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.

Welcome to cable calculator pro.

Within this site you will be able to check your designs against our simple cable calculator and for the professional electrician / engineer you may purchase our cable calculator pro. this stand alone program will be able to design a circuit from start to finish meeting all the criteria of the IET and BS 7671 the 17th edition.

Cable calculator free

It is envisaged that the cable calculator pro software package would pay for its self the first time it is used. To access any of the free cable calculators or tutorials you will have to create an account (free). Once you have an account you may log in and access most of the site,parts of the site will be for subscription only. The calculator will only run on windows for the time being, it will run on XP, Vista (yuk) and Win 7. You will require dot net downloaded onto your computer, as 95% of windows computers have this already it should not present a problem however for those that do not the latest version can be found here dot-net.I hope to release cable calculator pro in the first quarter of 2010.