Motor overload selection using nameplate FLA

[knoppdude]

Hello,
I am reviewing code calculations, and would like to know if anyone knows the reason that motor FLA table in article 430 are used for conductor sizing, overcurrent protection, etc., but to size overload elements, we have to use the motor nameplate information? I've checked in my code handbook, but it really doesn't give a reason for this, and the only thing I can think of is that the manufacturers develop the nameplate rating based upon motor winding insulation.

 

[nakulak]

the overloads are designed to protect the motor. everything else just protects conductors and other equipment (short ckt and ground fault protection)

 

[iwire]

the overloads are designed to protect the motor. everything else just protects conductors and other equipment (short ckt and ground fault protection)

Great answer, I am going to remember that one.

 

[DARUSA]

That's because the nameplate is showing the FLC ,that is the actually ampers rating for that specific motor.The table 430.247,248,249,250 showsthe maximum amp rating that any motor with tha same HP and the same voltage can draw. If you have to replace a motor , you will replace with another motor with tha same HP but maybe that motor will draw more ampers than the previous motor but never will be more than the table shows!!
You change the motor but you don't have to change all the branch circuit!! ( Braker and wire)!!!!

 

[knoppdude]

Thanks for the replies, when I get back to work, I am going to check a motors nameplate FLA for a particular HP motor, with what is in article 430's FLA tables. This is something I honestly have never done before, but it does make sense that the nameplate FLA value for the sizing of overloads, would be lower than the tables.

 

[DARUSA]

Table shows FLC , nameplate FLA both are the same but we use this for different calculations.