SWA Cable Calculator – Armoured Cable Current Rating, Size & Voltage Drop

This comprehensive SWA cable calculator performs all three checks required for correct cable selection: current-carrying capacity (with derating), voltage drop (per km from cable data), and short-circuit withstand (adiabatic equation). All three must pass — the most onerous determines the final cable size. Per NEC (PVC-insulated) and NEC (XLPE-insulated) armoured cables.

NEC Part 2 (XLPE-insulated steel wire armoured cables) is the preferred specification for new LT power distribution cables in the US. XLPE cables carry approximately 25–30% more current than equivalent PVC cables. Short-circuit rating check is critical for cables near the transformer (high fault levels). All three checks must pass; typically voltage drop governs for long runs, current rating for short runs near the transformer.

A 3-phase feeder carries a design current of 180 A over a 120 m run, with a fault current of 15 kA cleared in 0.3 seconds. Check 1: with derating factors (0.9 depth × 0.85 grouping) a 95 mm² XLPE/SWA cable rated 210 A gives an effective capacity of 210 × 0.9 × 0.85 = 161 A, which is insufficient, so 120 mm² (240 A rated, 184 A effective) is chosen. Check 2: at r = 0.196 Ω/km, voltage drop = 180 × 0.196 × 120 × √3/1000 ≈ 7.3 V (1.8%), well within the 5% limit. Check 3: required copper/XLPE area = (15,000 × √0.3)/143 ≈ 57 mm², comfortably less than 120 mm². The 120 mm² cable passes all three checks.