Wire Derating Calculator

The Wire Derating Calculator produces one core result: adjusted ampacity, the current-carrying capacity of a conductor after applying two independent correction factors to a base ampacity value. Adjusted ampacity is the number used downstream in conductor sizing, feeder and branch-circuit load review, and comparison against equipment terminal ratings. If the adjusted ampacity falls below the calculated load, the conductor size must increase or the installation condition must change before the circuit is finalized.

In this example, a User-supplied base ampacity of 100 A combined with an ambient temperature factor of 0.82 and a bundling factor of 0.8 produces an Adjusted ampacity estimate of 65.6 A. That figure represents roughly two-thirds of the conductor’s unadjusted rating once heat from ambient conditions and conductor bundling are both accounted for.

Derating Factors

The calculator applies exactly two inputs to the base ampacity:

• User-supplied ambient temperature factor — a decimal multiplier from 0.01 to 1 (example: 0.82) reflecting how ambient temperature at the installation location reduces the conductor’s rated capacity relative to its insulation temperature rating.
• User-supplied bundling factor — a decimal multiplier from 0.01 to 1 (example: 0.80) reflecting the effect of grouping or bundling multiple current-carrying conductors together, which restricts heat dissipation compared to a single conductor in free air.

Both factors are transparent, user-supplied decimals. The calculator does not select, look up, or reproduce factors from any adopted code table, manufacturer chart, or ambient temperature correction table. Every multiplier entered must be verified independently against the adopted NEC edition, AHJ direction, manufacturer conductor data, and actual project conditions before it is used in this field.

Current-Carrying Conductors

The Current-carrying conductor count field (3 in this example) is a review flag only. It documents the grouping assumption behind the bundling factor you entered, but it does not drive any calculation and does not automatically select or adjust the bundling factor value. If the conductor count changes — for example, from three to seven current-carrying conductors in the same raceway — the bundling factor must be re-verified and re-entered manually; the calculator will not update it for you.

This separation exists because bundling adjustment values depend on table lookups, spacing, raceway fill, and free-air conditions that vary by installation and by the adopted code edition. The calculator performs arithmetic on whatever factor you supply; it does not perform that lookup.

Calculation Example

The adjusted ampacity is the product of the base ampacity and both correction factors:

Adjusted ampacity = Base ampacity × Ambient temperature factor × Bundling factor

Applied to the example inputs:

\(100 \text{ A} \times 0.82 \times 0.8 = 65.6 \text{ A}\)

The calculator also reports the combined multiplier on its own, separate from the final amperage:

• Combined factor decimal: 0.656
• Combined factor percent: 65.6%

These two outputs express the same relationship — the net derating applied to the base ampacity — as a decimal and as a percentage, which is useful when comparing this installation condition against another circuit’s combined factor without recalculating amperage each time.

Temperature Ratings and Terminal Limits

Adjusted ampacity from this calculator reflects only the two factors entered. It does not account for the conductor’s insulation temperature rating (60°C, 75°C, 90°C) independent of the ambient factor you supplied, and it does not check the result against terminal rating limits at breakers, lugs, or other equipment. A conductor can show an acceptable adjusted ampacity here and still be limited by a lower equipment terminal rating on either end of the circuit. Terminal ratings, voltage-drop review, and raceway fill are separate checks that must be performed outside this calculation.

Field Verification

This calculator performs multiplication on the values entered — it does not select ambient temperature tables, does not select bundling or adjustment factor tables, and does not reference or reproduce any specific NEC article, manufacturer data sheet, or AHJ bulletin. The Derating factor source note field exists to record where each verified factor came from (adopted code table, manufacturer document, or AHJ directive) so the entry can be checked later against the actual project source.

Before using the 65.6 A result (or any result from this tool) in conductor sizing, feeder load calculations, or documentation submitted to an AHJ, confirm both input factors against the adopted NEC edition in force for the jurisdiction, the applicable ambient temperature correction table, the bundling/adjustment table matching the actual conductor count and raceway condition, and any local amendments. The arithmetic is fixed and repeatable; the correctness of the two decimal inputs is the responsibility of the person entering them.