Metric cable sizes on 230 V single-phase and 400 V three-phase, checked against the BS 7671 Appendix 4 limits: 3% for lighting, 5% for other uses — including the automatic long-run allowance beyond 100 m.
For an installation supplied from a public LV network, BS 7671 Appendix 4 (Table 4Ab) limits voltage drop to 3% for lighting (6.9 V on 230 V) and 5% for other uses (11.5 V) between the origin and any load point. Private supplies get an extra 2-point allowance, and runs over 100 m may add 0.005%/m (capped at 0.5%). Unlike the NEC's informational notes, these are requirements verified at design and inspection.
UK design tables express cable drop as millivolts per amp per meter. This tool computes the same physics from conductor area (ρ ≈ 0.0225 Ω·mm²/m copper at operating temperature) — for 2.5 mm² that yields 18 mV/A/m, matching Table 4D2B. Three-phase values are line figures using √3.
| I | load current, amps |
| L | one-way length, meters |
| ρ | resistivity ≈ 0.0225 Ω·mm²/m copper at operating temp |
| A | conductor cross-section, mm² |
Equivalent to the tabulated mV/A/m method: the two-core table value is exactly 2ρ/A × 1000. √3 replaces 2 for three-phase line values.
BS 7671 is the standard against which installations are certified — exceeding the Appendix 4 limits without justification fails design verification. In practice, yes: treat 3%/5% as binding.
For installations longer than 100 m, the limit may be increased by 0.005% per meter beyond 100 m, up to an extra 0.5% — relevant for farms, long drives and outbuildings.