Q. What is the maximum allowable ampacity of a 10 AWG THHN or THWN-2 insulated conductor installed in a schedule 80 PVC raceway (90°C rated) installed underground from the branch-circuit panelboard to the luminaires installed for parking lot lighting? There are four current-carrying conductors in this raceway. Disregard the terminals and the branch-circuit rating for this example.
10 AWG THHN or THWN-2 is a 90°C rated conductor per Table 310.13(A). From Table 310.16, 90°C column, a 10 AWG THHN or THWN-2 is a 40A rated conductor at 30°C. Per 310.8, 10 AWG THHN or THWN-2 is permitted to be used in dry, dry and damp, and wet locations (the THWN-2 rating is used for wet locations).
40A x 0.8 (an adjustment factor for four current-carrying conductors [from 310.15(B)(2)(a)]) = 32A maximum allowable ampacity, per this set of conditions. We are not considering the terminals [(110.14 (C)] or the branch-circuit overcurrent protective device rating or setting at this time.
The answer is based on changes in the 2008 NEC [300.5(B), 310.8, 310.13(A), 310.15(B)(2)(a), 310.16, 352.10, and 352.12]. Even though it has been known that conductors within raceways installed in wet location are considered to also be in a wet location, this change in the NEC makes that point clear.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at email@example.com.