NEC Conductor Derating Calculator
Derate conductor ampacity for bundled conductors and elevated ambient temperatures per NEC 310.15.
Results
Visualization
How It Works
When conductors share a raceway or bundle, they heat each other up, reducing their safe current-carrying capacity. Similarly, higher ambient temperatures reduce ampacity because the wire starts closer to its thermal limit. NEC 310.15 provides correction factors for both conditions that are multiplied together.
The Formula
Derated Ampacity = Base Ampacity x Bundling Factor x Temp Correction Factor
Temp Factor = √((Trating - Tambient) / (Trating - 30°C))
Temp Factor = √((Trating - Tambient) / (Trating - 30°C))
Variables
- Base Ampacity — Conductor ampacity from NEC Table 310.16 at 30\u00b0C ambient with 3 or fewer conductors
- Bundling Factor — Adjustment from NEC 310.15(C)(1) based on number of current-carrying conductors
- Temp Factor — Correction from NEC 310.15(B) for ambient temperatures above 30\u00b0C (86\u00b0F)
- T_rating — Insulation temperature rating (60, 75, or 90 degrees C)
Example
A 10 AWG THHN conductor (base 40A at 90\u00b0C) with 6 conductors in conduit at 104\u00b0F (40\u00b0C): Bundling = 0.80, Temp = sqrt((90-40)/(90-30)) = 0.913. Derated = 40 x 0.80 x 0.913 = 29.2A.
Tips
- Neutral conductors carrying only unbalanced load are not counted as current-carrying conductors in most cases.
- Equipment grounding conductors are never counted in the bundling calculation.
- Higher-rated insulation (90\u00b0C THHN vs 60\u00b0C NM) gives more derating headroom before the ampacity drops too low.
- Attic spaces in summer can easily reach 120-140\u00b0F -- always apply temperature correction for conductors in hot locations.
- When bundling and temperature both apply, multiply the factors together -- they compound, not add.