The most often used cable within domestic installations is the 70 deg thermoplastic PVC twin and earth which has a CPC built into it whose size has already been calculated using the formula method from regulation 543.1.3 and the normal sizes of the CPC in each cable are…
1.5mm2 T&E cable has a 1.0mm2 CPC
2.5mm2 T&E cable has a 1.5mm2 CPC
4.0mm2 T&E cable has a 1.5mm2 CPC
6.0mm2 T&E cable has a 2.5mm2 CPC
10.0mm2 T&E cable has a 4.0mm2 CPC
16.0 mm2 T&E cable has 6.0mm2 CPC
It is worth noting that although the CPC sizes are correct in most cases in certain instances a larger CPC may be required than the one provided in the cable. The most common example of this is when using a 16.0mm2 T&E to feed a remote distribution board, the 6mm2 earth conductor provided may need to be larger.
Also a little tip worth remembering is that the largest single strand conductor used is the 2.5mm2. This is useful when trying to tell the difference between a 6mm2 T&E and a 10mm2 T&E. If the CPC has just one single strand it’s a 6.00mm2 T&E but if the CPC is made of multiple strands it’s more than likely to be a 10.0mm2 T&E. However, in older red and black T&E cables multiple strand CPC conductors were common.
Hope you find this useful Dave