Cable Size Selection

I think you're asking how to determine what size of power cable you should use.

To size a power cable you need to know:

1. The load being supplied (i.e. 50 amp, 440V 3-phase load)
2. The length of the cable (i.e. 100m)
3. The permissible voltage drop from one end of the cable to the other (i.e. 2%)
4. The short circuit current the cable is required to withstand

You select the cable size so that the following three criteria are satisfied:

1. Current carrying capacity - the cable can continuously carry the maximum demand current without overheating.
2. Voltage drop - the cable resistance (ohms/km) does not result in excessive voltage drop at full current.
3. Short circuit withstand - the cable can withstand the maximum prospective fault current until a protective device operates. (If the closes protective device could fail to operate, use the operating time of the next backup protection device.)

In Australian practice this entire procedure is explained, with examples, in the cable selection standard AS3008.1:2009, Electrical Installations - Selection of Cables - Part 1: Cables for alternating voltages up to and including 0.6/1kV.