Supply sets the BS 7671 Appendix 4 volt-drop limits for twin & earth circuits: 3% lighting and 5% other uses from the public network, or 6% and 8% where the installation has its own private supply (e.g. its own transformer). The max voltage drop is a job-wide cap on top of that (5% is a common general guide - some circuits need tighter limits). Length allowance covers routing slack, drops and terminations, added on top of each circuit's straight-line length when totalling cable to order.