Guide · NEC 2023
How to size wire per NEC 310.16
Formula: derated ampacity = base (insulation column) × temp factor × bundle factor, then min(derated, terminal column). Breaker ≤ ampacity and ≤ 240.4(D) cap.
Example 1 — attic A/C (hot + bundled)
#10 THHN copper, 6 current-carrying conductors, 122°F attic, 75°C terminals. Base (90°C) = 40A. Temp (114–122°F, 90°C) ×0.82 = 32.8A. Bundle (6) ×0.80 = 26.2A. Terminal (75°C) = 35A. Derated 26.2A governs. Max breaker 25A (standard size ≤ 26.2A; 240.4(D) allows 30A for #10 but ampacity caps at 25A).
Example 2 — 60A EV charger (continuous)
Continuous loads size at 125%: 60A × 1.25 = 75A minimum conductor ampacity. #6 THHN copper at base conditions: 90°C = 75A, derated none, terminal (75°C) = 65A → 65A. Fails 75A. Step to #4 copper: 90°C = 95A, terminal 85A → 85A ≥ 75A. Use #4 Cu THHN on a 70A or 80A breaker per 110.14(C) and equipment listing. Aluminum equivalent: #3 or #2 Al — check the calculator toggle.
Example 3 — 100A subpanel feeder
#3 THHN copper: 90°C = 115A, terminal (75°C) = 100A → 100A. Max breaker 100A. Passes exactly. Field-friendly: pull #2 copper (115A terminal) for headroom and lower drop. If aluminum: #1 Al (100A terminal) minimum, #1/0 Al preferred.
The #14 / #12 trap (240.4D)
#14 THHN copper has 25A (90°C) and 20A (75°C) ampacity — but the breaker is capped at 15A. #12 THHN has 30A / 25A ampacity — breaker capped at 20A. That is why a 20A kitchen circuit needs #12 minimum, even though #14 “carries 20A” in the 75°C column.