Ampacity & Derating Calculator
Table 310.16 ampacity, corrected for ambient temperature and bundling — the two adjustments everyone forgets on hot attic runs and packed conduits.
Table ampacity
25 A
Corrections
× 1.00 × 1.00
Adjusted ampacity
25.0 A
Per NEC Table 310.16 with 310.15(B)(1) ambient correction and 310.15(C)(1) bundling adjustment. Remember 110.14(C): the circuit is also limited by its termination ratings (usually 75°C — 60°C for many devices on 15/20 A circuits), and small-conductor rules in 240.4(D) cap #14/#12/#10 breakers at 15/20/30 A regardless of this math. Verify against the current code.
The three-step check
- Start at the table. Table 310.16 gives the base ampacity for the conductor and its insulation rating.
- Correct for heat. Ampacities assume 30°C (86°F). Attics, rooftops, and boiler rooms run hotter — multiply by the 310.15(B)(1) factor.
- Adjust for company. More than three current-carrying conductors in a raceway or cable bundle share heat — multiply by the 310.15(C)(1) percentage.
Then remember the ceiling: terminations are usually rated 75°C (110.14(C)), so even 90°C wire lands on the 75°C column at the ends — the 90°C column is your derating headroom, not free ampacity. And 240.4(D) still caps #14, #12, and #10 at 15, 20, and 30 amps for most applications no matter what the math says.