Wire Ampacity Calculator — NEC Table 310.16
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Look up allowable ampacity the way the code book does it: pick the insulation temperature column, then apply conduit-fill derating and ambient-temperature correction. OhmBase shows every step, plus the minimum breaker after the NEC 240.4(D) small-conductor caps.
Open the free calculator →How the math works
Start from the NEC Table 310.16 ampacity for your conductor size, material, and insulation rating. Multiply by the adjustment factor for more than three current-carrying conductors (310.15(C)(1)) and by the ambient temperature correction (310.15(B)). The overcurrent device is then selected per 240.4 — remembering that 14, 12, and 10 AWG copper are capped at 15, 20, and 30 A by 240.4(D) regardless of the table value, unless a listed exception applies.
12 AWG copper · 75°C column · 1–3 conductors · 86°F ambient
- Table 310.16 base ampacity: 25 A
- No derating or correction applies
- Breaker: capped at 20 A by NEC 240.4(D).
FAQ
- Why is 12 AWG limited to a 20 A breaker if the table says 25 A?
- NEC 240.4(D) caps overcurrent protection for small conductors — 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG copper — regardless of insulation rating. The higher table value still matters for derating math.
- When do I use the 90°C column?
- Only as the starting point for derating calculations when every part of the circuit (conductor insulation AND terminations) supports it. Terminations are usually rated 75°C, so the final ampacity can’t exceed the 75°C value.
- Do motor circuits follow 240.4(D)?
- No — motor branch circuits follow Article 430 and are exempt per 240.4(G). That’s why a motor circuit can have a breaker far above the wire’s table ampacity.