WARNING I am still developing these calcs check your answer!

(Iz) = current carryng capacity of the cable where it is installed. (It) = tabulated current for a single circuit at ambient temp of 30°
(Ib) = the actual current to be carried. (In) = is the Circuit Breaker rating in Amps.(I2) = the current at which the CB opens.
(Ca) = correction factor for Ambient temp. (Cg) = correction factor for grouping. (Ci) = correction factor for thermal insulation.
One method to determine the Potential short circuit current if you do not have a Loop Impedance Tester.
Find the volt drop of the system V1-V2 Divided by the Load gives Zs. Where V1 is the open circuit voltage,
V2 is the voltage under load and load is the amperage of the system.
IE: (V1)240 - (V2)238 = 2 divided by the Load 40Amps = 0.05 Ω.
Then PSC = System voltage (Uo) / (Zs) 240 ÷ 0.05 = 4800 Amp.
Enter different values in the boxes below and click on Send, you can use the Tab button to move between boxes.

V1  V2  Load in Amps  Calculate 
Cable Selection, Copper Only, up to 25mm², above this Cable Reactance must be considered.
Enter your proposed cable size,type of insulation,Voltage,ambient temp (30 is norm) your cable length and circuit load in Amps.

Cable size
Abient Temp
Cable length in Mtrs
Design Amperage
Circuit Breaker
Pvc covered
25 mm²