Motor disconnecting means

[Njarhead]

I'm preparing to take my Master Electricians test and am a little confused about one of the questions in my study guide. I don't understand why I'm supposed to use this formula rather than Table430.52 (2014 NEC) If it's not appropriate for this question, when is it supposed to be used? Here's the question:

An inverse time circuit breaker used as the disconnecting means for a 50 hp, 480-volt, three-phase continuous-duty, induction-type type ac motor shall have an ampere rating of at LEAST _____.

Here's the answer:

For general motor applications in regard to 3-phase motors, in compliance with the requirements listed in Section 430.6(A)(1), when sizing the motor disconnecting means the FLC values shown in Table 430.250 shall be used. Section 430.110(A) mandates the motor disconnecting means shall have an ampere rating of not less than 115% of the full-load current rating of the motor:

FLC of motor = 65 amperes x 115% = 74.75 amperes

My question is.....Why don't I use Table 430.52 (2014NEC) in this situation?

 

[augie47]

My question is.....Why don't I use Table 430.52 (2014NEC) in this situation?

Due to the details of the question itself. They are asking for the size of disconnecting means , the "keyword" in this case, which would be covered by 430.110 for the minimum rating.
430.52 gives you the maximum size of the GFSC protection.


In every day applications you would be far more lively, to use 430.52 but you need to carefully watch the wording on test questions.