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$$ A = \frac{2 \times \rho \times L \times I}{V_{drop_allowed}} $$
However, the rated ampacity ($I_{rated}$) of a cable depends on several correction factors. The practical formula for selecting a cable based on heating is: formula for cable size calculation
$$ V_d = \frac{2 \times L \times I \times (R \cos\phi + X \sin\phi)}{1000} $$ $$ A = \frac{2 \times \rho \times L