In article ,
Ian Stirling writes:
I have 3 2.5mm^2 T+E cables going to the garage, in a duct.
They are about 30m long.

Assuming 16 ohms per Km, that's .55 ohms.
Or around 40A, on voltage drop alone, so that's not a concern.

Why 2.5mm^2?
I had planned to put in one 2.5mm^2, but I realised as I was about to
drag it through the conduit, that I had all this spare cable...

The duct is buried in the ground, and is 36mm in diameter.

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.
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?

How do you protect against overload in the case of one or two
broken neutrals?

You possibly haven't considered a break in one live conductor
near the supply end which grounds only the longer leg, which
is backfed via both the other conductors fused at 13+13A.

Conductors in parallel are never as simple as they seem.
I suggest you use them as a draw wire to pull through the
size of conductor you really need. Failing that, use them
to supply different circuits without paralleling up (which
would certainly help avoid lighting fluctuations when you
switch heavier loads on the other circuits).

--
Andrew Gabriel