Glossary Cable Sizing
A jargon-free explanation of what ampacity actually means, for anyone who has heard the term without a clear mental picture.
Ampacity is one of the first terms anyone encounters in cable sizing, and it's worth a plain-English explanation for anyone who's absorbed the word without a fully clear mental picture.
Ampacity is simply the maximum current a conductor can safely carry continuously, under specific stated conditions, without exceeding its insulation's safe operating temperature. It's a current rating tied directly to heat — too much current generates too much heat, which degrades or destroys insulation over time (or immediately, in extreme cases).
A cable's ampacity isn't a fixed, universal number — it's specific to the conditions the table assumes: a particular ambient temperature, a particular installation method, a particular conductor count sharing the same space. Change any of those conditions, and the cable's real, safe ampacity changes with it — which is exactly why derating exists (see the dedicated derating article).
Ampacity answers the question "how much current can this cable handle continuously, right here, in these real conditions, without overheating" — not an abstract, condition-free number.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks