Electrical cable size

A cable has to pass three checks, and this tool does only the first. Current capacity is worked out here as Ib, the design current, divided by every derating factor, then matched against the smallest tabulated size that clears it. Voltage drop on long runs and earth-fault loop impedance for disconnection times are separate checks with their own tools, and on a long run the voltage drop often picks a bigger cable than capacity does. The capacity tables cover multicore cable clipped direct on a wall at 30 °C: BS 7671 tables 4D1A and 4E4A on the IEC side, NEC table 310.16 on the other. Buried runs, cables in thermal insulation, and sunlit surfaces are not modelled; price them into the extra factor. IEC aluminium uses the 0.78 × copper rule of thumb rather than a tabulated series, and NEC 90 °C columns still have to fit the 60 or 75 °C terminal rating of the equipment they land on, which the result flags but does not enforce. Grouping is the factor people forget: size for the number of circuits that will eventually share the space, not the ones there today. Published tables only, for common cases; the standards themselves are the authority, and this page is not a compliance certificate.