SWA Cable Sizing Calculator

Steel Wire Armoured (SWA) cables are used for underground, direct-buried, and exposed outdoor installations where mechanical protection is required. Sizing SWA cables involves checking both current-carrying capacity (with soil thermal resistivity and grouping derating) and voltage drop over the cable route. This calculator follows NEC (for PVC-insulated armoured cables) and NEC (for XLPE armoured cables).

SWA cables are standard for LT power distribution underground in US buildings and infrastructure. NEC Part 1 (PVC insulated) and NEC Part 2 (XLPE insulated) cover armoured cables. XLPE insulated cables (NEC) can carry more current than PVC cables of the same size due to higher permissible conductor temperature (90°C vs 70°C). All buried cables must be at minimum 600 mm depth per NEC; major roads require 900 mm depth with protective slabs.

Example: A 4-core XLPE/SWA cable feeds a sub-station load of 150 A at 415 V, direct-buried at 0.8 m depth in soil with resistivity 1.5 K·m/W, run 120 m to the panel. Table ampacity for the chosen 70 mm² cable is 178 A; applying k_soil = 0.95 and k_depth = 0.98 gives derated capacity = 178 × 0.95 × 0.98 ≈ 165.7 A, above the 150 A demand. Checking voltage drop: ΔV = 150 × (0.52×0.85 + 0.09×0.53) × 120/1000 ≈ 8.6 V, about 2.1% of 415 V - well inside the usual 5% limit, so the 70 mm² XLPE/SWA cable is confirmed.