Thanks Goose for the insight for me
Anyway let get to the topic right now related to the showerhead unit.,, if i recall the UK reguation required the showerhead instat heater unit it must be wired on seperated RCD circuit [ it can not tie into anything else ]
and for wire size you need at least 6mm2 X2 plus earth conductor flat cable.
how many KW that unit have i know most useally are about 3 to 6 KW range [ you may have to check with the latest UK reguation it may change a bit due i am more famiuir with both NEC and French codes.]
you will need either 32 or 48A [ if have one ] RCD breaker [ that area i will recomned to get a UK electrician to propely hook up because the customer unit hook up is somehow different than what we used to see it here on this side of " pond " ]