S=(√I²t)/k
S is the csa of the earthing conductor/cpc that you need to find
I is the prospective fault current at that point in in the circuit (normally PSC for the installation)
t is the disconnection time for the protective device you are using at the fault current I, if this is a new installation and the device is a BSEN60898 MCB and the fault current is higher than the "instantaneous" trip time shown in appendix three of the BGB (5x the MCB rating for type B MCBs) then this value is 0.1s (though the actual trip time may be quicker)
k is the correction factor for the material of the conductor, since in domestic situations the conductor is invariably copper then you only need to consider if the conductor is enclose din a cable or not. If it is in a cable then the value of k is 115. If it is not in cable then the value of K is 143.
The csa of the earthing conductor in a TNCS system should be selected from table 54.8 as it is also a bonding conductor, the csa of a cpc that is not incorporated in cable and not mechanically protected must be at least 4mm².
The value S that is obtained is increased to the next available size of conductor, so if you get a value of 3.4 then the conductor would need to be 4mm² minimum.