NCEES 106

[EEVA PE]

This problem is fuse sizing to protect conductors against short circuit and ground faults with a motor in the circuit.

The way the answer is given in the book is not to consider the ampacity of the conductors when a motor is in the circuit since the conductors are protected against overload by the motor-overload device and to use table 430.52 for fuse selection with the only concern for short circuit and ground faults.

Now if I change this problem a little by eliminating the motor from the circuit, I now must consider the ampacity of the conductors for fuse selection.

So my thought process for fuse selection criteria is if a motor is in the circuit be concerned with short circuit and ground faults. If there is no motor in the circuit than be concerned with the ampacity of the conductors. Is that correct?

 

[Flyer_PE]

^It's mostly correct. Since the conductors should be sized based on 125% of the motor FLA, if the motor overload protection is sized correctly to protect the motor, the conductors are also protected. In this case, the purpose of the fuses is to protect against fault current. The important detail is that separate motor overload protection hast to both be there and be sized correctly.

 

[kris7o2]

Is there a right answer as to what size the fuses should be to protect the conductors against short circuits or ground faults. The answers provided are just the incorrect responses, and so wondering if there is a right answer?