Dear DIYers,

I have recently bought my first house and I am wiring an electric
shower (9.5kw) into the bathroom. The house has an en-suite bathroom
in an extension next to the main bathroom and I am going to take the
existing electric shower circuit from the en-suite and re-use it in
the main bathroom. The existing circuit is made up of 10mm twin &
earth cable, a 45amp double-pole switch and a 45amp MCB at the
consumer unit, the consumer unit also has a RCD device built into it.

I understand that I can only have one shower unit using the circuit
and that is not a problem since I have removed the electric shower
from the en-suite and chucked it and I intend to put a mixer shower in
there later on.

The problem is that the cable (twin & earth) which used to go to the
double pole switch in the old bathroom does not reach the position
where the new double pole switch is going to be, so I was wondering
how I can safely connect some new cable to the old cable in the loft
above the bathroom bearing in mind that it is going to be pulling
approx 40 amps and if connected incorrectly would get pretty hot. I
had thought of using the old double pole switch positioned in the
attic and left on which seems safe because I will have a second double
pole switch between that and the shower. I do not want to run a new
cable back to the consumer unit since I already have a perfectly good
one which comes 9/10s of the way!

Any advice will be very much appreciated, thanks in advance,

Ross.