Cable Sizing Without the Guesswork
Four factors, in order, and one of them is the reason so many installations run warm.
Current-carrying capacity
Start with the design current, apply the correction factors for ambient temperature, grouping, and thermal insulation, and only then read the table. The order matters, and grouping is the factor most often ignored because it depends on how the job actually gets installed rather than how it was drawn.
Volt drop is where designs fail
A cable can be perfectly adequate thermally and still deliver unusable voltage at the far end of a long run. On agricultural and industrial jobs with distances involved, volt drop sets the size far more often than current does.
Insulation is a thermal problem
A cable buried in loft insulation cannot shed heat. The derating is severe and it is routinely missed on retrofits, where the insulation arrives years after the wiring did.
Worth remembering when quoting work in a house that has been through an energy-efficiency scheme.
