Not this again - see the overheated shower cable thread.. There are more than one size of CPC fitted. It is up to the designer to select the right one, depending on application.
The 'normal' read cheapest is indeed 10mm +4mm, but both 10mm+ 6mm and 10mm+ 10mm flavours are available, and may be needed to meet Ze on longer runs.
If in doubt assume 4mm.
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