NEC Derating
The NEC’s ambient temperature correction table works alongside Table 310.15(B)(16), not in place of it.
NEC Table 310.15(B)(1) provides ambient temperature correction factors — a genuinely separate table from the base ampacity figures in Table 310.15(B)(16), used together rather than as alternatives to each other.
Corrected ampacity = Base ampacity (Table 310.15(B)(16)) × Correction factor (Table 310.15(B)(1))
Table 310.15(B)(1)'s correction factors are keyed to the insulation temperature rating (matching the same 60/75/90°C column concept from the base table) and the actual ambient temperature of the installation — not a single universal multiplier applied regardless of insulation rating.
It's easy to correctly pull a base ampacity figure from Table 310.15(B)(16) and then forget the separate correction step entirely, especially since the two tables live in different parts of the code and aren't visually adjacent on the page.
Hot attic spaces, rooftop conduit runs, and outdoor equipment enclosures in warm climates are exactly the scenarios where skipping this correction produces a meaningfully wrong — and unsafe — result.
OhmWorks applies both tables automatically in the NEC Cable Sizing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks