How do I count current-carrying conductors?

Phase conductors and the neutral on a non-linear load (a neutral that carries harmonic currents) count. Equipment grounding conductors do not. The neutral on a balanced three-phase wye does not count unless the load is non-linear, per NEC 310.15(E).

When can I skip derating?

NEC 310.15(C)(1) Exception 1 lets you skip conductor count adjustment in nipples 24 inches or shorter. Temperature correction is required whenever the ambient differs from the table baseline of 30°C.

Do I have to apply both factors?

Yes, when both apply. Multiply them together - they are independent and cumulative. A hot, crowded raceway can derate ampacity by 30-50%.

NEC Derating Calculator

NEC 310.15

Apply NEC temperature correction and conductor bundling adjustment factors to find derated ampacity.

Conductor

Wire Gauge

Insulation Type

Conductor Material

CopperAluminum

Conditions

Ambient Temperature

°C

Current-Carrying Conductors

Results

Base Ampacity

40A

Temp Correction Factor

1.00

Bundling Factor

1.00

Derated Ampacity

40.0A

Ampacity Waterfall

Base Ampacity40.0 A

40.0 A

After Temp Correction40.0 A

40.0 A

After Bundling Adj.40.0 A

40.0 A

Derating Steps

1Base Ampacity

10 AWG at rated temperature

40.0 A

2Temp Correction

Ambient temperature factor

× 1.00

40.0 A

3Bundling Adj.

Conductor fill factor

× 1.00

40.0 A

About this calculator

When you bundle current-carrying conductors in a raceway or cable, or run them in hot ambient temperatures, the heat each conductor produces is harder to dissipate. NEC 310.15(B) and (C) require you to derate the conductor ampacity to account for this. Get derating wrong and the conductors run hotter than the insulation can handle - leading to slow degradation and eventual failure.

Formula

Adjusted ampacity equals the base Table 310.16 ampacity multiplied by the ambient temperature correction factor (Table 310.15(B)(1)) and the conductor adjustment factor (Table 310.15(C)(1)). Both factors are unitless multipliers less than or equal to 1.0.

Adjusted ampacity = base ampacity × temp correction × adjustment factor

Reference: NEC 310.15(B), 310.15(C), Tables 310.15(B)(1) and 310.15(C)(1)

How to use

Start with the base ampacity from NEC Table 310.16 for your wire size, material, and insulation column.
Enter the ambient temperature where the raceway or cable will run.
Enter the number of current-carrying conductors in the raceway.
Pick the insulation temperature rating - this determines which row of the correction table applies.
The calculator applies both factors and reports the adjusted ampacity available for your load.

Worked example

Setup

#10 AWG copper THHN, six current-carrying conductors in a raceway in a 40°C attic.

Calculation

Base ampacity for #10 copper THHN at the 90°C column: 40 A. Temperature correction at 40°C ambient for 90°C insulation: 0.91. Adjustment for 4-6 conductors: 0.80. Adjusted ampacity = 40 × 0.91 × 0.80 = 29.1 A.

Answer

29.1 A is the maximum continuous load this conductor can carry under those conditions. Note that the breaker still has to match a 60°C or 75°C termination - in practice, this conductor would protect at 30 A under standard terminations.

Related study guides

Frequently asked questions

How do I count current-carrying conductors?: Phase conductors and the neutral on a non-linear load (a neutral that carries harmonic currents) count. Equipment grounding conductors do not. The neutral on a balanced three-phase wye does not count unless the load is non-linear, per NEC 310.15(E).
When can I skip derating?: NEC 310.15(C)(1) Exception 1 lets you skip conductor count adjustment in nipples 24 inches or shorter. Temperature correction is required whenever the ambient differs from the table baseline of 30°C.
Do I have to apply both factors?: Yes, when both apply. Multiply them together - they are independent and cumulative. A hot, crowded raceway can derate ampacity by 30-50%.