how we can calculate cable size for any load?

Cable size calculation for carrying load current has to satisfy 3 conditions:

a)      Ampacity

b)      Short circuit withstanding capability

c)       Voltage drop

1. Ampacity: Current carrying capacity after considering conditions like temp, grounding, spacing etc..
2. Short circuit with standing capability : When fault is occurred at load, before the Circuit breaker trips the Cable has to withstand the fault current (25 KA,40KA etc..)
3. Voltage: You can not lay the line for km as it is satisfied above two conditions, because the cable will have R and X, which will have drop of voltage,so IR drop needs to consider.
   This drop should be 5% during running of load and 15% during starting of load.

If any condition is not satisfying, go to higher size than you selected.