Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of derating. 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
Every published rating assumes a reference condition: 30 °C ambient for cable ampacity, 25 °C for many component ratings, a specified mounting for power devices. Derating corrects for the difference.
Cable derating factors stack — ambient temperature, number of conductors in a raceway, and whether the load is continuous all apply at once, and the product can be well under half the nominal rating.
Common misuse
Applying only one derating factor. They multiply, and the cumulative effect surprises people who expect a 20% adjustment.