NEC Table 430.250

[TLCPA]

Is there a formula for calculting amperes based on HP for the NEC Table 430.250? The table only goes to 500 HP and I have a 520 HP motor @ 575 VAC.

 

[dpc]

The table is based on conservative data taken from actual motor nameplate information.

For a particular motor, you should be able to get the full load amperes from the motor nameplate or motor supplier. Generally assuming 1 kVA per horsepower will be conservative. However, a 520 hp motor sounds somewhat non-standard, so you should try to get full load amp data from the manufacturer. Especially if it is submersible, hermetic, or unusually low rpm.

 

[waross]

The table is based on less efficient motors with a little higher than average FLC. Then if an efficient motor is replaced with a less efficient motor with higher current the conductors will still have adequate ampacity. Ampacity is 125% of the assumed full load current of the motor. Many times motor circuit conductors are oversized due to voltage drop considerations. If voltage drop is an issue then even one size larger conductors than 125% of FLC should make most AHJs happy.

Bill
--------------------
"Why not the best?"
Jimmy Carter