Christian McArdle wrote:
Or you might discover that the other wire supplies your rarely-used
immersion heater, in stark contravention of the Regs (an imm. heater
wants its *own* final circuit, not sharing with owt else, (...)

Well, within reason. I've shared the immersion circuit with the central
heating and boiler circuit. This is for two reasons. Firstly, it takes one
less way on the consumer unit. Secondly, it means I can combine the controls
for both without worrying about having two points of isolation. I believe
this is allowed because I've designed with 100% of both expected loads (13A
for immersion, 3A for CH into a B16A MCB) and not applied any diversity.

It's the way things were (and are) wired at our house as well, a
dedicated 16 amp MCB feeds the immersion heater and the CH controls.
(The CH controls are on a FCU with a 3 amp fuse actually). I didn't
do it like this but it seems to me a very reasonable and sensible
arrangement so when I moved some of the controls to make them more
accessible I left the circuit arrangement as it was.

--
Chris Green )