Hi i have been asked to install a new shower supply cable for a 8.5kw shower. The cable run is approx 15 metres long with about 3 metres through a cupboard with a boiler in it and 8 metres through a loft. The cable will be clipped directly to joists by itself and avoids all insulation or going through walls. taking into account all factors will a 10mm cable be sufficient? Also the consumer unit is an old(ish) one with an rcd covering just the sockets. Will a 40 amp mcb on the rcd side of the c.u be ok?