FAQ

16. Cable size calculation
enerally speaking, cable size selection is based on the three factors: Current carrying capacity, Voltage drop and Short circuit rating
The current carrying rating is determined by the conductor size, the installation method and thermal heating of the cable.
Voltage drop mainly depends on the length of cable being installed. The longer the cable install, the bigger the voltage drop occurs.
Short circuit ratings are based on the maximum current withstand capability of the cable in a short circuit condition. The cable should be capable of withstanding this current without thermal damage until the fault condition can be switched to safety through a device like a circuit breaker of fuse.
Short-circuit rating is the maximum short- circuit current the cable can safely withstand when protected by a specific overcurrent protective device or for a specified time. 
The specific data for the above factors can be find in BS 7671.