Ampacity is the maximum current, in amps, that a conductor can carry under stated conditions without overheating. This calculator starts with NEC Table 310.16, applies NEC Table 310.15(B)(1) ambient correction and NEC Table 310.15(C)(1) conductor adjustment, then applies NEC 110.14(C) and 240.4(D) limits.

Which insulation temperature column should I use?

Use the insulation rating for the starting ampacity column, then separately choose the termination rating. THHN is normally a 90°C insulation, while marked equipment commonly limits the final result to the 75°C column. NM-B and unmarked equipment use the 60°C termination limit.

How do I count current-carrying conductors?

Count ungrounded conductors and neutrals that carry current under normal conditions, including harmonic current from non-linear loads. Count spare conductors. Do not count equipment grounding conductors, grounding electrode conductors, or conductors that cannot be energized simultaneously.

Is the continuous-load rule part of derating?

No. Ambient correction and conductor adjustment reduce usable conductor ampacity because of heat. NEC 210.19(A)(1) and 215.2 separately require 125% sizing for a continuous load. This calculator shows the two checks separately.

🔥 Ampacity & Derating Check

Start with the insulation column, correct for heat and bundled conductors, then see exactly which safety limit controls.

Wire material

Wire size

AWG = American Wire Gauge. It is printed on the wire jacket.

Insulation temperature rating

Use the conductor insulation rating for the starting column. THHN is normally 90°C. This is not the termination temperature rating.

Termination temperature rating

Use 60°C for NM-B or unmarked equipment. Use 75°C or 90°C only when the equipment is marked and listed for that temperature. The termination limit is applied after both derating factors.

Ambient temperature

°C

Enter the surrounding air temperature at the conductors. NEC Table 310.15(B)(1) publishes bands through 85°C; a “—” cell is not permitted and this calculator will not substitute a value.

Number of current-carrying conductors

conductors

Count ungrounded conductors and any neutral carrying current under normal conditions, including neutrals carrying harmonics from non-linear loads. Do not count equipment grounding or grounding electrode conductors. Count spare conductors. Do not count conductors that cannot be energized at the same time.

Current it needs to carry

Load duration

A load expected to run for 3 hours or more is sized at 125%. This is a separate load-side check, not a derating multiplier.

Pass

final permitted ampacity

Conductor-side derating

The two factors multiply the insulation-column ampacity first. Only after that do the termination and small-conductor limits apply.

Base ampacity (insulation column)

Ambient correction

Ambient table row used — The published ambient band containing the entered temperature is used.

Current-carrying conductor adjustment — not binding

110.14(C) termination limit — not binding

240.4(D) small-conductor cap — not binding

FINAL PERMITTED AMPACITY

Binding limit:

Load-side check — separate calculation

Continuous-load sizing and conductor derating answer different questions. The 125% rule sizes the circuit for a long-running load; it is not multiplied into the conductor derating factors.

Entered load

Load under 3 hours: no 125% continuous-load multiplier

The entered load is within the final permitted ampacity.

What this checks

This U.S. edition uses the verified NEC Table 310.16 base ampacities, Table 310.15(B)(1) ambient factors, Table 310.15(C)(1) conductor factors, 110.14(C) termination limits, and 240.4(D) small-conductor caps. It never interpolates a missing value.

Ampacity and voltage drop are separate checks — a wire must pass both.

Want to know why you should trust this result? Read how VoltDrop checks its numbers.