Motor Circuit Protection

[dvcraven052256]

I have a question relative OCPD of motors.

As an example 40HP motor rated a 480/3. NEC Table 430.52 for Inverse Time Breaker say 250% of FLA 52 x 250% = 130A thus a 150CB.
My Square-D motor calculator says 90 Thermal Magnetic trip rating. Obviously their must be some characteristics of the Square-D type CB that allows for the
90ampere rating but I am not sure what that is.

 

[ptonsparky]

I have a question relative OCPD of motors.

As an example 40HP motor rated a 480/3. NEC Table 430.52 for Inverse Time Breaker say 250% of FLA 52 x 250% = 130A thus a 150CB.
My Square-D motor calculator says 90 Thermal Magnetic trip rating. Obviously their must be some characteristics of the Square-D type CB that allows for the
90ampere rating but I am not sure what that is.

Some motors and loads are hard to start and under those conditions the 250% is allowed. Typically the motor will start with the 90 suggested. Most likely even smaller.