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.
What are the five checks a cable calculation has to pass?
Overload coordination (Ib ≤ In ≤ Iz, Regulation 433.1.1); current-carrying capacity, where the tabulated rating is derated by the ambient, grouping and thermal-insulation factors; voltage drop against the Appendix 4 limits of 3 per cent for lighting and 5 per cent for other uses; earth-fault loop impedance Zs against the maximum for the device and its disconnection time; and the adiabatic check on the CPC, k²S² ≥ I²t. A cable is only compliant when it clears all five — passing four is not a pass.
Do I derate the load current or the device rating?
The device rating, In. Regulation 433.1.1 requires Ib ≤ In ≤ Iz, so the cable has to clear the protective device rather than merely the load. Appendix 4 §5 puts it plainly: the rating factors are applied to In as divisors. A 25 A load on a 32 A breaker is sized on 32 A, and dividing by the load instead is one of the commonest ways to undersize a cable.
Why does my cable get bigger when I add circuits to the same containment?
Because of the grouping factor Cg from Table 4C1. Bunching cables together means each one sheds heat less easily, so every cable in the group loses capacity — nine circuits bunched in trunking keep only half their tabulated rating. Note 2 to the table is the useful get-out: where the horizontal clearance between adjacent cables exceeds twice their overall diameter, no grouping factor need be applied at all. Spacing the cables out is often cheaper than upsizing them.
Is 5% voltage drop always the limit?
No. Appendix 4 Table 4Ab gives 3 per cent for lighting and 5 per cent for other uses, measured from the origin of the installation — so a sub-main and its final circuit share one budget, and the final circuit does not get 5 per cent of its own. There is also an allowance for installations longer than 100 m from the origin. And the limits are for a public supply; a private supply from a transformer or generator gets a larger allowance.
Does this replace ProDesign or Amtech?
No. It is a first-pass design aid for one distribution board — useful for checking a circuit, sanity-checking a contractor's submission or sizing a job before it goes into the modelling package. It does not model a whole distribution network, produce a discrimination study, or generate a stamped submission pack. Every value it uses is an indicative subset of BS 7671 behind a VERIFY register and must be confirmed against your own copy of the standard before issue.
Where does the reference data come from, and can I rely on it?
The current-carrying capacities, voltage-drop figures and rating factors are transcribed from BS 7671:2018+A4:2026 Appendix 4, and the maximum Zs values from Chapter 41. They are an indicative subset, not the whole standard — a limited range of cable types, installation methods and devices. Treat every figure as something to verify: the printed report carries a VERIFY register naming exactly which values need confirming against the standard and the manufacturer data for the products specified.