Andy Wade wrote:
Ian Stirling wrote:

The easy approach is to put a 40A fuse at the close end, and parallel
all the wires.
Of course, this isn't very safe, as there are unfortunately joins in the
cable.
snip
one of them [see Reg. 473-02-05]. You'd need to check carefully that
the 1.5 mm^2 CPCs are adequately protected by the 40 A fuse.

I expect not, though I do need to run the numbers.

My current plan is to common all the L/N/E, put a 13A fuse on each
live, and at the garage end, have a RCD CU, with a seperate earth spike.

Anything obvious that I've missed?

Hmm, well, the use of multiple devices for overload & fault protection
of parallel conductors is permitted [see 473-01-06, et. seq. and
473-02-05] but usually this would only be done in a high current
situation where there'd be monitoring to draw attention to the opening
of any one device. With what you propose, one fuse could fail, reducing
your affective cable rating without you being aware of it. So if you
must do it I'd suggest using a 3-pole (linked) 16 A MCB, so if one leg
trips, all cables are disconnected.

Is this an issue?

In the case of a fuse failing, I don't care that the effective cable
rating is lower.

I care that the remaining active cables do not fail due to overload in
the time the other fuses take to trip.

With one fuse protecting each cable,

Quite frankly though, I think the best advice is not to be so
penny-pinching - go out and buy some 10 mm^2 cable and do the job in a
more conventional manner!

Sigh.
Well, yes.
Unfortunately, I have filled the conduit with other wiring (all
isolated, so if it goes live there is no safety issue), and it's going
nowhere.
Fixing it 'properly' would involve ripping the lawn up, along a ~10m
track, which is really, really annoying to do, and get level again.

Maybe in a few years I'll do this, and put a 100mm conduit in, as I have
for the bit going under the house wall.

Thanks.