BS 7671 Cable Sizing Calculator
Build a distribution board circuit-by-circuit and check each one to BS 7671:2018+A2:2022 — current-carrying capacity with derating, voltage drop, earth-fault loop impedance and the adiabatic CPC check — then print a submission-style calculation report. A first-pass design aid, not a substitute for a full ProDesign network model.
This is a desktop tool — the circuit schedule is wide, so on a small screen it scrolls sideways. Every value is indicative; verify against your own copy of BS 7671 before issue.
This calculator is a first-pass design aid to BS 7671:2018+A2:2022. Its reference data (current ratings, mV/A/m, correction factors, k-values) is an indicative subset of BS 7671 Appendix 4 / Chapter 41–54 — every value marked VERIFY must be confirmed against your own copy of the standard and manufacturer data before a calculation is issued. It does not model a whole distribution network or produce a discrimination study; the design remains the responsibility of the engineer.
The Five Checks in a Compliant Cable Calculation
For every circuit, BS 7671 effectively requires five things to be demonstrated, and this calculator runs all five as you type: the Ib ≤ In ≤ Iz coordination between load, protective-device rating and cable capacity; current-carrying capacity corrected for installation method, grouping (Cg), ambient temperature (Ca) and thermal insulation (Ci); voltage drop against the 3 % lighting / 5 % power limits; earth-fault loop impedance Zs against the maximum for the device and disconnection time; and the adiabatic (thermal withstand) check that sizes the CPC for fault energy (S²k² ≥ I²t).
Auto-Sizing That Respects Voltage Drop
Leave a cable on "auto-size" and the tool picks the smallest conductor whose corrected capacity clears the required tabulated rating, then steps the size up until the voltage drop also passes — so the suggested cable satisfies both current-carrying capacity and volt drop, which is exactly where quick single-circuit checks usually go wrong on longer runs. XLPE terminated on 70 °C terminals is sized on the 70 °C column automatically (Reg 512.1.5).
Board Totals & a Printable Report
Add the whole board and the tool rolls up connected load, maximum demand with per-circuit diversity, three-phase balance and a suggested incomer rating. Print / Save as PDF produces a submission-style report — supply data, the full circuit schedule, per-circuit arithmetic, board summary, the standards used and a VERIFY register. For a full distribution network model, discrimination study or a stamped submission pack, see our ProDesign cable calculation service. Background reading: cable sizing to BS 7671 and how the derating factors work.