Correction factors

Multipliers applied to a cable's tabulated current rating to reflect how it is actually installed. High ambient temperature, grouping with other loaded circuits, contact with thermal insulation and the type of protective device all de-rate it. The cable size that survives every applicable factor — and still beats the design current — is the one you install.

Where to look it up: BS 7671 Appendix 4.

Taught and practised in the Installation section.