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Wire Ampacity Calculator

Calculate the safe current-carrying capacity (ampacity) of a conductor, adjusted for ambient temperature and the number of current-carrying conductors bundled together.

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These calculators provide estimates and educational results. Always verify critical engineering calculations against applicable standards and qualified professional review.

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What Is It

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. Base ampacity tables assume a 30C ambient and no more than 3 current-carrying conductors bundled together - real installations often need correction factors applied to those base values.

How to Calculate

Select the wire gauge and material, the ambient temperature range at the installation, and how many current-carrying conductors are bundled in the same raceway or cable. The calculator applies the ambient temperature correction factor and the conductor bundling adjustment factor to the base 75C ampacity.

Example

A #10 AWG copper conductor has a base ampacity of 35A. Installed in an attic where ambient reaches 41-45C, alongside 5 other current-carrying conductors: adjusted ampacity = 35 × 0.82 × 0.8, about 23A - well below the unadjusted 35A rating.

Recommendations

Always apply both correction factors together when they apply - ambient temperature and conductor bundling compound multiplicatively, not additively. A conductor that looks adequate at the base table value can be significantly undersized once real installation conditions are considered.

Frequently Asked Questions

Why is my wire's real ampacity lower than the table value?

Ampacity tables assume a 30C ambient with no more than 3 bundled conductors. Hotter environments (attics, rooftops) and more bundled conductors both reduce the safe current rating, and the two effects multiply together.

Do I need to derate for conduit fill?

Conduit fill (physical space) and ampacity derating (heat dissipation) are separate checks - you need to satisfy both the fill percentage limit and the adjusted ampacity for your conductor count.

What's the difference between the 60C, 75C and 90C columns?

They correspond to the conductor's insulation temperature rating. Most equipment terminals are only rated for 60C or 75C, so even a 90C-rated wire is often limited by the terminal rating it connects to, not its own insulation rating.

Last reviewed: 2026-09-12