What is conductor derating and when is it required?

Conductor derating reduces the allowable ampacity of a conductor when conditions exceed the standard assumptions of NEC Table 310.16 (30°C ambient, up to 3 current-carrying conductors). Derating is required per NEC 310.15 when ambient temperature exceeds 30°C or when more than 3 current-carrying conductors share a raceway, cable, or bundle.

What is the NEC 310.15 conductor derating formula?

Derated Ampacity = Base Ampacity × Temperature Correction Factor × Bundling Adjustment Factor, then limited by the termination temperature rating per NEC 110.14(C). Temperature correction factor = √((T_rated − T_ambient) / (T_rated − 30)). Bundling factors: 4–6 conductors = 0.80, 7–9 = 0.70, 10–20 = 0.50, per NEC Table 310.15(C)(1).

How do I use this conductor derating calculator?

Select conductor material (copper or aluminum), insulation type, ambient temperature, number of current-carrying conductors, and terminal rating. In Check Wire mode, select a wire size to see its derated ampacity and whether it passes for your load. In Find Minimum Size mode, enter your load current and the calculator finds the smallest conductor that passes all derating.

What counts as a current-carrying conductor for derating?

Current-carrying conductors (CCCs) include all ungrounded (hot) conductors and any grounded (neutral) conductor that carries unbalanced current. Equipment grounding conductors (EGC/ground wires) never count. Neutrals in 3-phase 4-wire systems with primarily nonlinear loads (harmonics) may count as CCCs per NEC 310.15(E).

Can I use the 90°C ampacity column from NEC Table 310.16?

You may use the 90°C column as the starting ampacity for derating calculations — but the final result must still be limited by the termination temperature rating per NEC 110.14(C). Most equipment terminals are rated 60°C (for equipment ≤100A) or 75°C (for equipment >100A), so the terminal limit often governs the final ampacity.

How does conductor derating affect continuous loads?

For continuous loads (operating for 3 hours or more), NEC 210.19(A) requires the conductor to be sized at 125% of the continuous load current. This means the conductor's derated ampacity must be at least 1.25 times the load, or equivalently, the maximum continuous load is 80% of the conductor's derated ampacity.

Frequently asked questions about conductor derating

Common derating questions: Apply both temperature and bundling factors together — they multiply. The neutral counts as a CCC if it carries significant current (unbalanced loads). Rooftop conductors may require an adder of up to 60°F (33°C) to ambient temperature per NEC 310.15(B)(3)(c). Always apply the termination limit after derating — this step is commonly missed and often determines the final ampacity.

⚡ Electrical Calculator

Conductor Derating Calculator

NEC 310.15 · Temperature × Bundling × Termination Limit · Table 310.16 · PASS/FAIL with code citations

⚙️ Installation Conditions

Conductor Material

Insulation Type

Ambient Temperature (°C)

Standard is 30°C (86°F). Attics can reach 50–60°C in summer.

Current-Carrying Conductors (CCCs)

Count hots + unbalanced neutrals. Do NOT count EGC (ground).

Equipment Terminal Rating (NEC 110.14(C))

Most modern panels ≤100A are 60°C rated. Panels >100A are typically 75°C.

Load Type

Continuous Load

Operates for 3+ hours continuously (NEC 210.19(A))

🔌 Wire to Check

Conductor Size (AWG / kcmil)

Load Current (A) — optional for PASS/FAIL

Please fill in all required fields.

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Base Ampacity
(Table 310.16)

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Derated
Ampacity (A)

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Max Amps
Allowed

📊 Step-by-Step Calculation (NEC 310.15)

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Temperature correction: CF = √((T_ins − T_amb) / (T_ins − 30)) | Bundling: NEC Table 310.15(C)(1) | Termination: NEC 110.14(C)
Base ampacity from NEC Table 310.16 (copper and aluminum, in raceway or cable, not more than 3 CCCs, 30°C ambient).
Results are indicative. Always verify with a licensed electrical engineer before installation.

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Conductor Derating Calculator

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