For three-phase:
A = (144 × 50) / (56 × 12) = 107 mm²
Incorrect cable selection is the leading cause of premature insulation failure, nuisance tripping, and energy waste. This post breaks down the engineering behind the tables: the five mandatory checks, the hidden correction factors, and the iterative reality of professional cable sizing.
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).