| Copper, XLPE, 90°C, 30°C ambient, free air | 1.5 mm² → 24 A | 2.5 mm² → 32 A | 4 mm² → 42 A | | Aluminum, PVC, 70°C, buried | 16 mm² → 70 A | etc. |
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. how to size a cable
Example : 230 V single-phase, L=80 m, I=20 A, cosφ=0.85, 4 mm² Cu (R=4.6 Ω/km, X=0.09). Vd = [2×80×20×(0.0046×0.85 + 0.00009×0.526)] / 1000 = 12.8 V → 5.6% > 3%. Fail. Increase to 6 mm². During a short circuit, heat is generated faster than it can dissipate (adiabatic process). The cable must survive until protection clears the fault. | Copper, XLPE, 90°C, 30°C ambient, free air | 1
$V_d = \frac2 \times L \times I \times (R \cos\phi + X \sin\phi)1000$ (L in meters, Vd in volts) Vd = [2×80×20×(0
$V_d = \frac\sqrt3 \times L \times I \times (R \cos\phi + X \sin\phi)1000$
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).
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: