wig wrote:

I'd run it from a lighting cable. What is the socket going to be used
for? and how often?

Just Say No.

Take a feed from an appropriate circuit. An appropriate circuit is one
wot's got other sockits onit. Not the shower, not the light. Either one
of those risks overloading the cable - and for those who believe that an
MCB rated at 6A will instantly blow if you pull 6.01A, 6.1A, or 7A - no,
it won't. It'll let 9-10A pass for 15-20 minutes at least. *If* you're
lucky, the lighting cable's 1.5mmsq and can dissipate heat easily, so
drawing 9A (full lighting load + 1kw fan-heater warming you in the loft)
won't matter at all. If it's wired in the more normal (and cheaper, so
bound to be used in any mass-built house) 1mmsq, and passes through good
thick thermal insulation for any substantial part of its run, it'll warm
up good and proper. No, it won't burst into flames; yes, the PVC
insulation *will* soften, and anywhere there's a bit of pressure on it
or a bend in the cable, the softened insulation *will* flow slowly away,
leaving bare conductors if you're unlucky.

How are you for black cats and paired magpies?

Similar thought tells you why putting a 13A socket on your shower cable
is a no-no. Again, the cable and MCB/fuse have been sized for the shower
you have (or for the shower that was initially installed; possibly
replaced later by a higher-current model). If the shower's a dinky
little 7.5kW job (draws 31A), and it's wired in 10mmsq because that's
what the sparks had on the van that day, and it takes a thermally-kind
route - no practical problem. (Still a Regs violation, of course.) If
the shower's a 9.6kW job (40A), run on a 6mmsq cable which was OK for
the previous one which a handyperson's replaced with the new whizzy
model, running through thermal insulation so it's already running at or
over the sustained-temp limit of 70 degrees, with a 40A or 45A MCB
fitted by the same handydroid to Uprate the shower, and you fit a 13A
socket on it, wh'appen when it's *really* cold oop in t'loft and you
wind the fanheater up to 3kW, while someone else takes a shower? The
current's now up to 52A. Your 40A MCB doesn't mind, it can pass that
current for hours; your cable's insulation, on the other hand, is now
enjoying new heights of devil-may-care flexibility...

Y'see, the Regs is written to reduce - not eliminate, reduce - the need
for detailed analysis, and so that almost all foreseeable installations
which conform with them are safe in practice, in normal use *and* under
fault conditions. Going beyond the envelope of the Regs puts you in
territory where you *may* still be lucky, or you *may* not.

Again, how many rabbit's feet do you have to hand?

There's plenty of ways of arranging a 'temporary' safe-enough feed up to
the loft, which others have already outlined - if it's genuinely
occaisional use, a flex running up a corner of some room or up a
built-in wardrobe or airing cupboard up to a 4-way extension block which
you plug in to a handy-enough socket is safer than fannying about with
putting 13A sockets on lighting or (shudder) shower circuits. Doing a
surface run of cable as a spur - again, taking some
aesthetically-acceptable route - with a good length of slack up in
t'loft ready for when you next redecorate and can sink the cable
properly into the wall, or run it through the studwork wall if that's
how your place is built, is a fully Regs-conformant solution. In short,
there's enough ways of doing the job properly which don't require wall
butchery at an inconvenient time that there's no excuse for bodgery...

Stefek