Cable Ampacity Calculator

Ampacity is the maximum continuous current a cable can carry without the conductor exceeding the temperature its insulation is rated for. It depends on cross-sectional area, conductor material, installation method, ambient temperature and grouping with other cables. This calculator starts from typical BS 7671 style base ratings at 30°C in air and applies correction factors, so the figure you get is a design capacity, not a guaranteed rating for every installation.

The base rating assumes 30°C ambient air. Hotter surroundings reduce capacity, and the calculator models that with Ca = sqrt((90 - Ta) / (90 - 30)). Cables run together also heat each other, so a grouping factor Cg applies on top: roughly 0.80 for two cables, 0.70 for three, down to 0.57 for six. Design capacity is the base rating multiplied by Ca and Cg, and both factors compound, so a hot, grouped run loses capacity quickly.

Aluminium has higher resistivity than copper, so an aluminium conductor of the same cross-sectional area carries less current: this calculator rates it at about 78 percent of the equivalent copper figure. Aluminium is lighter and cheaper per amp on large distribution runs, which is why network operators use it widely, but terminations need more care. For most internal commercial wiring in the UK, copper is the default choice.

As a starting point, 4 mm² copper clipped in free air carries around 32 A at 30°C, but the honest answer is that it depends: run the same cable in conduit, through thermal insulation or grouped with others and the capacity drops sharply, sometimes below the breaker rating. Voltage drop over long runs matters too. Final cable selection must be done to BS 7671 by a qualified electrician.

A cable's rating is set by how fast it can shed heat. Clipped to a surface in free air it cools well. Inside conduit or trunking the surrounding air is still, and capacity falls by roughly a quarter. Buried in the ground it can rate higher because soil conducts heat away. Enclosed in thermal insulation it performs worst, often carrying barely 60 percent of its free-air figure, which is why loft and cavity runs need particular care.