Conductors, Cables, Ampacities, and Derating
Core concepts
Ampacity comes from Table 310.16, which gives values in 60°C, 75°C, and 90°C columns for copper and aluminum. The column you may use is limited by 110.14(C): the lowest temperature rating of any terminating device in the circuit caps the usable ampacity, even when the conductor itself is rated higher.
Two adjustment factors reduce ampacity. Ambient temperature correction (310.15(B)(1)) uses the ambient at the conductor, adjusted for rooftop temperature adders where applicable. Conductor count adjustment (310.15(C)(1)) applies when more than three current-carrying conductors share a raceway or cable: 80 percent for four to six, 70 percent for seven to nine. Neutral conductors count as current-carrying on wye systems with nonlinear loads such as harmonic-rich electronic ballasts.
Parallel conductors (310.10(H)) are permitted at 1/0 AWG and larger, must be the same length, material, size, and insulation, and must be terminated the same way.
Traps
- Taking the 90°C ampacity as the final answer when terminations are 75°C rated. The correct method is to start with the 90°C value for derating, then compare the result to the 75°C value and use the lower of the two.
- Forgetting to count a shared neutral as a current-carrying conductor.
- Applying the conduit-fill adjustment to conductors in a nipple 24 in. or less long.
Worked example
Six current-carrying 10 AWG THHN copper conductors in a raceway at 40°C ambient. 90°C ampacity = 40 A. Ambient correction at 40°C = 0.91; six-conductor adjustment = 0.80. 40 × 0.91 × 0.80 = 29.1 A. The 75°C termination value is 35 A, so the final ampacity is 29 A.