The base layer most national wiring rules build on: metric cable, ρ/A physics, and a suggested limit of 4% where no national value applies (IEC 60364-5-52, Annex G). Your country's rules then tighten or restate it.
A non-exhaustive tour: the UK (BS 7671) sets 3% lighting / 5% other; Australia/NZ (AS/NZS 3000) a hard 5% total; India (IS 732:2019) harmonizes with IEC 60364, with project specs commonly adopting 3%/5%; Germany commonly applies DIN 18015's ~3% from the meter for dwellings alongside VDE 0100; Canada sits outside the IEC family entirely (CEC, AWG-based, mandatory 5%). When a project spec cites IEC 60364 with no national overlay, design to 4% — or 3% where lighting flicker or electronics matter.
The math is identical everywhere: Vd = 2 × I × L × ρ / A single-phase, √3 for three-phase, ρ ≈ 0.0225 Ω·mm²/m copper at operating temperature. Size-by-size charts live under metric cables.
It's the informative default of Annex G in 60364-5-52 — binding only where a contract or national code adopts it. National rules usually supply the enforceable number.
This one (or the generic metric calculator) — pick the limit your spec states. For UK or AU/NZ projects, use their dedicated pages so the right limit and conventions apply.