Quick Hits

EPISODE 11 — The Main Ampacity Tables: In Raceway/Cable/Earth & Free Air (310.16, 310.17)

Slide 1 — Hook "THE table everyone knows — Table 310.16. Today: how to actually read it, plus its free-air cousin, Table 310.17."

Slide 2 — 310.16 — the classic table

• Applies when: conductors rated 0-2000V, rated 60°C/75°C/90°C, installed in a 30°C ambient, with 3 or fewer current-carrying conductors.

• Structure: conductor size down the left/right columns, three temperature-rating columns (60°C/75°C/90°C) across, split into copper and aluminum/copper-clad aluminum sections.

• Example pattern to know: a 12 AWG copper conductor is rated 20A at 60°C, 25A at 75°C, 30A at 90°C — but remember, 240.4(D) caps 12 AWG's OVERCURRENT PROTECTION at 20A regardless of which insulation temperature rating you're using, UNLESS a specific exception applies.

• Higher-temperature columns (90°C) exist for INSULATION rating and ampacity ADJUSTMENT/CORRECTION purposes — you often can't actually use that higher number unless terminations support it too (per 110.14(C), covered back in episode 8).

Slide 3 — 310.17 — free air

• Same voltage/temperature/ambient conditions as 310.16, but for single-insulated conductors installed in FREE AIR instead of raceway/cable/earth — notice it does NOT carry the "3 or fewer conductors" condition, since free-air spacing already handles heat dissipation differently.

• Free-air ampacities run noticeably HIGHER than the same conductor size in Table 310.16 — open air dissipates heat much better than a crowded raceway or buried cable.

Slide 4 — Quiz + CTA Q: "For the same conductor size and insulation type, would you expect the free-air ampacity (Table 310.17) to be higher or lower than the raceway/cable ampacity (Table 310.16)?" A) Lower — free air traps more heat B) Higher — free air dissipates heat much better C) Identical in all cases D) There's no meaningful difference

✅ Answer: B — free-air installation allows significantly better heat dissipation, so Table 310.17 ampacities run higher.

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