Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of ampacity. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Ampacity is not a property of the conductor size alone. The same 2.5 mm² conductor carries 20 A with 60 °C insulation, 25 A with 75 °C and 30 A with 90 °C in a raceway — and different figures again in free air or bundled.
That is why a single ampacity number beside a cable size is meaningless without its basis: insulation rating, installation method, ambient temperature and how many conductors share the raceway.
Common misuse
Reading an ampacity table without its installation basis. The same cable has four or five legitimate ratings depending on how it is installed.