A journeyman electrician is installing control wiring for a motor control center. The control circuit conductors are protected by a 10-ampere overcurrent device. What is the minimum size copper control circuit conductor permitted for a Class 1 control circuit, assuming they do not extend beyond the control enclosure?
- A12 AWG
- B18 AWG
- C16 AWG
- D14 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 18 AWG
According to NEC 430.72(B), for Class 1 control circuits, the overcurrent protection for the conductors shall not exceed the values specified in 430.72(A). However, for conductors not extending beyond the control enclosure, 18 AWG copper conductors are permitted to be protected by a 10-ampere overcurrent device, and 16 AWG by 20-ampere OCPD. NEC Table 430.72(B) explicitly states 18 AWG for 10A OCPD when conductors are within the control enclosure.
Why the other options are wrong
- A. 12 AWG is permitted for 40-ampere OCPD inside the enclosure, or generally 20A for branch circuits.
- C. 16 AWG is permitted for up to a 20-ampere OCPD when within the control enclosure.
- D. 14 AWG is permitted for 30-ampere OCPD inside the enclosure, or generally 15A for branch circuits.
Motor Control Circuit Conductor Sizing (Class 1)
For Class 1 motor control circuits, the minimum conductor size and associated overcurrent protection depend on whether the conductors extend beyond the control enclosure. Smaller gauges (e.g., 18 AWG) are permitted for conductors entirely within the control enclosure with appropriate overcurrent protection.
- NEC 430.72(B) and Table 430.72(B) govern control circuit OCPD and conductor sizing.
- Conductors within an enclosure have different rules than those extending outside.
- 18 AWG copper is permitted for 10A OCPD if within the control enclosure.
Memory trick: Control wires need specific protection; inside the box, smaller wires are okay.