Doubling up underground cable.

[timegoesby]

I am considering installing a cylinder and boiler in a garage. I may use two 3 kilowatt immersions. I need to take some cable from the Consumer Unit to the garage underground. Supplying the boiler, immersion, lights and sockets in there is more than what a direct burial 2.5mm cable can provide. Doubling up two of these cables is cost effective as heavy direct burial cable is expensive. Is it within regs to have these cables on one 42A mcb at the CU and take it to a garage CU in the garage and then take all the circuits off the Garage CU? Makes sense to me.

Or is it best to have each cable on an mcb at the CU and no garage CU with just two isolators as the cable enters the garage?

Any views?

TIA

[Andy Burns]

If you mean using conductors in parallel on the same circuit, that is=20 generally frowned on.

1.5mm 2 core SWA =A30.88/m 2.5mm 2 core SWA =A30.91/m 4.0mm 2 core SWA =A31.20/m 6.0mm 2 core SWA =A31.60/m

From

seems proportionately cheaper to me

If you really want to use multiple cables then that would appear to be a =

better way of doing it.

[timegoesby]

Thanks Andy. The 6mm 3 core will be difficult to bend. I intend to take the cable up from the ground and through the wall. Dark coloured plastic conduit will make it neater rising and going through the wall. Do you have any recommendations on the conduit to use?

TIA.

[Roger Mills]

Surely, one 6mm SWA cable would be best - and probably cheaper than 2 x

2.5mm cables. The problem with doubling is that if a poor contact develops where they are connected to the CUs, one cable can end up taking all the current - and will then overheat.

[Steve]

But is this any different from a ring main in essence?

Agreed.

Steve.

[Andy Cap]

Just drill upward at about 60 degrees, below where you want to mount the CU. Then cold chisel out the inside bends

Andy

[Dave Plowman (News)]

Good grief.

At TLC prices 2.5mm TW&E is 0.97 gbp a metre. Assuming you want twice the current carrying capacity, 6mm SWA is 1.60 a metre. And TW&E will require additional protection.

Seems to me you've been listening to 'two combis' dribble too much.

[cynic]

two 2.5mm cables in a ring main can be protected by a 32A MCB not 42 if you really meant that. Or did you mean 45? IME you should use one 6mm subject to volt drop considerations. Why do you plan to use two immersion heaters especially if you have a boiler? You need to investigate diversity I think. Have you thought out your earth / protection system?

[ARWadsworth]

I was wondering how the heating pipes were going to be laid between the house and the garage, if the boiler is going to be used to heat the house.

Adam

[Andrew Gabriel]

You have to conform to the "conductors in parallel" part of the regs. In this case, you could probably do it by using a pair of ganged breakers at the CU (so if one trips, it switches off the other too). For more than two cables, you would normally need a set of ganged breakers at both ends of the parallel run. However, you will have to sit down and work out all possible fault scenarios, and that the protective devices are effective. The harder ones to meet are a short in the cable near the CU, where the fault current is being back- fed via the other conductor.

Another factor is that you will have to derate the cables if you lay two together (derating factor) as they'll heat each other up.

As someone else already said, you really don't want to go this route...

[Doctor Drivel]

You need to eff off. This is for your own good.

[Dave Plowman (News)]

You need to seek treatment if your only contribution to a thread is this. We all know you had some form of breakdown when you threatened to 'set' the police on me. Perhaps it's time to go back to the doctor.

[Doctor Drivel]

You really do need to eff off. It will do you the world of good.

[timegoesby]

I agree. Dave Plowman should eff off as he is only of nusiance value.

[timegoesby]

Thanks to all. I will use one 6mm cable and a garage CU. I assume this can enter via an angled hole through the footings rather than up from the ground and through the wall. Is that so?

I am thinking of taking the water pipes underground through a 110mm plastic pipe, with the water pipes being plastic, fully insulated and

500 mm deep. It will be simple enough to do, apart from some digging, and save a lot of space in the house. It looks to be well worth it for the space gains.

Thanks and happy new year to everyone on uk.d-i-y.

[timegoesby]

Another minor point. If the mains water pipe enters a detached garage, then underground to the main house will it need a stop c*ck entering the house as well as the garage?

[Andy Hall]

A few tips here (because I've done it)....

- 110mm pipe will not be large enough to fit two 22mm pipes with a respectable amount of insulation. The recommended amount of insulation is at least the diameter of the pipes so this comes to 66mm all up for each one. Standard insulation thicknesses appear to be 9, 13, 19 and 25mm. You would have to go down to 9mm, which is not enough. I used 160mm and there was then plenty of room.

- I used plastic barrier pipe in coil form and threaded it through so that there were no joints in the underground section. There needed to be one pipe elbow at one end and two at the other. To assemble everything, it proved easiest to insulate and tape each pipe and then to tape the two together at regular intervals to make a bundle. This was easy to thread through the underground soil pipe. The elbows were put on afterwards with the pipe having been warmed wih hot water first. It's much easier this way than trying to thread the individual pipes through or to push or pull the bundle though.

[Doctor Drivel]

Matt, you have been on the new years pop. You said you taped the pipes together and pushed it through. Is that so? You can get larger plastic undergound pipes than 110mm. Put on the thickest lagging you can and fill the large conduit pipe with vermiculite. It would be beneficial to cover the underground conduit pipe with underground Jablite foam to prevent heat loss to the colder earth on the top of the pipe.

[Andy Hall]

Boot on other foot or in mouth?

Why don't you read the post before commenting?

1) I proposed insulating each pipe first, *then* taping the two insulated pipes together. The latter was simply for convenience of threading the pipes through the drainage pipe. If you try to feed them separately, the insulation sleeves inevitably rub against one another over part of the way and it becomes difficult to push the second pipe through. How do I know this? It's what I tried to begin with. In the end, I took out the single first pipe and taped the pair together. After that, feeding the bundled pipe through was easy.

2) I suggested use of 160mm pipe precisely because it allows more insulation around the pipes.

3) Have you ever done any of this?

[Doctor Drivel]

Matt, not like you I'm glad to say.