Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsHard
A technician is installing a new 480-volt, 3-phase motor circuit using 3 AWG copper conductors with THHN insulation. The conductors will be run in an ambient temperature of 45°C (113°F). What is the maximum allowable ampacity for these conductors, assuming no more than three current-carrying conductors in the raceway?
- A90 Amperes
- B85 Amperes
- C100 Amperes
- D95 Amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 90 Amperes
From NEC Table 310.16, 3 AWG THHN copper conductor has a base ampacity of 115 A (90°C column). For an ambient temperature of 45°C, Table 310.15(B)(2)(a) (now Table 310.16 for ambient temperature correction factors) provides a correction factor of 0.82. Therefore, 115 A * 0.82 = 94.3 A. The closest standard overcurrent protective device rating would be 90A, per 240.4(B) if the calculated ampacity does not correspond to a standard OCPD size and is not above the next standard size.
Why the other options are wrong
- B. Incorrect. This value is too low after applying the correction factor.
- C. Incorrect. This value is too high and does not account for the ambient temperature derating.
- D. Incorrect. This value is too high; the calculated ampacity is 94.3 A, and you cannot round up to 95 A as it's not a standard OCPD size in this range.
Conductor Ampacity Derating
Conductor ampacity must be adjusted (derated) for ambient temperatures exceeding 30°C (86°F) and for more than three current-carrying conductors in a raceway or cable.
- NEC Table 310.16 provides base ampacities at 30°C.
- NEC Table 310.15(B)(2)(a) (now integrated into 310.16 tables as correction factors) provides ambient temperature correction factors.
- NEC Table 310.15(B)(3)(a) provides adjustment factors for the number of current-carrying conductors.
- Always use the lowest appropriate temperature column (e.g., 90°C for THHN) for the conductor's insulation rating before applying correction factors.
Memory trick: Base, then adjust, for heat and conductor trust!