Cable Sizing & Selection Calculators
Five essential calculators for cable selection and verification to BS 7671 IET Wiring Regulations. All calculations run in your browser — no data leaves your device.
Determine the minimum cable size from the design current, protective device rating and correction factors. Each cable type is rated from its own BS 7671 Appendix 4 column (single-phase, copper); a combination BS 7671 does not rate is refused rather than estimated.
Length of the run surrounded by insulation (Appendix 4 §2.6, cables up to 10 mm²). A cable with one side against a thermally conductive surface is rated by its tabulated installation method, not by a factor.
Results
NOT TABULATED
Ca (temp)
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Cg (grouping)
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Ci (insulation)
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Required It
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Suggested cable
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Tabulated Iz
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Disclaimer
These calculators use simplified reference data from BS 7671:2018+A4:2026 and the IET On-Site Guide. Results are for guidance and educational purposes only. Always verify against the full regulations and manufacturer data for your specific installation. Cable current-carrying capacities and mV/A/m values are read from the BS 7671 Appendix 4 column for the selected cable type and installation method (copper, single-phase unless stated); the column is cited with each result.
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