Journeyman Electrician Exam (NEC)Equipment and DevicesHard
A journeyman electrician is installing control wiring for a motor control center. The control circuit operates at 120 volts and uses No. 14 AWG copper conductors, which are protected by a 15-ampere overcurrent device. According to NEC 430.72(B), what is the maximum rating of the overcurrent protective device permitted for the control circuit, assuming the control circuit power is derived from a separate control power transformer?
- A20 amperes
- B15 amperes
- C25 amperes
- D10 amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 20 amperes
NEC 430.72(B) Table 430.72(B) specifies the maximum rating of the overcurrent protective device for motor control circuits. For No. 14 AWG copper conductors, when protected by an overcurrent device that is part of the motor control circuit, the maximum rating is 20 amperes. This applies when the control circuit is derived from a separate control power transformer.
Why the other options are wrong
- B. 15 amperes is a common OCPD size, but 20 amperes is permitted for No. 14 AWG control circuit conductors under NEC 430.72(B).
- C. 25 amperes exceeds the maximum permitted rating for No. 14 AWG control circuit conductors.
- D. 10 amperes is below the maximum permitted for No. 14 AWG control circuit conductors.
Motor Control Circuit Overcurrent Protection
The overcurrent protective device for the conductors of a motor control circuit, sized according to NEC 430.72, to protect against overcurrents specific to control wiring.
- NEC 430.72(B) and Table 430.72(B) govern sizing.
- Conductor size dictates maximum OCPD rating.
- Different rules apply if derived from the motor branch circuit vs. separate source.
Memory trick: Control Wire: Size Conductor, Check Table, Pick Max.