Power Cable: Reading the Construction Layers and the Four Sizing Calculations
A power cable looks like one object and behaves like four. Each layer answers a different engineering question, and each is verified by a different calculation. Treating selection as a single ampacity lookup is where most installation failures originate. Conductor. Copper or aluminium, stranded to IEC 60228 classes. Copper reaches 100 % IACS conductivity; aluminium about 61 %, so an aluminium conductor needs roughly 1.6 times the copper cross-section for equal resistance — but weighs roughly half as much for equal resistance per metre, which is why transmission lines are aluminium and riser cables often are not. Insulation. Thermoplastic PVC limits the continuous conductor temperature to 70 °C; thermoset XLPE or EPR raises it to 90 °C and survives 250 °C for the five seconds a short circuit may persist. That 20-degree gap is real cable, not marketing: the same 10 mm² core carries meaningfully more current in XLPE once installation correction factors are applied. Screen and armour. Metallic screens confine the electric field and carry fault current in medium-voltage designs. Steel wire or tape armour protects against direct mechanical impact in buried or industrial runs, and its corrosion protection — galvanised, or polyethylene over a sacrificial layer — determines burial life more than any electrical rating. Sheath. Polyethylene for underground moisture resistance, low-smoke halogen-free compounds where people evacuate, and UV-stabilised jackets for exposed tray. Four calculations then size the cable: current capacity from IEC 60287 corrected for grouping, ambient temperature and soil thermal resistivity; voltage drop, kept within 3 % for lighting and 5 % for other uses per IEC 60364-5-52 and BS 7671; short-circuit withstand via the adiabatic equation k²S² ≥ I²t; and thermal stability of the screen and armour separately. Pass all four, and a power cable quietly outlives the equipment it feeds. Miss one, and the layer that fails is always the one nobody checked.
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