Calculation — For Cable Size

Below is a comprehensive guide and the formulas used in electrical engineering.

For 3-phase power (400V/415V), the formula changes slightly because there is no "return" wire in the same way; the voltage vectors are 120 degrees apart. The factor "2" changes to $\sqrt3$ (approx 1.732). calculation for cable size

For a 15 Amp load, you check the standard tables: Below is a comprehensive guide and the formulas

$$I_cable \ge I_load$$