Additional Consumer unit to feed two showers

[angusmac]

I am at installing a second shower. The shower fitted at the moment is a 9.5 kw and the one I want to add in an additional shower is a 8.5kw. At present there is a 10mm cable which runs from the main consumer unit to a 45amp fuse box thereafter a 6mm cable which runs to the shower unit. I want to replace the 45amp fuse box with a 63am 30ma RCD unit to run the two showers from. The question is will I have to replace the 10mm cable which runs from the main consumer unit with a higher rating and if so what rating? or is this not viable.
Hope this makes sense.

 

[breezer]

the size and cost (not sure if you can get bigger than 10mm twin & earth) would be very expensive, cheaper to run a new 10mm cable from your consumers unit and put it on its own mcb (if you have one spare) if not get the consumers unit changed