Conductors, Raceways and Wiring Methods
What the exam targets
Chapter 3 wiring methods plus Chapter 9 tables: conductor ampacity and derating, conduit fill, bending radius, support spacing, burial depth, and selecting the right raceway or cable for the environment.
Concepts to own
- Start ampacity in Table 310.16, then correct for ambient temperature (310.15(B)(1)) and adjust for current-carrying conductors in a raceway (310.15(C)(1)): 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%.
- Conduit fill, Chapter 9 Table 1: one conductor 53%, two 31%, three or more 40%. A nipple 24 in or shorter may be filled to 60%.
- Support: EMT within 3 ft of a box and every 10 ft; RMC/IMC every 10 ft; PVC Schedule 40 every 3 ft; ENT 3 ft and 10 ft.
- Burial depth (300.5): 24 in direct burial for residential and driveways; 18 in for a 120 V, 20 A GFCI-protected residential branch circuit.
- Pull box sizing (314.28): 8× the largest raceway trade size for a straight pull, 6× for an angle or U pull.
Traps
Treating the 90 °C column as a final ampacity instead of a derating starting point; applying adjustment factors to the 75 °C value; counting a neutral as current-carrying on a balanced 120/240 V multiwire; using 40% fill for a two-conductor raceway; ignoring that different systems need separate raceways.
Worked example
Six 12 AWG THHN current-carrying conductors in one conduit at 100 °F (40 °C), 75 °C terminations. 90 °C ampacity 30 A × 0.80 (6 conductors) = 24 A, then × 0.91 (ambient) = 21.8 A. The usable ampacity is 21.8 A; a 20 A overcurrent device keeps the circuit protected without relying on the next-size-up allowance.