Extending ring main in conduit?

[Geoff Mills]

Hi, I'm planning to cut into the ground floor power cable ring and extend it into the adjacent, integral garage. The concrete block walls of the garage will make it necessary to run the cable in conduit (plastic).

I intend to have one vertical run of 2.5M, with 3 x 2 gang boxes and sockets contained within this run. I then want to elbow the top and place another 2 gang box and socket at the end of a 1.5M horizontal run, which will then form an inverted L shape. To maintain the ring structure I'll need to return the cable to the extension starting point.

Is it common practice to return the cable down the same conduit that it came up in, or is it better to form a conduit loop? The latter option is going to require twice as much conduit, so I'd rather use the former method if practical.

[David Hansen]

On Wed, 16 Sep 2009 16:09:50 +0100 someone who may be Geoff Mills wrote this:-

Yes.

Assuming metal conduit, if there is not a copper protective conductor drawn into the conduit then you need to be particularly careful about making proper joints to maintain the continuity of the trunking which forms the protective conductor.

[Roger Mills]

He explicitly said he'd be using plastic conduit!

[nightjar]

It is acceptable, but if you are using twin and earth cable, rather than individual conduit wires, you may find it a bit of a tight fit around elbows in 20mm conduit. I prefer 25mm conduit if I am running two 2.5mm T&E cables.

Colin Bignell

[David Hansen]

On Wed, 16 Sep 2009 16:44:53 +0100 someone who may be "Roger Mills" wrote this:-

Apologies. I didn't spot the orphaned word.

[<me9]

Wouldn't the bunching reduce the current carrying below 20A before allowing for any insulation?

[Geoff Mills]

Thanks, I hadn't considered metal conduit before, nor that the conduit itself could be used for earthing. Lots to learn.

[Geoff Mills]

Thanks, I started out with the idea that I'd be using T&E cable though perhaps individual wires are more suitable. But then again, if that is so, why would anyone be running 2 T&E cables through conduit?

[nightjar]

Individual wires is the better choice. However, it probably depends upon whether you are doing enough to justify buying three reels of conduit wire.

Colin Bignell

[nightjar]

Metal conduit is excellent if you need more mechanical protection than heavy duty plastic offers, but it is not easy to work with. You need a pipe threader and, if you want to make pretty shapes, which you can do with care and a hot air gun in PVC, a pipe bender. As noted, the earthing it offers can be unreliable, so I always run earth wires in it as well.

Colin Bignell

[Geoff Mills]

Are individual wires not usually sold by the metre? I like the idea of these because only one earth wire would be needed. Trouble is the girl who answered the phone at the local Peco said she'd never heard of using individual wire. She always sells T&E for the purpose and this with 20mm conduit, as 25mm is special order and only sold in bulk. Fortunately there is a local Screwfix now.

[Dave Plowman (News)]

Much stronger if in the sort of place where it could get bashed. The minimum you'd need is a die to thread the cut ends. Assuming you can do it all in straight runs.

[Dave Plowman (News)]

Saves buying the singles? They have thicker insulation than stripped TW&E. And IIRC only come in 100 mtr drums - dunno anyone that sells cut lengths.

[Dave Plowman (News)]

TLC cut most cables - but not this one. Probably no demand. You could check Ebay, though. Just for info you used to be able to get it in stranded too - which made pulling through conduit that much easier. Dunno if you still can.

[David Hansen]

On Thu, 17 Sep 2009 08:40:14 +0100 someone who may be "nightjar" wrote this:-

If it is installed and maintained improperly. However, that is true of other ways of doing the same thing.

[nightjar]

I always buy frrom my local electrical wholesaler, who would probably give me a very odd look if I asked to buy anything other than armoureed cable in cut lengths. I usually buy conduit wire in 100m reels. I think 50m is also available, but I used more than one of the 100m reeels last time I did a factory rewire, so it wasn't worth buying less.

Again, my local electrical wholesaler (Newey & Eyre) will supply 25mm in single lengths. It is to order only, but delivery is next day, to my address.

Colin Bignell

[nightjar]

For the marginal cost of an extra wire run, it is safer to assume that it will, at some point, not be properly maintained.

Colin Bignell

[Dave Plowman (News)]

Yes. Conduit into BESA boxes will usually maintain good conductivity for ages. The problem is with ordinary surface mount steel boxes etc where you rely on a backing ring or nut.

[Geoff Mills]

Thanks for the leads. I'll follow them up.

Does oval marry up with the surface mounted back boxes in a similar way to conduit?

That is a state of excellence which I'm unlikely to achieve in this lifetime. Fascinating to read though.

[Geoff Mills]

Good points. I'm usually a belt and braces man but the relative ease of working with plastic conduit is going to swing it on this occasion.