In most circumstances a 6mm cable would be suitable for a 7Kw, but I would recommend having a 9.5Kw shower fitted as 7Kw will struggle to give decent flow at comfortable temperature especially in winter. Anything above 9.5Kw will need a 10mm cable. The cable can be installed by any competent person, but the electrical connections at isolator/fuse board and RCD protection have to be made by an electrician, tested and a certificate issued.
I disagree with Ian, a 6mm twin and earth cable has a current carrying capacity of 47amps, which is in excess of the max power rating of a 9.5Kw shower. I always specify a 10mm cable to for new shower installs but it is untrue to say that 6mm is always inadequate for a 9.5KW shower. Ross, HNC electrical, mechanical & hydraulic engineering.
I would disagree with the previous comments you will definitely need 10mm cable for a 9.5kw shower and 6mm will only run around 7_7.5kw, you are at risk of the cable over heating and staring a fire, my advice get a electrician in to check all your installation
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