Do the factors apply to the base ampacity or to the termination limit?

They apply to the conductor base ampacity. The termination limit is a separate ceiling that is not adjusted by these factors.

Does a conduit in sunlight need extra derating?

Yes, when it is on a roof. NEC 310.15(B)(2) adds 33°C (60°F) to the outdoor ambient where a raceway or cable is exposed to direct sunlight and the bottom of the raceway is less than 3/4 in above the roof. NEC 2023 uses 3/4 in; the 2020 edition used 7/8 in. This is a code rule, not only a local amendment. This calculator does not add it automatically, so raise the ambient input by 33°C when the rule applies, then confirm the edition your jurisdiction enforces.

What if my ambient exceeds the table range?

The tables stop at a maximum ambient for each insulation class. Beyond that, the insulation is not suitable and you must change the conductor or the installation.

Guide

Ampacity Correction Factors in Practice

Correction factors are multiplicative. Two moderate factors can cost you a third of the ampacity, which is usually one or two conductor sizes.

Ambient temperature

Every conductor is rated at a reference ambient. When the actual ambient is higher, the allowable current falls, and the penalty accelerates as the temperature rises.

The relevant temperature is where the raceway runs, not the outdoor weather report — a roof deck or an attic can be dramatically hotter.

Conductor count

Bundling conductors traps heat. Once more than three current-carrying conductors share a raceway, the allowable ampacity is reduced in steps as the count rises.

Note the word current-carrying: a grounding conductor does not count toward this adjustment, although it does count toward conduit fill.

Combine them, then apply the termination cap

Multiply the base ampacity by both factors, then compare the result with the termination-limited ampacity and take the smaller number.

Frequently asked

Do the factors apply to the base ampacity or to the termination limit?: They apply to the conductor base ampacity. The termination limit is a separate ceiling that is not adjusted by these factors.
Does a conduit in sunlight need extra derating?: Yes, when it is on a roof. NEC 310.15(B)(2) adds 33°C (60°F) to the outdoor ambient where a raceway or cable is exposed to direct sunlight and the bottom of the raceway is less than 3/4 in above the roof. NEC 2023 uses 3/4 in; the 2020 edition used 7/8 in. This is a code rule, not only a local amendment. This calculator does not add it automatically, so raise the ambient input by 33°C when the rule applies, then confirm the edition your jurisdiction enforces.
What if my ambient exceeds the table range?: The tables stop at a maximum ambient for each insulation class. Beyond that, the insulation is not suitable and you must change the conductor or the installation.

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Cite this page

Use this reference in a report, article, course note or dataset. Calculated examples use this site's engine; reference data and explanatory text have separate sources and review status.

ElecCalcs. “Ampacity Correction Factors in Practice.” Updated October 29, 2026. https://eleccalcs.com/guides/ampacity-correction-factors

Values reflect NEC 2023 and are pending review. Cite the edition you verified against.

Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.