Ok first post on here so be gentle
. Cable calcs would be as follows;
Ib - 10000/230 = 43.5 amps so a 45 amp mcb in one of the spare ways
It - 45/1.03 = 46.35 amps made some assumptions that the ambient temperature would be 20C and the cables would be by them selves not grouped together. Using reference method c this would size the cable at 10mm.
Checking volt drop on the cable would be 4.4 x 43.5 x 3 / 1000 = 0.57v and 4.4 x 43.5 x 18 / 1000 = 3.4v these both come within the allowed 6%.
Might as well check Zs by calculation while I'm at it which would be R1+R2 per meter for 10mm is 6.44 milli ohms so 6.44 x 18 / 1000 = 0.11 ohms. Ze for tncs would be a maximum of 0.35 ohms so worst case would be 0.46 ohms. On a 45 amp mcb the max Zs would be 0.78 so again all good.
Isolators could be vertical 2 gang 50 amp double pole switches mounted out side the bathroom door.
As for making sure about not going over the max demand I can think of two ways of doing it. The simplest and cheapest way would be a subtable rated change over switch mounted some where accessible by the CU so only one shower can be used at a time. The other way that I can think of but maybe wrong (educate me) would be the use of two suitably rated contactors wired as you would for forward reverse motors (few less wires as not swapping two phases just switching the line from one contractor to another). Each contractor would then power one shower. Momentary switches could then be wired by each bathroom to turn on each shower, but not the other.
