Martin Angove wrote:

If the shower has been there some time, chances are that it will be
wired in 6mm2 T&E (this stuff is about half an inch wide altogether)
[...]

[...] one you can afford so you should also be factoring in the cost of
running a new length of 10mm2 cable (it's about an inch wide:

Your suggestion that 10 mm^2 cable is about twice the width of 6 mm^2
didn't quite ring true. According to the Pirelli data sheet 6 mm^2 T&E
is approx. 13.1mm wide and 10 mm^2 is approx. 16.8 mm wide - so "just
over half an inch" and "getting on for eleven-sixteenths of an inch" is
nearer the mark.

While you're at it, you need to think about the shower disconnection
times and whether or not an RCD is required for it. This will largely
depend on the type of earthing your system has and the length of cable
from CU to shower.

And on the type of overcurrent protective device. For a 45A shower
circuit the maximum voltage-drop limited circuit length is 49 m. With a
PME supply there is no need for an RCD with any device type (other than
Type C or old Type 3 MCBs on circuits longer than 23 m - but these
device types aren't appropriate for a shower in any case).

For a TN-S earthed supply:

- with a BS 1361 cartridge fuse an RCD would be required for a circuit
longer then 10 m;

- with a re-wireable fuse (or old Type 1 MCB) the full 49 m is OK
without an RCD;

- for a Type B MCB the length limit without RCD is 35 m;

- other MCB types will require the RCD, regardless of length.

In all cases running a separate parallel 10mm^2 circuit protective
conductor to the shower (taking the same route as the 10 mm^2 T&E) could
also be considered.

Don't forget to make sure your supplementary bonding
is up to scratch too.

Absolootly :-)

--
Andy