SWA & Armoured Cable Size Calculator

How SWA cable is sized to BS7671

Steel wire armoured cable — PVC/SWA/PVC to BS6346 or XLPE/SWA/PVC to BS5467 — is sized the same way as any other multicore cable in BS7671 Appendix 4. The armour provides mechanical protection and can serve as a circuit protective conductor; it doesn't get its own current-rating table. What matters for sizing is the conductor material, the insulation type (PVC 70°C or XLPE 90°C), and the installation method — most commonly method C (clipped direct to a wall, tray or surface) or method D1/D2 (in a buried duct, or direct buried).

The sizing sequence

1. Design current Ib — the actual load current the circuit will carry.
2. Device rating In — the MCB, MCCB or fuse protecting the circuit; In ≥ Ib.
3. Tabulated rating It — read from Appendix 4 for the insulation, configuration and installation method.
4. Correction factors — Ca (ambient temperature), Cg (grouping with other circuits), Ci (thermal insulation contact) and Cc (protective device type) are multiplied together and applied to It to get Iz, the actual current-carrying capacity.
5. Compliance check — Iz must be ≥ In, and the voltage drop over the cable run must sit within the 5% limit in BS7671 Section 525 for non-lighting circuits (3% for lighting).

Why voltage drop often decides the size, not current rating

SWA cable is frequently used for longer sub-main and outbuilding runs — garages, workshops, garden offices, EV chargers. On a 25–40m run, the cable size needed to keep voltage drop within the Section 525 limit is often larger than the size the current rating alone would require. Always check both, which is exactly what the calculator above does in one pass.

Worked example

The preset loaded in the calculator above — a 32A single-phase radial circuit, XLPE/SWA/PVC, clipped direct (method C), 40m run, 30°C ambient, no grouping, no thermal insulation — returns a minimum compliant size of 6mm², at 4.06% voltage drop against the 5% limit. 4mm² already carries the current comfortably (its tabulated rating is 42A, well over the 32A design current), so it's the voltage drop check over the 40m run that pushes the result up to 6mm², not the current rating. Shorten the run in the calculator above and watch the recommended size drop back down once voltage drop stops being the binding constraint.