i won't fit a 10.5 kw shower on nothing less than 10mm sometimes 16mm.

Hi Steve,

Its not simple and straight forward - there are factors to take into account including the length of the cable run.
Are you asking because you this something you are thinking of upgrading to a 10.5 Kw shower and want to use a circuit that is already there , or possibly you are thinking of installing a whole new circuit? Be aware if you are and you are looking at doing the work yourself then what you are about to do could well be a notifyable activity to your Building Control Authority ( unless you are suitably accredited / Part P registered and can self certify - but then I suspect that you would not be asking this question if you were). Arguably if you are replacing a fixed appliance to an existing circut like for like this may not warrant BC notification but a shower of an increased rating surely will.
I would be interested to hear other posters understanding of DIY ( and non part P reg) shower fit / refit activity and to what extent is is allowed under regs .