hi my name is Gerry and i am putting together the parts required to fit my new shower which is a Triton 8.5 kw electric shower my consumer box is a MEM 2000 AD and i am not sure which RCD to purchase my friend is a sparky and is on holiday in Spain at the moment and i would like to have everything ready for him fit when he returns as he is doing me a favor, could somebody please advise me on the correct one to purchase, and also if possible as i am interested the formula used to work this out, regards Gerry..