How three phase system reduces the copper loss

Consider a 5KW load.If a single phase system it then we require 2 wires,one carrying the current towards the load and other carrying current away from the load. Assume that the applied voltage is 100V  then the current is 30A.if we see the copper losses in it(if resistance of each wire is 1 ohm) the Total copper loss is 1800 watts (900+900).
Now consider a three phase balanced supply.It has three wires each carrying 10 amps current then calculate the losses which are less when compared to that of single phase.