Calculating Wire/Cable Size formula for Three Phase Circuits. Wire Circular mils = √3 x 2 x ρ x I x L / (% Allowable Voltage drop of source voltage ) Note: the Value of ρ = Specific resistance or resistivity of Conductor isCustom here for copper and aluminum is 11.2 and 17.4 respectively at 53° C (127° F) Also check the “Good to know.