Simplicalcs

    Cable size calculator

    A
    m
    Minimum cable size
    โ€”
    All sizes compared
    Size CCC CCC โœ“ Volt drop VD โœ“
    Worked example โ€” 200A farm submain, 130m buried
    A 200A three-phase submain across a farm, buried direct in ground. The tool identifies 95mmยฒ as the minimum passing size on CCC alone, but 150mmยฒ was specified to give comfortable headroom โ€” 287A rated vs 200A design current, with a volt drop of only 3.22V (0.81%).
    Load this example โ†’
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    Selecting a cable involves two checks that must both pass. First, the cable must carry the design current without overheating โ€” its current-carrying capacity (CCC) must exceed the load. Second, the voltage lost along its length must stay within BS 7671 limits: 3% for lighting, 5% for power circuits. A cable that passes on current but fails on volt drop is undersized for the run; a cable that passes on volt drop but fails on current is a fire risk.

    CCC values are from BS 7671 Appendix 4 for copper conductors, adjusted for installation method. The grouping factor accounts for heat buildup when multiple cables run together โ€” grouped cables can carry less current than a single cable in free air. Resistance values for the volt drop calculation are from BS 7671 Table B1 and manufacturer data.

    This tool identifies the smallest cable that passes both checks and shows the full picture for all sizes above and below, so you can make an informed choice between minimum compliance and practical headroom.