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Temperature and Conduit-Fill Derating Factors Explained

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NEC Table 310.16 lists ampacity values — the maximum current a conductor can safely carry — but those values assume two specific conditions: an ambient temperature of exactly 30°C (86°F), and no more than three current-carrying conductors sharing a raceway. Real installations often violate one or both assumptions. When they do, NEC 310.15 requires you to reduce (derate) the table ampacity before selecting a wire size. This page explains why derating exists, how each factor works conceptually, and when to use the calculator to handle the math.

Why Conductors Generate Heat

Every conductor has some resistance, and when current flows through resistance, heat is produced (I²R losses). The wire must be able to shed that heat into its surroundings. Two conditions make heat-shedding harder:

  • Hot surroundings — A conductor in a 120°F attic has less temperature difference between itself and the air around it, so it cannot dissipate heat as fast. The wire runs hotter at the same current, shortening insulation life and raising fire risk.
  • Neighboring conductors — Pack multiple current-carrying conductors into the same conduit and they all contribute heat to the same confined space. Each one now operates in a warmer microenvironment than the NEC table assumes.

Derating is the NEC's answer to both problems: apply a multiplier less than 1.0 to the table ampacity, forcing you to choose a larger (lower-resistance, lower-temperature) conductor than the table alone would suggest.

Ambient Temperature Correction — NEC Table 310.15(B)(1)

Table 310.16 baselines ampacity at 30°C (86°F) ambient. Any installation warmer than that requires a correction factor from NEC Table 310.15(B)(1) (NEC 2023). The correction factor is always a decimal less than 1.0 for temperatures above 30°C, and greater than 1.0 for cooler environments (a modest bonus rarely used in practice).

Common situations that push ambient above the baseline include attics, rooftop conduit, boiler rooms, outdoor runs in direct sun, and any mechanical space with poor ventilation. As of 2026, the NEC also requires an added temperature adder for raceways or cables exposed to direct sunlight on or above rooftops — consult NEC 310.15(B)(3)(c) and verify with your AHJ for current requirements.

Conceptual worked example — high-ambient attic run:

Suppose a 10 AWG copper THHN conductor has an ampacity of 40 A in the 90°C column of Table 310.16. The wire passes through an unconditioned attic that regularly reaches 38°C (100°F). The NEC 310.15(B)(1) correction factor for that temperature range with a 90°C-rated conductor is approximately 0.91 (source: NFPA 70 NEC 2023; verify in the current edition for your jurisdiction).

Corrected ampacity = 40 A × 0.91 ≈ 36.4 A

That same wire can now only be loaded to about 36 A in that location — roughly a 10% reduction. For larger temperature excursions the reduction is steeper, and you may need to move up a wire gauge to stay within safe limits. Always use the calculator or consult a licensed electrician rather than relying on a mental estimate.

Conduit-Fill Adjustment — NEC 310.15(C)(1)

When more than three current-carrying conductors share a raceway or cable assembly for more than 24 inches, NEC 310.15(C)(1) (NEC 2023) requires an adjustment factor from NEC Table 310.15(C)(1). This is sometimes called "bundling derating" or — loosely — "conduit fill derating" (technically the fill percentage rules live in NEC Chapter 9, Table 1; the thermal adjustment is a separate concept).

What counts as a current-carrying conductor?

Only conductors that actually carry load current count toward the tally. Generally:

  • Phase conductors always count.
  • Equipment grounding conductors (EGCs — the green or bare wire) do not count; they carry current only during a fault, not under normal operation.
  • Neutral conductors on a balanced three-phase wye system generally do not count because the phase currents cancel. However, neutrals on a circuit serving non-linear loads (computers, variable-frequency drives) can carry significant harmonic current and may need to be counted — consult NEC 310.15(E) and a licensed electrician for non-linear load situations.
  • The 24-inch rule: short conduit nipples under 24 inches are generally exempt from the adjustment, as referenced in NEC 310.15(C)(1) exceptions.

Adjustment Factor Reference (NEC Table 310.15(C)(1))

The table below reflects the conductor groupings and adjustment factors as published in NFPA 70 NEC 2023. Always verify against the edition adopted by your local authority having jurisdiction (AHJ).

Current-Carrying Conductors in Raceway Adjustment Factor Ampacity Remaining
1–3 1.00 (no derating) 100%
4–6 0.80 80%
7–9 0.70 70%
10–20 0.50 50%
21–30 0.45 45%
31–40 0.40 40%

Source: NFPA 70, NEC 2023 Table 310.15(C)(1). Verify with the edition adopted in your jurisdiction. Values above 40 conductors are listed in the full NEC table.

Applying Both Factors Together

When a run is both in a hot environment and shares a conduit with more than three current-carrying conductors, you apply both corrections. The general approach is to multiply the table ampacity by the temperature correction factor and then by the conduit-fill adjustment factor (or vice versa — multiplication is commutative). The result is the derated ampacity that your actual load must not exceed.

Because stacking two derating factors can significantly shrink effective ampacity, conduit design and routing choices matter: splitting circuits across separate raceways, keeping runs out of unconditioned spaces, or choosing conductors with higher temperature ratings can all reduce or eliminate derating penalties. These are judgment calls best made with a licensed electrician who knows your specific installation.

What the Wire Size Calculator Handles

The Wire Size Calculator applies standard NEC Table 310.16 ampacity at 30°C with up to three current-carrying conductors — the baseline case. If your installation has elevated ambient temperature or more conductors in the conduit, you should calculate your derated ampacity first, then enter that derated value as your design load into the calculator to find the correct wire size. Alternatively, consult a licensed electrician who can work through the full NEC 310.15 analysis for your specific conditions.

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