Cable Sizing to BS 7671: Spreadsheets vs ProDesign

Every electrical designer sizes cables, and plenty of jobs still get done on spreadsheets or manufacturer tables. This guide walks through what a compliant BS 7671 cable calculation actually has to prove, where spreadsheet approaches typically fall down, and what network software like Trimble ProDesign (which many contractors still know by its former name, Amtech ProDesign) automates.

The Five Checks in a Compliant Cable Calculation

For every circuit, BS 7671:2018+A2:2022 effectively requires five things to be demonstrated:

• Coordination of design current and protection — the classic I_b ≤ I_n ≤ I_z relationship between load, device rating and cable capacity.
• Current-carrying capacity — the tabulated cable rating corrected for installation method, grouping with other circuits, ambient temperature and thermal insulation.
• Voltage drop — not just per circuit, but cumulative from the origin of the installation to the final load, against the limits in Appendix 4.
• Earth fault loop impedance — Z_s low enough for the protective device to disconnect within the required time (shock protection).
• Thermal withstand (adiabatic) check — the conductor and CPC must survive the fault energy let through before the device clears (S²k² ≥ I²t).

Where Spreadsheets Go Wrong

None of those checks is hard in isolation — which is exactly why spreadsheets feel adequate. The problems are systemic, not arithmetic:

Cumulative effects. Voltage drop and fault levels are network properties. The voltage drop at a final circuit depends on every cable upstream; the fault level at a distribution board depends on the supply and everything between. Spreadsheets handle circuits one at a time, so the network-level picture is stitched together by hand — or missed.

Change doesn't propagate. When the submain gets longer at construction stage, every downstream circuit's voltage drop and loop impedance change. In a spreadsheet, someone has to remember that and re-check every affected row. Most design errors we're asked to fix trace back to exactly this.

Discrimination is invisible. A spreadsheet can verify one device against one cable, but it can't show you whether the upstream and downstream devices discriminate — you need the actual time–current curves overlaid, with the fault levels calculated at each point.

What ProDesign Automates

ProDesign models the whole distribution network — supply characteristics, transformers, submains, boards and final circuits. The regulation tables, correction factors and manufacturer device data (with real time–current curves) are built in. Change a cable length, a board location or the supply fault level, and every affected circuit is re-validated automatically against BS 7671. Discrimination studies come from the same model, and the output is a calculation pack that an approving engineer can audit: inputs, methods, results, per circuit.

To be fair to spreadsheets: for a quick single-circuit check or an early feasibility estimate, they're fine. The gap opens up on real distribution networks — multiple boards, mixed installation methods, design change during construction — which is most real projects.

What a Submission-Ready Pack Contains

Whether produced in-house or outsourced, a calculation pack that gets accepted first time typically includes: the modelled single-line diagram, the design basis (supply data, demand assumptions, diversity), per-circuit cable sizing results covering all five checks, fault level results at each board, discrimination curves for the protective device chain, and a results summary an approving authority can review without re-running the software.

Don't have a ProDesign licence or a spare designer? We produce BS 7671-compliant calculation packs from your drawings and load schedules — see our ProDesign cable calculation service.