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.
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