Wire Sizing with Temperature and Conduit Fill Derating

Every electrician knows Table 310.16 (or 310.15(B)(16) in some editions) — the ampacity table that tells you how much current each wire size can carry. A 10 AWG THHN copper conductor has an ampacity of 40A at 90°C, 35A at 75°C, or 30A at 60°C.

But Table 310.16 assumes two specific conditions:

1. An ambient temperature of 30°C (86°F)
2. No more than three current-carrying conductors in a raceway

When either condition is different — and it usually is — the allowable ampacity must be adjusted. Running conductors through a hot attic (50°C+ ambient) or bundling 10 circuits in a conduit requires derating the ampacity, sometimes significantly.

When the ambient temperature exceeds 30°C (86°F), the allowable ampacity decreases because the conductor can’t dissipate heat as effectively. NEC Table 310.15(B)(1) provides correction factors:

Example: A 6 AWG THHN copper conductor in a hot attic (50°C ambient). The 90°C column ampacity from Table 310.16 is 75A. The correction factor for 46–50°C at 90°C insulation is 0.82.

Corrected ampacity = 75A × 0.82 = 61.5A

But wait — per NEC 110.14(C), if the equipment terminations are rated 75°C (most breakers and panels), you must use the 75°C column for termination purposes. So the comparison is: 65A (75°C base) vs. 61.5A (90°C derated). The 90°C derated value of 61.5A controls.

When more than three current-carrying conductors share a raceway, heat buildup reduces the allowable ampacity. NEC Table 310.15(B)(3)(a) provides adjustment factors:

What counts as a current-carrying conductor? Equipment grounding conductors do not count. Neutral conductors that carry only unbalanced current do not count (for most circuits). But neutrals on shared-neutral multi-wire branch circuits and circuits with harmonic loads (nonlinear loads) do count.

Example: A conduit with 8 current-carrying conductors (four 2-wire circuits). The adjustment factor is 0.70. If the base ampacity of 12 AWG THHN at 90°C is 30A:

Adjusted ampacity = 30A × 0.70 = 21A

When both temperature correction and conduit fill adjustment apply, multiply them together:

Derated ampacity = Base ampacity × Temperature factor × Conduit fill factor

Example: 10 AWG THHN copper, 45°C ambient, 6 conductors in conduit.

1. Base ampacity (90°C column, Table 310.16): 40A
2. Temperature correction (41–45°C, 90°C insulation): 0.87
3. Conduit fill adjustment (4–6 conductors): 0.80
4. Derated ampacity: 40A × 0.87 × 0.80 = 27.84A

Compare against the 75°C termination limit: 35A (from Table 310.16, 75°C column). The derated value of 27.84A controls.

If this circuit carries a 20A continuous load (25A after the 125% multiplier per NEC 210.19(A)(1)), the 27.84A derated ampacity is sufficient. But a 30A continuous load (37.5A) would require upsizing to 8 AWG.

Wire sizing with derating involves checking four constraints and selecting the wire that satisfies all of them:

1. Ampacity requirement: Load current (with 125% for continuous loads) must not exceed the derated ampacity
2. Temperature correction: Apply Table 310.15(B)(1) factor for the actual ambient temperature
3. Conduit fill adjustment: Apply Table 310.15(B)(3)(a) factor for the actual conductor count
4. Voltage drop: Calculate VD% and verify it’s within 3% (branch) or 5% (total)

The required wire size is the largest size demanded by any of these four constraints. A professional NEC calculator should show all four constraints side by side and highlight the controlling factor — so you know exactly why the wire size is what it is.

This multi-factor convergence display is what separates a professional tool from a basic calculator. When the inspector asks “why 6 AWG instead of 10 AWG?” you need to point to the specific constraint that required upsizing.