Multiple long cables in parallel.

[Ian Stirling]

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?

[Andrew Gabriel]

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).

[Andy Wade]

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.

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!

[Ian Stirling]

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

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,

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.

[Ian Stirling]

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'.

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.

[meow2222]

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

[Ian Stirling]

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.

[meow2222]

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.

NT

[meow2222]

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

[Andy Wade]

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.