Motor question

[SonofAdam]

Got an exam question on my masters...

240V 3 phase motors : What is the overcurrent protection required for the following.

one 50 HP 3 phase, 2 10HP motors and one 50 hp motor that has a 5 minute rated short duty motor.


I know that it's 125% of the largest motor plus 100% of the others... the 5 minute thing is throwing me for a loop

 

[Jraef]

Red herring? I have had instructors do this sort of stuff on purpose, just to make you think “Why is that there?”, when in fact it’s totally superfluous information.

The 5 minute rating might be important for selecting CONDUCTOR size, as per 430.22(E), but has no bearing on the OCPD issue as far as I know.

 

[Elect117]

430.33

 

[Elect117]

Also see 430.24 exception 1. Or Maybe 430.53? It really depends on how the question was worded. If this is branch circuit or feeder protection. If it is feeder:

You would be using 430.62 for the calculation. (technically 430.63 but it points you back to 430.62). Since 430.62 points to conductors sized based on 430.24, I am assuming you would use the reduced ampacity rating for the 50HP 5min rated motor.

Largest protective device + FLA of all other devices on the branch circuit.

50HP (largest OC protective device) + 5min rated FLA of 50HP motor + FLA of 10HP motor + FLA of 10HP motor

They should have specified a type of overcurrent protection.