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Feeder Conductor Ampacity Derating Calculator

Calculate a conductor's derated ampacity after applying ambient temperature and conductor-count correction factors.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Derated conductor ampacity

91.52 A

Check your inputs

Verify both correction factors directly against the current NEC tables for your specific ambient temperature and number of current-carrying conductors — this calculator applies factors you supply, it does not look them up for you.

Running these inputs gives 91.5 as the derated conductor ampacity. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 91.52 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Derated ampacity = base conductor ampacity × ambient temperature correction factor × conductor bundling/count adjustment factor, per NEC 310.15(B) temperature correction tables and NEC Table 310.15(C)(1) adjustment factors for more than 3 current-carrying conductors in a raceway

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Frequently asked questions

Why does a conductor need to be derated below its table ampacity?
The NEC's base ampacity tables (like Table 310.16) assume a specific ambient temperature and a limited number of current-carrying conductors. When the actual ambient temperature is higher, or more current-carrying conductors are bundled together in the same raceway or cable, both effects reduce the conductor's heat dissipation, so NEC 310.15(B) and Table 310.15(C)(1) require applying correction and adjustment factors to find the safe derated ampacity.
Where do I get the correction and adjustment factor values?
The ambient temperature correction factor comes from the NEC 310.15(B) temperature correction table for your conductor's insulation rating and the expected ambient temperature. The conductor count adjustment factor comes from NEC Table 310.15(C)(1), based on the number of current-carrying conductors in the same raceway, cable, or earth. This calculator applies the factors you enter — it does not look them up for you.
Can I apply both factors at once?
Yes — when both elevated ambient temperature and conductor bundling apply to the same run, NEC 310.15 requires applying both correction factors together (multiplying the base ampacity by each), exactly as this calculator does.