A journeyman electrician is installing control wiring for a motor control center. The control circuit conductors are protected by a 15-ampere overcurrent device. What is the minimum allowable ampacity of a No. 18 AWG control circuit conductor, assuming it is insulated with 75°C rated insulation and used for Class 1 control circuits?
- A10 amperes
- B2 amperes
- C5 amperes
- D7 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 7 amperes
According to NEC Table 402.5, for Class 1 control circuits, the ampacities for conductors No. 18 AWG through No. 14 AWG are determined by 725.43. NEC 725.43(A) refers to Table 402.5 for ampacity. For a No. 18 AWG conductor, the maximum overcurrent protection is 7 amperes (as per 725.43(A)(1)). The ampacity of a No. 18 AWG conductor in Table 402.5 is not directly listed for 75°C, but the maximum protection is limited to 7A for 18 AWG. This implicitly limits the working ampacity to 7A for this application. NEC 725.43(A)(1) states that for No. 18 AWG, the overcurrent protection shall not exceed 7 amperes. This effectively limits the circuit's usable ampacity to 7A for conductor protection, regardless of the theoretical ampacity from other tables if the OCPD itself is limited.
Why the other options are wrong
- A. This is too high for the maximum overcurrent protection of No. 18 AWG in Class 1 control circuits.
- B. This is too low for the typical application of No. 18 AWG in control circuits.
- C. This is the maximum overcurrent protection for No. 16 AWG conductors in Class 1 circuits, not No. 18 AWG.
Class 1 Control Circuit Conductor Sizing
Determining the minimum allowable conductor size and corresponding overcurrent protection for Class 1 control circuits, which are typically used for motor control or other similar applications, as per NEC Article 725.
- Class 1 circuits have power limitations and voltage limitations.
- Ampacity and overcurrent protection are specified in NEC 725.43(A).
- Smaller conductors (No. 18, 16 AWG) have specific OCPD limits.
Memory trick: Control circuits, Class One, 18 gauge, 7 amp run!