How can you make sure you are focusing on the key word or phrase in an exam question that will help your find the code article that answers the question?
Example:
What is the minimum size inverse time breaker required to protect a circuit supplying a 5 hp 230 volt single-phase a.c. motor?
Key word:
Motor
This key word reveals that the code rules found in Article 430 will apply because when it comes to motor calculations the rules found in Article 430 take precedence over rules found in other parts of the code.
Key phrase:
Inverse time breaker
This key phrase reveals that what you are really looking for is the overcurrent protection rating for the breaker that will protect this motor branch circuit.
Key phrase:
Minimum size
This word indicates that there is a minimum sizing method for determining the size of overcurrent protection required.
Solution:
Refer to NEC Article 430-52 (B) Rating or Setting for Individual Motor Circuit-All Motors. “The motor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor.”
Since the starting or locked-rotor current of a motor is six (6) times its f.l.c. (full load current) and the f.l.c for this motor is 28 amps, the locked rotor current for this motor is 168 amps.
Since a breaker will allow three (3) times its rating to flow for starting purposes, dividing 168 amps by 3 = 56 amps. Since 56 amps is not a standard size breaker, the next higher standard size is 60 amps.
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