Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsMedium
A commercial building requires a new 208-volt, 3-phase, 60-amp feeder circuit to a subpanel. The circuit will be run using 1 AWG THHN copper conductors. What is the maximum number of current-carrying conductors allowed in a 1-inch trade size Rigid Metal Conduit (RMC) before derating factors must be applied?
- A4 conductors
- B3 conductors
- C9 conductors
- D5 conductors
Show answer & explanationAnswer & explanation
Correct answer: B. 3 conductors
According to NEC Table 310.15(C)(1), ampacity adjustment factors must be applied when the number of current-carrying conductors in a raceway or cable exceeds three. Therefore, up to 3 current-carrying conductors can be installed without applying a derating factor.
Why the other options are wrong
- A. Incorrect. Derating factors apply for 4 to 6 current-carrying conductors (80% adjustment).
- C. Incorrect. Derating factors apply for 4 or more current-carrying conductors (70% for 7-9 conductors).
- D. Incorrect. Derating factors apply for 4 or more current-carrying conductors.
Conductor Ampacity Derating (Number of Conductors)
The reduction in a conductor's allowable ampacity when the number of current-carrying conductors in a raceway, cable, or bundle exceeds a specified limit.
- Caused by heat buildup from adjacent conductors.
- Governed by NEC Table 310.15(C)(1) (formerly Table 310.15(B)(3)(a)).
- Applies when more than three current-carrying conductors are present.
Memory trick: Three is Free, More is Less.