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?

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

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.

The presence of joints shouldn't affect your thinking here - after all,
all joints must be sound. However there may be a different problem: in
a situation like this you must make sure that each cables in thermally
protected in the event of an phase-neutral or phase-earth fault on any
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.

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.

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!

--
Andy

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.

Andrew Gabriel andrew@a17 wrote:
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.

snip
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?

Oh dear.
True.

The obvious solution is to then fuse each neutral too, at both ends.
But this leads to unexpected behaviour in case of faults, as it can
still be live, though being 'dead'.

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.

Again true.
Oh well. Something to consider for the next rewire - I'm wanting to put
solar panels on the garage, so want lagged pipes in conduit, and I can
put the bigger cable in then.

For the moment, I think I'm paralleling them all, and sticking on a 16A
MCB - with the joins to the 2.5mm outside, where them going on fire
would at most destroy the buried conduit.

I don't need that much power at the moment - the main reason for putting
them all in initially was to reduce voltage drop - to be able to run a
3Kw kiln up there, and to hit it operating temperature spec, rather than
sort of wandering up there.

Thanks.

Ian Stirling wrote:
Andy Wade wrote:
Ian Stirling wrote:

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?

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.

Why not run the kiln on one 2.5 and everything else off the other? You
could even hack up a split CU so its running 2 separate circuits.


NT

wrote:
Ian Stirling wrote:
Andy Wade wrote:
Ian Stirling wrote:
snip
Why not run the kiln on one 2.5 and everything else off the other? You
could even hack up a split CU so its running 2 separate circuits.

Because on one 2.5 - assuming I've got the sums right, it loses ~7.5% of
its heating ability, and it is a bit aneamic anyway.

Ian Stirling wrote:
wrote:
Ian Stirling wrote:
Andy Wade wrote:
Ian Stirling wrote:

Why not run the kiln on one 2.5 and everything else off the other? You
could even hack up a split CU so its running 2 separate circuits.

Because on one 2.5 - assuming I've got the sums right, it loses ~7.5% of
its heating ability, and it is a bit aneamic anyway.

ok, then 2 cables in parallel for the kiln alone would lose 2% heat
output, and since youve only got the kiln on that pair it can be mcb'ed
at 16A. Other TnE for everything else. Now there is no danger if one
cable goes oc or they dont share well.

Another option is a small boost transformer (a few volts 13A) to add
your lost couple of volts back. Fully compliant.


NT

Ian Stirling wrote:

For the moment, I think I'm paralleling them all, and sticking on a 16A
MCB - with the joins to the 2.5mm outside, where them going on fire
would at most destroy the buried conduit.

I don't need that much power at the moment - the main reason for putting
them all in initially was to reduce voltage drop - to be able to run a
3Kw kiln up there, and to hit it operating temperature spec, rather than
sort of wandering up there.

In that case it all gets much easier - it was the mention of a 40 A fuse
that started all the problems. If you can set the design current at 30
A or less then use just a 30 or 32 A fuse or MCB at the supply end and
hard-parallel the three cables. By analogy with the standard ring
circuit you know that the cables, including their CPCs, will be
adequately protected.

Whatever you do, do not put fuses in any neutrals - that would violate
one of the fundamental safety principles [Regs. 131-13-01 and -02].
(CBs in neutrals are only allowed if they are linked, so as to break all
associated phase conductors.

--
Andy