Ampacity Calculator

Find wire ampacity with temperature and bundling derating per NEC 310.15 for safe conductor sizing.

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How It Works

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC 310.15 requires derating for high ambient temperatures and bundled conductors.

The Formula

Adjusted Ampacity = Base Ampacity × Temp Factor × Bundle Factor
Temp Factor = √((Trated - Tambient) / (Trated - 30°C))

Variables

  • Base Ampacity — NEC 310.16 table value for the wire gauge and insulation type
  • Temp Factor — Correction for ambient temperatures above 30C (86F)
  • Bundle Factor — Derating for multiple current-carrying conductors in a raceway

Example

12 AWG THHN at 104F (40C) with 6 conductors: Base = 30A. Temp factor = sqrt((90-40)/(90-30)) = 0.913. Bundle = 0.80. Adjusted = 30 x 0.913 x 0.80 = 21.9A.

Tips

  • Neutral conductors carrying only unbalanced current do not count for bundling derating.
  • Equipment grounding conductors do not count as current-carrying conductors.
  • The 80% continuous load rule applies on top of ampacity derating.
  • THHN/THWN-2 (90C) wire has the highest base ampacity of common insulation types.
  • In hot attics or rooftops, ambient temperature can exceed 120F, requiring significant derating.