Wire Ampacity Calculator

About the Wire Ampacity Calculator

This wire ampacity calculator looks up the maximum current-carrying capacity of insulated copper or aluminum conductors per NEC Table 310.16, the standard reference for residential, commercial, and industrial wiring in the US and Canada. Pick the AWG, conductor material, and insulation rating to get the ampacity at 30°C ambient.

For non-standard ambient temperatures or more than 3 current-carrying conductors in a raceway, apply the NEC 310.15 derating factors (typical reductions: 80–100% for ambient, 70–80% for raceway fill).

How wire ampacity is determined

Ampacity (table value) × Temperature derate × Adjustment factor

The base value comes from NEC Table 310.16, which lists ampacity by AWG, conductor material (Cu/Al), insulation temperature rating (60/75/90°C), and assumes 30°C ambient. Real-world ampacity gets reduced by ambient temperature corrections (NEC Table 310.15(B)(1)) and conductor count adjustments (NEC Table 310.15(C)(1)).

Worked example

#10 AWG copper THHN (90°C insulation) has a base ampacity of 40 A per NEC Table 310.16. However, terminations on most equipment are limited to 75°C, so the practical ampacity is 35 A (the 75°C column). For a 30A motor circuit, #10 is fine; for 35A, you need #8.

When to use this calculator

• Sizing motor branch circuits to NEC 430 requirements
• Selecting feeder conductors to subpanels
• Verifying that existing wiring can support a new load
• Choosing copper vs aluminum for large feeders
• Quick reference during inspections and code reviews
• Estimating wire cost for new construction or renovation

Why insulation temperature matters

The temperature rating is what allows the conductor to safely dissipate heat at full load. Higher-rated insulation (90°C THHN, XHHW) gives higher ampacity than lower-rated (60°C TW). However, NEC 110.14(C) limits the practical ampacity to whatever the connected equipment terminals are rated for — usually 75°C in modern equipment, 60°C in older or residential equipment under 100A. So the 90°C value is rarely usable in practice.

Frequently Asked Questions

What is wire ampacity?

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 lists ampacity by wire size, material, and insulation rating at 30°C ambient.

What gauge wire for a 30-amp circuit?

#10 AWG copper at 75°C ampacity = 35 A (sufficient for 30 A continuous). For aluminum, you’d need #8. For long runs, voltage drop may require upsizing.

What’s the difference between 60°C, 75°C, and 90°C ratings?

The temperature rating is what the insulation can handle continuously. THHN/XHHW are 90°C; THW/THWN are 75°C; TW/UF are 60°C. Higher-rated insulation = higher ampacity, but you’re limited by the connected equipment’s terminal rating (usually 75°C).

Should I use copper or aluminum?

Copper has higher ampacity per gauge and easier connections, but costs more. Aluminum is widely used for large feeders (#4 AWG and larger) where cost matters. Avoid aluminum on circuits under 30 A unless using approved AL/CU connectors.

Do I need to derate for ambient temperature?

Yes if ambient is above 30°C. At 40°C, multiply by 0.91 (75°C insulation). At 50°C, 0.82. The full table is in NEC 310.15(B)(1). Common scenarios: rooftop conduit (often 50+°C), boiler rooms, attics.

What about more than 3 conductors in a conduit?

Apply NEC 310.15(C)(1) adjustment factors: 4–6 conductors = 80% of ampacity, 7–9 = 70%, 10–20 = 50%. This prevents heat buildup when many conductors share a raceway.

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