Cable Sizing Derating

What "Derating" Actually Means — A Plain-English Explanation

A conceptual explanation of derating for anyone who has heard the term without a clear picture of what it actually represents.

"Derating" is one of those terms that gets used constantly in electrical work without always being clearly explained. Here's the plain version.

The Core Idea

A cable's ampacity rating printed in a reference table represents its capacity under specific, standardized conditions — a particular ambient temperature, installed on its own rather than bundled with others. Real installations rarely match those exact conditions. Derating is simply the process of adjusting that table figure downward to reflect the cable's actual safe capacity in its real installation environment.

The Two Most Common Derating Factors

• Temperature derating — capacity drops as real ambient temperature exceeds the table's reference point
• Grouping derating — capacity drops as more current-carrying conductors share the same enclosed space, since they heat each other

Why It's Not Optional

Skipping derating means trusting a capacity figure the cable doesn't actually have in its real installation — the table number was never meant to be used without this correction step whenever real conditions differ from the reference assumptions.

See derating applied automatically and explicitly broken out in any of the OhmWorks Cable Sizing calculators.

Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks

← All articles