$V_d = \frac2 \times L \times I \times (R \cos\phi + X \sin\phi)1000$ (L in meters, Vd in volts)
If calculated Smin > ampacity size, the cable must be upsized for fault survival. For TN systems, a fault between phase and earth must draw enough current to trip the OCPD quickly. The maximum cable length is limited by: how to size a cable
$V_d = \frac\sqrt3 \times L \times I \times (R \cos\phi + X \sin\phi)1000$ $V_d = \frac2 \times L \times I \times
Example : 2.5 mm² PVC copper (30 A tabulated), ambient 45°C (k₁=0.79), 4 circuits (k₂=0.65) → effective = 30×0.79×0.65 = 15.4 A. For a 16 A load, this cable fails. Increase to 4 mm². Voltage drop reduces torque in motors, causes flicker in lights, and wastes energy. For a 16 A load, this cable fails
Example : Isc = 3 kA, t = 0.1 s (breaker trip), Cu/XLPE, k=143. $S_min = \sqrt(3000^2 × 0.1) / 143 = \sqrt900,000 / 143 = 948 / 143 = 6.6 mm²$. Minimum = 6.6 → choose 10 mm² (next standard size).
Cable sizing is not merely about matching a conductor to a load current. It is a multi-variable optimization problem that ensures safety, reliability, efficiency, and longevity of an electrical installation. An undersized cable causes overheating, voltage drops, energy losses, and fire hazards. An oversized cable wastes material, increases installation costs, and may create termination difficulties.