The correct size is the smallest cross-section that satisfies checks simultaneously – not the largest, and certainly not the first guess from a table.
The condition:
[ V_d = 2 \times L \times I \times (R_ac \cos\phi + X \sin\phi) ] cable selection calculation
For three-phase:
Yes – a 10kA fault requires a cable just to survive 0.4 seconds. In practice, faster protection (e.g., current-limiting fuses with 0.01s clearing) reduces this dramatically. Earth Fault Loop Impedance: Ensuring the Fuse Blows This is the most commonly skipped calculation, and it's a killer. The cable's earth conductor (or armour, or separate PE) must have low enough impedance that a phase-to-earth fault draws enough current to trip the protective device within the required time (0.4s for final circuits per IEC 60364). The correct size is the smallest cross-section that