Which temperature column do I use?

Start from the conductor insulation rating, then verify the termination is rated for the same temperature. If not, use the lower of the two.

Do I count the neutral as a current-carrying conductor?

A neutral that carries only the unbalanced current from other conductors of the same circuit is not counted. A neutral in a 4-wire, 3-phase wye circuit supplying nonlinear loads, where significant harmonic current is present, is counted (NEC 310.15(E)). The equipment grounding conductor is never counted for this adjustment.

Why is my conductor suddenly not adequate in summer?

High ambient temperature reduces the correction factor. A raceway on a roof or in an unconditioned space can be far hotter than the air temperature.

Guide

How to Read an Ampacity Chart

An ampacity chart gives you the starting number, not the answer. The answer comes from applying the ambient, conductor-count and termination adjustments to it.

Four steps

Find the row for the conductor size.
Pick the column for the insulation temperature rating.
Apply the ambient temperature correction factor for the installation location.
Apply the conductor-count adjustment if more than three current-carrying conductors share a raceway.

Then stop and check the termination

The corrected ampacity is only half the story. The device the conductor lands on sets its own ceiling, and for circuits of 100 A or less that ceiling is often the 60°C column.

Take the lower of the two numbers. That is the ampacity you are allowed to use.

Frequently asked

Which temperature column do I use?: Start from the conductor insulation rating, then verify the termination is rated for the same temperature. If not, use the lower of the two.
Do I count the neutral as a current-carrying conductor?: A neutral that carries only the unbalanced current from other conductors of the same circuit is not counted. A neutral in a 4-wire, 3-phase wye circuit supplying nonlinear loads, where significant harmonic current is present, is counted (NEC 310.15(E)). The equipment grounding conductor is never counted for this adjustment.
Why is my conductor suddenly not adequate in summer?: High ambient temperature reduces the correction factor. A raceway on a roof or in an unconditioned space can be far hotter than the air temperature.

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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. “How to Read an Ampacity Chart.” Updated October 29, 2026. https://eleccalcs.com/guides/how-to-read-ampacity-chart

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.