Oval conduit <> metal box + Q's..

[T i m]

Hi All,

I had a quick look at the wall chaser, the 32mm oval conduit, the roll of ,5m T&E and the metal boxes that I'm supposed to be playing with to halp my mate over the weekend and have some questions.

First some observations:

The oval is due to carry some 2.5mm down from above the false ceiling line, to double sockets avove the worktop and back up again (in several places) to become the 'ring'.

The wall chaser has a maximum slot width of 28mm.

The conduit he has is 32mm wide (but can be swapped out).

Looking at the section of the trunking and the 'knock outs' in metal box it's sorta square peg / round hole(s) thing?

Q's

So, should we use 25mm wide trunking (plastered in the wall) for two runs of 2.5mm T&E and (if enough room etc) and which of the 4 holes in the top of the box would *you* bring the cables into (for a std double socket) and why please?

Do 'people' use the grommets? (I asked him to get a box of them and would use them myself anyway).

Do you twist the earths together (or not) and sleeve as one or sleeve individually?

Do we still *not* twist the conductors where they enter the screw terminals in the back of the socket?

Does it matter if you use flex or solid between socket and metal box (as long as it's the right colour and gauge etc)?

I dare say the sparks will (should) know all this when he comes to do the main hook-up / testing etc but I'd like to make sure the background stuff is as right as it can be please?

All the best ..

T i m

[Owain]

I'd run the chaser down the wall twice, bash out the ridge in the middle, and put in 2 x ovals with a gap between them. Probably more likely to hold plaster better when you make good.

The ones up to which you abut the ends of the ovals. That's probably going to be the middle pair.

Of course :-)

If the cables were coming through the same box hole the earths might be sleevable as one, but as you'll be using separate holes and 2.5mm probably better sleeving separately.

I'm a twister. Opinions vary.

Although if you thread the cable through before beginning you might manage without cutting the conductors at all.

Yes, flex and cable in the same terminal probably won't hold a firm connection. Anyway, why buy flex when you'll have plenty of offcuts of earth from your reel of 2.5mm.

Do check for continuity end to end and polarity at the sockets before the sparky comes. Less embarrasing if you find your own faults first.

Owain

[Chip]

On Thu, 19 Jan 2006 18:56:10 GMT,it is alleged that T i m spake thusly in uk.d-i-y:

*prepares to answer by donning flameproof suit*

Yes, I always [this is the reason for the flameproof suit] use a pair of pipe grips to 'round' the oval conduit and just jam it in with a grommet, never had any pop out yet.

25mm + 2x 2.5mm T&Es is a little tight but doable.

And whichever box knockout fits

I do, they're cheap enough and useful to have around.

Personally, I don't twist, but sleeve together.

I was always taught not to twist, so I don't. I do however twist the ENDS of the earth wires simply because it feels more solid, (1.5mm earths on ring circuits usually if using T&E)

Flexible conduit cable is fine, the right gauge of *flex* may not be correct though, due to voltage rating. In practice it's not going to matter - especially on an earth - but it could fail an inspection.

[T i m]

Hmm .. 'two ovals'. With this chaser I'm gonna have to bash out the ridge left between the blades anyway and if it's as dusty a job as you guys suggest I think one pass will be sufficient thanks! As long as I can get (easily / legally) 2 x 2.5mm T&E's up/down a ~1.5m length of

25mm oval I think I'll go with that.

lol . I was thinking there might be an advantage say bringing the cables in at the end rather than near the middle as they cables may lay better in the box or summat?

Good ;-)

Ok. But even if run side-by-side would you sleeve the earths separatly?

"Twisting the wire a-way .. twisting .. twisting .." em .. ar .. sorry ;-(

Hmm .. I have done that before and I like the concept .. just a bit more effort required to get it right.

Yep, good point .. noted ;-)

Indeed. I generally work fairly methodically, checking each move / step as I go so generally I check for mistakes as I go. Still might do the test as you suggest though.

Al the best ..

T i m

[T i m]

Hmmm .. imaginitive .. squeese the square peg till it's round! ;-)

Well, of course there would be more room in 32mm oval but there's that conflict at the box? Seems silly to then take both T&E's into one grommeted hole but I don't think it's wide enough to get one leg through two holes?

ok lol ;-)

Ok .. good ..

Ok .. understood. With the 'double sleeving' solution wouldn't there be more chance of some exposed earth core near the end of the outer sheath whereas individually sleeving would allow the sleeving down between the L / N cores and into the outer sheath?

Understood

As in 'singles' ?

Are you suggesting it *should* be flex Chip (ie *not* solid) ?

All the best and thanks for the input ..

T i m

[somebody]

In message , T i m writes

I am no sparky, so do check this for yourself. I was recently informed that the grommets are a building regs issue.

I have no idea how true that is and I am a little surprised that the usual posters on this group who have copies of the regs in front of them haven't already replied to it.

Maybe it's a mute point?

Personally for the extra fractions of a penny, just put them in.

[T i m]

Hi and I agree .. 'hapenth of tar' etc ;-)

A thought that came out of the grommet end was the use of round plastic trunking?

I believe that interfaces directly with the boxes but not sure how it would handle 2 x 2.55mm T&E rather than singles?

At least there would be a common section between the trunking and the box hole and would potentailly just need a narrower (but possibly a touch deeper) slot in the wall?

All the best ..

T i m

[chris French]

In message , T i m writes

It should be ok I think.

It's probably more like double the depth at least of chase required. Oval conduit is what no more than 10 mm deep? And often oval conduit will only require the plaster to be chased, rather than the brick or blockwork as well.

I've never bothered to try to get the ovasl conduit to mate with the backbox, I just terminate the conduit at the outside edge of the box

[T i m]

Ok, thanks.

I guess ..

These walls seem to have a very thin skim over a sand cement render of (as yet) unknown thickness. I'm not too bothered about the depth of cut as I will be using the wall chaser ;-)

Understood Chris .. and by 'terminate' I don't really mean a real engineering solution, just one where half the oval isn't looking at some galv steel rather than hole?

Ok, the reason for the conduit there is in the unlikely even you need to replace the wiring it can be done without tearing slots up the room. It's just here we have a clean slate, the tools and the chance (for minor cost differences) to make a 'neat / easy' job of it?

*I* like the mecanical solution where a round pipe enters a round hole and all sits neat n tidy and I know that is in direct contrast to many sockets I have come across in the past where you have to rip half the plaster / wall away around the box to pull the cables though (where the trunking stops 2" short of the box and the gap filled with render / bonding / skim etc?). ;-(

It's also my thought that it might be easier to 'fill' a slightly deeper chased slot filled with a round trunking *because* you have more depth for any filler to attach to (someting wider and flatter (and still plastic) being worse)?

I'm also doing 15 of such drops so I want it to be as predictable as possible?

All the best ..

T i m

[John Rumm]

The gromit is just there to protect the wires from the sharp edge of the box. Thermal expansion etc can result in the box slowly cutting through the insulation otherwise.

If you are using conduit in wall chases (by no means a requirement - you can bury directly if desired), it is not a requirement to join it to the back box. Abutting it ought to be enough. The threadded conduit fixings to which you refer are more commonly used with exposed conduit when terminating to surface mounted (often metal clad) accessories.

[chris French]

In message , T i m writes

That's east enough to arrange, usually I find it lines up ok, if not little bit of packing does the job.

I've replaced cables in oval conduit I've put in - as long as it matches up with the conduit box hole enough so that plaster etc. doesn't hold it in place.

no, sorry I think that's a load of rubbish :-)

All the more reason for not over engineering it then....?

[Paul]

snip

Round conduit into round hole on box. I personally found it easier to feed two TE down the round than the oval.

Do you need to go down and up on each point? Could you chase horizontally to link some of the sockets and reduce the amount of cable required and also the joy of getting two pieces of cable down one piece of conduit.

Paul

[chris French]

In message , Paul writes

I find it easier justto put in two bits of smller oval conduit than try to get 2 cables down one bit.

[Owain]

I would avoid trunking on the premise that you have already bought the oval stuff so why spend money on trunking.

Of course, you don't actually need oval, AIUI it doesn't count as mechanical protection for buried cable so you still have to run within the zones.

Depends how the cables fit (and how cheap I'm feeling) :-) But I woudln't twisy them (except at the termination)

Owain

[T i m]

Ok, understood. Do they make boxes with OVAL knockouts!

Me too .. and then theres that are a pain .. ;-)

Oh? From a mechanical point of view I can't see how you could get as good a bond to the sides of a 10mm deep slot as you could to the sides of a 20mm deep cut ? (I'm not saying there would be an issue with the former but in my experience it's easier (as in to get the filler to stay in place) to 'fill' a narrow deep crack than a shallow wide one?)

Good point .. ... cept I see it as 'easier engineering' ...?

1) I find the idea of cutting a 20 x 20 slot (and breaking out the centre rib) being easier than a 40 x 10 slot and breaking out the middle to leave it flat and consistant, pleasing. 2) Knowing that my round tubing is mecanically attached to the box and centered in the hole (and with no rubber grommets falling out) gives me pleasure. 3) Being able to freely slide the cable through the trunking freely (as a pair, often easy or one beside the other, often less easy), rewarding. 4) Knowing that I can access any of it to replace / upgrade / move later on, satisfying.

But no, I'll do what everyone else does because it's quicker, cheaper or 'will do', in spite of that fact that none of those give me satisfaction ..?

But I can't help it .. I'm a Virgo!

All the best ..

T i m

[T i m]

Don't .. just when I was agreeing with the 'nail it to the surface and paper over it' boys! (I personally found it easier to

I have done quite a bit of that in the past (sheds etc) and it does seem 'easier'. I've also done quite a bit of single in steel as well and that's everso nice .. ;-)

Well I think it's 6 of 1 and 1/2 doz of the other Paul.

The drops are about half the distance of that between the sockets but there's all sorts of other crap inbetween (joggles in the wall, windows, data points etc etc). Plus all the existing feeds are coming in from 'above' and will end up above their new ceiling ..?

All the best ..

T i m

[T i m]

Ah, didn't think of the 'thermal expansion' bit John. When I have seen grommets in the past if you actually pull the cable down the trunking and out of the box they normally end up threadded on the cable rather than in the fitting! ;-(

(ok, thanks for that)

Ok, understood. I just like the 'mechanical' preciceness of it all ..poke wire in open end of trunking and watch it pop out into box (not snag everywhere!) ;-)

All the best ..

T i m

[Chip]

On Fri, 20 Jan 2006 01:22:40 GMT,it is alleged that T i m spake thusly in uk.d-i-y:

[snip]

I should probably have qualified: I double sleeve when the cables are close enough for this not to happen :-)

But yes, it would be a worry if the 2 weren't side by side, if in doubt, I sleeve everything individually.

Yes, but also see Owain's sensible point, one that I'd missed: flexibles may not grip well in combination with solid core, offcuts of the normal 2.5 T&E earth cable should be fine.

No, merely that if you do use flex, it should be a compliant type (flexible conduit single, not stripped out flexible cord).

[ARWadsworth]

Grommets are not a requirement. What is a requirement is that the cable is protected from any sharp edges. I suppose you could start deburring the back boxes etc but it is faster to fit a grommet.

Individually. Earth sleeving is so cheap

Some do some don't.

It does not matter if you do not earth the metal back box with a seperate flylead as long as one of the lugs you connect the front plate to is fixed. I have not seen new back boxes with 2 adjustable lugs for a long time now.

Adam

[John Rumm]

Its nice in theory ;-) Feeding T&E through conduit however it a PITA IME. Not too bad on short runs like drops to light switches it has to be said.

How much use it is depends on how accesable the top of the conduit will be in future (i.e. if there is any possibility of getting at it to replace a wire should there be a requirement).