Voltage Drop & SWA Cable Size Calculator
Cable sizing guidance using BS 7671-style checks and manufacturer product data.
The calculator checks Ib, In, corrected current-carrying capacity, and voltage drop using the mV/A/m method. It is scoped to clipped-direct / in-air reference data and does not cover buried cable conditions, thermal insulation, harmonics, special locations, manufacturer terminal limits, or fault-loop compliance.
BS 7671:2018+A4:2026 is now published. During the transition from A3:2024, verify final values against the edition being applied to the job and the cable manufacturer datasheet.
Voltage drop is calculated as mV/A/m x design current x route length / 1000, then compared with the selected percentage of the supply voltage. Current-carrying capacity is checked against the protective device rating after the selected ambient and grouping factors are applied.
Protective Device
The result uses Ib <= In <= corrected Iz, so a low load on a larger breaker cannot accidentally recommend an undersized cable.
Adiabatic Check
The optional adiabatic calculation compares the required copper CPC size against the selected or integral CPC only. SWA armour as CPC needs a separate manufacturer-backed check.
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