Does the 80% rule apply to a 15 A lighting circuit?

It applies to the continuous portion of the load. If the load runs three hours or more, it is limited to 80% of the circuit rating.

Does 125% change the wire or just the breaker?

The 125% is applied to the termination column and to the overcurrent device. The corrected ampacity only has to cover the actual load (NEC 210.19(A)(1)). The conductor often still goes up a size because the termination column has to cover the larger number.

Is the 80% rule the same as the 40% conduit rule?

No. The 40% is about physical space for conductors in a raceway; the 80% is about electrical loading over time.

Guide

The 80% Rule for Circuit Loading

The 80% rule is shorthand for the continuous-load requirement: a load expected to run for three hours or more must be limited to 80% of the circuit rating, which is the same as sizing the circuit at 125% of the load.

One rule, two ways of saying it

Sizing the circuit at 125% of a continuous load, and limiting a continuous load to 80% of the circuit rating, are the same statement from opposite ends.

A 20 A circuit carrying a continuous load is limited to 16 A. The breaker is still 20 A; what changes is how much load you may put on it.

What counts as continuous

The test is duration: three hours or more of maximum current. Lighting in a commercial space, EV charging, water heating and similar long-running loads are the usual examples.

If a load is not continuous, the 125% factor does not apply.

Frequently asked

Does the 80% rule apply to a 15 A lighting circuit?: It applies to the continuous portion of the load. If the load runs three hours or more, it is limited to 80% of the circuit rating.
Does 125% change the wire or just the breaker?: The 125% is applied to the termination column and to the overcurrent device. The corrected ampacity only has to cover the actual load (NEC 210.19(A)(1)). The conductor often still goes up a size because the termination column has to cover the larger number.
Is the 80% rule the same as the 40% conduit rule?: No. The 40% is about physical space for conductors in a raceway; the 80% is about electrical loading over time.

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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. “The 80% Rule for Circuit Loading.” Updated October 29, 2026. https://eleccalcs.com/guides/80-percent-rule-electrical

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.