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Ampacity Derating

The table ampacity is not the ampacity. Derate it.

The number in Table 310.16 assumes 86°F air and no more than three current-carrying conductors — conditions that vanish in attics, rooftops, and stuffed conduits. This calculator runs the full NEC 310.15 method: start at the conductor's insulation column, apply ambient temperature correction and bundling adjustment, then cap the result at the termination-temperature column per 110.14(C), because the weakest link governs. It also applies the 240.4(D) small-conductor breaker caps and the 125% continuous-load rule.

Why people use this

Electricians in hot climates use it every summer — an attic at 122°F quietly strips a third of a conductor's rating. Solar installers and EV-circuit runners use it to prove the 125% continuous-load math holds after derating, which is the first thing a plan reviewer checks.

How it works

1. Pick wire gauge (copper) and insulation rating — 90°C for THHN/THWN-2, 75°C for THWN/XHHW, 60°C for TW/UF.
2. Set the termination rating (most modern equipment is 75°C) and the real ambient temperature at the installation.
3. Enter current-carrying conductor count and your load; mark continuous loads for the 125% rule.
4. Read final allowable ampacity, PASS/FAIL against required ampacity, and the maximum legal breaker.

Common questions

Why is the final ampacity sometimes lower than the temperature-derated value?

The termination cap. NEC 110.14(C) says the circuit ampacity cannot exceed the column matching the lowest-rated termination in the circuit — usually 75°C for modern breakers. So even though 12 AWG THHN starts at 30A in the 90°C column, it can never exceed the 25A in the 75°C column after derating. You derate from the high column, then cap at the low one.

Do neutrals and grounds count as current-carrying conductors?

Grounds never count. Neutrals only count when they carry substantial harmonic current (nonlinear loads like VFDs and LED drivers on 3-phase systems). For a typical single-phase circuit, only the ungrounded (hot) conductors count.

What does the 125% continuous-load rule do to my wire size?

A continuous load (3+ hours: lighting, HVAC, EV charging) must be multiplied by 1.25 before comparing to conductor ampacity — a 20A continuous load requires conductors rated for 25A after all derating. On a 12 AWG run at 75°C terminations, that uses the entire 25A allowance with zero margin.

My ampacity came out between breaker sizes — can I round up?

Often yes: NEC 240.4(B) allows rounding to the next standard breaker size when the ampacity doesn't match one, but this calculator reports the largest breaker not exceeding your final ampacity — the conservative answer — and applies the 240.4(D) hard caps of 15/20/30A for 14/12/10 AWG copper, where rounding up is not allowed.

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