Voltage Drop Calculator
BS 7671 Appendix 4 voltage-drop calculation from first principles:VD = (mV/A/m × Ib × L) / 1000. You supply your own mV/A/m value from the standard - no BS 7671 tables are republished here.
Results
Worked example
230 V single-phase, 2.5 mm² cable, mV/A/m = 18, Ib = 20 A, L = 15 m, lighting circuit.
VD = (18 × 20 × 15) / 1000 = 5.4 V
5.4 / 230 = 2.35 % - passes the 3 % limit (6.9 V).
⚠️ Educational design aid. Verify all results against the current BS 7671 edition. Not a compliance tool - does not replace professional engineering judgement or BS 7671 table lookups.
Table columns: BS 7671 Appendix 4 voltage drop tables (e.g. Table 4A2) have separate columns for single-phase and three-phase mV/A/m values. Ensure you enter the value from the correct column for your supply type. The three-phase column already includes the √3 factor in the tabulated value.
Regulation reference: Per BS 7671 Appendix 4, Table 4A2 for voltage-drop values per metre. The 3 % / 5 % limits are from BS 7671 Table 4A2 (or 4B1 for the older layout).