Sizing motor branch circuit conductors.

[JdoubleU]

I know that you have to use the table in the NEC to size the motor conductors and not the name plate, I just can not remember why. Would someone be able to shed some light on this for me.


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[GoldDigger]

The generally accepted reason is that at some time in the future the motor may be replaced with one of the same HP but different efficiency. If the circuitry is sized from s table which uses a worst case motor, then any replacement will be OK.
Now if the motor is part of a system, such as an A/C, you can assume that any replacement will be a match for the original. So, in those cases the appliance nameplate is authoritative, rather than the tables.

 

[ActionDave]

430.1 430.6

 

[kwired]

430.1 430.6

430.6 tells us we need to use the tables to determine motor current for calculations, but OP is asking why it wants us to use the table instead of actual motor nameplate.

 

[petersonra]

Who knows. There may have been a reason when the requirement was put in place or it may have been something that someone just decided would be a good idea.

 

[ActionDave]

... OP is asking why it wants us to use the table instead of actual motor nameplate.

Because the NEC says so, and knowing that and where to look in the codebook will get you at least one or more questions right on you continuing ed or j-mans test.

 

[JRW 70]

Future Changes

Future Changes

Goldigger has the correct answer. The cables need to be
sized for the worst motor configuration possible for a given
horsepower rating

 

[JdoubleU]

thank you for your replies