Determine the derated current-carrying capacity of a cable in its installed conditions.
Derating factors multiply rather than add. A cable derated 12 per cent for temperature and 20 per cent for grouping retains only 70 per cent of its tabulated rating, not 68 per cent by subtraction. Grouping and thermal insulation are the most commonly overlooked deratings, and both routinely cost more capacity than ambient temperature does.
Wire Ampacity
Derated ampacity = tabulated ampacity × ambient factor × grouping factor × insulation factor
Derated ampacity = tabulated ampacity × ambient factor × grouping factor × insulation factor Derating factors multiply rather than add. A cable derated 12 per cent for temperature and 20 per cent for grouping retains only 70 per cent of its tabulated rating, not 68 per cent by subtraction.
Grouping and thermal insulation are the most commonly overlooked deratings, and both routinely cost more capacity than ambient temperature does.
This calculator takes 5 inputs: Tabulated ampacity, Derating for ambient temperature, Derating for grouping, Derating for thermal insulation, Design current. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.