a difference.

Your motor may be highly energy efficient but its future replacement may be a real power hog. By sizing the circuits to the NEC values, instead of the actual nameplate, we are assured that the motors can be replaced without affecting the branch circuit wiring.