Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsHard
A commercial building requires a new 20-amp, 120-volt branch circuit for general-purpose receptacles. The circuit will use 12 AWG THHN copper conductors in Electrical Metallic Tubing (EMT). What is the maximum number of current-carrying conductors permitted in the conduit before ampacity derating is required?
- A6
- B3
- C5
- D4
Show answer & explanationAnswer & explanation
Correct answer: B. 3
According to NEC Table 310.15(C)(1), ampacity adjustment factors apply when there are more than three current-carrying conductors in a raceway or cable. Therefore, up to 3 current-carrying conductors can be installed without requiring derating due to bundling. The question asks for the maximum number *before* derating is required.
Why the other options are wrong
- A. 6 current-carrying conductors require a 70% derating factor.
- C. 5 current-carrying conductors require a 70% derating factor.
- D. 4 current-carrying conductors require a 80% derating factor.
Conductor Ampacity Derating (Bundling)
The reduction in the allowable current-carrying capacity of conductors when more than three current-carrying conductors are installed in a raceway, cable, or bundle, due to reduced heat dissipation.
- Applies when more than three current-carrying conductors are present.
- Adjustment factors are found in NEC Table 310.15(C)(1).
- Neutral conductors are generally not considered current-carrying unless unbalanced or on a 3-phase, 4-wire wye system with nonlinear loads.
Memory trick: Three's a crowd, four's a derate: count the 'current carriers' for heat's sake.