What will be the cable size for 300 A if the 3 phase voltage is 220 volts?

At least 50mm but with all the de-rating factors it might be 70mm that's if its 2 cable single phase non armoured clipped direct. Check out the tables dopey!!!!

FYI 50 mm = 2 inches, 70 mm = 2.75 inches these are not wire sizes, sound more like cable outside diameters.

Correct answer

The wire size is dependant on the insulation factor on the wire. A few common insulations and corresponding wire sizes for 150 amps are - Size 0 R90 good for 155 amps, also with insulations of RW 75, TWH, TW75, good for 150 amps. Size 000 TW good for 165 amps.