Quick Hits

EPISODE 9 — Ampacity Tables General Rules & Temperature Correction (310.15(A)–(B))

Slide 1 — Hook "Today: how to actually READ the ampacity tables — including the correction factor for a hot ambient temperature, and a sneaky rule about rooftops."

Slide 2 — 310.15(A) General

• Ampacities for 0-2000V conductors come from Tables 310.16 through 310.21, as modified by 310.15(A)-(F) and 310.12.

• Under engineering supervision, sizes NOT shown in the tables can be found by interpolating between adjacent conductor sizes, based on circular mil area.

• Corrected/adjusted ampacity can't exceed what the TERMINATION is rated for, per 110.14(C) — the table ampacity is a ceiling, not a guarantee you can actually use that number.

Slide 3 — 310.15(B) Ambient temperature correction

• Tables 310.16/310.17 assume a 30°C ambient; Tables 310.18-310.21 assume 40°C. If your actual ambient is different, you MULTIPLY the table ampacity by a correction factor from Table 310.15(B)(1)(1) or (2) — hotter ambient = smaller factor (reduces ampacity); colder ambient = larger factor (increases ampacity, within limits).

• Rooftop rule: for raceways/cables exposed to direct sunlight on/above a rooftop, if the gap between the raceway/cable and the roof is LESS than ¾", add 33°C to the outdoor temperature before applying the correction factor — because that thin airspace traps serious heat. (XHHW-2 is exempt from this specific rooftop adder.)

Slide 4 — Quiz + CTA Q: "For a raceway exposed to sunlight on a rooftop with less than ¾ in. clearance from the roof surface, what temperature adder applies before using the correction tables?" A) 10°C B) 20°C C) 33°C D) 50°C

✅ Answer: C — 310.15(B)(2): add 33°C (60°F) to the outdoor ambient temperature in that specific scenario.

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