S = Isquared x T then hit square root and finally divided by K i think lol cant show exactly asdont know how to on the laptop its in the regs pg128
S = is the nominal cross sectional area of the conducter
I squared is the value in amperes of fault current of negligable impedence which can flow through the protective device , due account being taken fof the current limiting effect on the circuit impedences and the limiting capability I2t
T the operating time of the disconnecting device in seconds corresponds to the fault current I amperes
K is the factor taking into account of the resistivity ,temperature coefficient and heat capacity of the conductor material , and the apropriate initial and final temperatures