What should be the size of a wire for 3 phase connection?

The 3 phase wire size can be calculated by using formula Wire Circular mils =√3 x 2 x ρ x I x L / (%Allowable Voltage drop of source voltage) Where; ρ = Specific resistance or resistivity of Conductor D = Distance in Feet (One way) i.e. ½ the total circuit length I = Load Current Note: the Value of ρ = Specific resistance or resistivity of Conductor is used here for copper and aluminum is 11.2 and 17.4 respectively at 53° C (127° F)

more than 150mm if load 300 A

thans a lot  and yes load will be above4 kwh

it depends on the load it will be carrying.