how to calculate the power cable size for 120 meter?

Question

the load is 80 Amp. distance between the transformer to the dist board is 120 meter.

Answer 1

See http://en.wikipedia.org/wiki/American_wire_gauge. There are two factors to consider: temperature rise in the cable and voltage drop. You have to know the rating of the cable called ampacity which is based on the temperature rating of the insulator (see http://en.wikipedia.org/wiki/Electrical_cable ). As far as voltage drop, that depends on how low the voltage can be allowed to get at the distribution board, and the resistance of the cable. The resistance can be found in a wire-guage table http://en.wikipedia.org/wiki/American_wire_gauge