The UK approach: metric cable (mm²), runs in meters, and drop expressed as millivolts per amp per meter from Appendix 4 — with limits of 3% for lighting and 5% for other circuits that are verified at certification, not merely recommended.
Drop = (mV/A/m) × I × L ÷ 1000. The tabulated value already includes the round trip — for two-core copper it equals 2ρ/A, which is exactly how this calculator computes it (ρ ≈ 0.0225 Ω·mm²/m at operating temperature). Three-phase tabulated values are line figures using √3.
| Cable | mV/A/m (1Ø) | Rating A (Method C) |
|---|---|---|
| 1.5 mm² | 30.0 | 20 |
| 2.5 mm² | 18.0 | 27 |
| 4 mm² | 11.2 | 37 |
| 6 mm² | 7.5 | 47 |
| 10 mm² | 4.5 | 64 |
| 16 mm² | 2.8 | 85 |
| 25 mm² | 1.8 | 112 |
| 35 mm² | 1.3 | 138 |
Limits come from Appendix 4 Table 4Ab: 3% lighting / 5% other from the origin for public LV supplies, +2 points for private supplies, and a 0.005%/m allowance beyond 100 m (capped +0.5%). Unlike the NEC's notes, exceeding these fails design verification. Deeper dive on the UK calculator page; size-by-size charts under metric cables.
BS 7671 Appendix 4, tables 4D1B–4J4B by cable construction and installation method. This calculator reproduces the copper conductor-resistance component, matching the tables within rounding for thermoplastic cables.
BS 7671 budgets the whole installation from the supply origin (the cutout) to the load point — so distribution-board submains consume part of the 3%/5% before final circuits begin.