Size conductors using NEC Table 310.16 with full temperature correction (310.15(B)) and conduit fill derating (310.15(C)) applied automatically.
NEC Table 310.16 lists the allowable ampacity for conductors rated 0–2000V in a raceway, cable, or earth with not more than three current-carrying conductors. Before selecting a conductor size, two correction factors must be applied: temperature correction (310.15(B)) and conduit fill adjustment (310.15(C)).
For continuous loads, NEC 210.19(A) and 215.2(A) require the conductor ampacity to equal or exceed 125% of the continuous load — not just the load itself. This means the conductor must be sized for the corrected ampacity, then also checked against the 125% rule.
NEC 310.15(B)(2) provides temperature correction factors. Attic spaces can reach 50–60°C in summer. For THWN-2 (90°C rated), the correction factor at 50°C is 0.82, and at 60°C is 0.71. This significantly reduces the allowable ampacity and often requires one or two sizes larger than the base table would suggest.
NEC Table 310.15(C)(1) derating applies when more than three current-carrying conductors are in a conduit: 4–6 conductors = 80%, 7–9 = 70%, 10–20 = 50%, 21–30 = 45%, 31–40 = 40%, over 40 = 35%. Neutral conductors carrying only unbalanced current do not count as current-carrying.
Yes. NEC 110.14(C)(1) limits termination temperature. Most equipment is rated for 60°C or 75°C terminals, so 90°C insulated wire must be derated to the 75°C column for the final ampacity. However, the 90°C column can be used when applying correction and adjustment factors — the result must not exceed the 75°C column value for the selected size.
A neutral that carries only the unbalanced load of a wye system does not count as a current-carrying conductor for conduit fill purposes (NEC 310.15(E)). However, a neutral in a 4-wire, 3-phase system serving nonlinear loads (switching power supplies, variable frequency drives) does count, as it can carry significant harmonic current.