Armeg Scutch Comb mini-review / Oval conduit and socket boxes

[Luke]

Two parts to this post. The first part informational.

I've started rewiring the living room which involves getting rid of the two surface mounted double sockets and replacing with 8 doubles. (I'm pretty sure I understood the OSG correctly where it recommends 6 double sockets for a room of its size plus an extra two doubles for "home entertainment areas"!).

After spending a long time reading the pros and cons of various ways of chasing out socket boxes I settled on an Armeg SDS Scutch Comb chisel from Screwfix =A329.59

carefully marked out the perimeter of the cut with a pencil, then scored round with a Stanley knife, then delicately took off the skim coat with the Armeg chisel. I did attempt to "stitch drill" the perimeter first, but realised after the first box that this was not necessary. Once through the skim coat I went hard at it with the Armeg, down to the brick. Then I gradually ground away at the brick until the hole was the correct depth.

This was my first attempt at cutting out socket boxes and I was pretty pleased with the end result. I am sure a pro would have cracked them out a fair bit quicker than me but it was a neat job. Compared to the flat SDS chisel bit I tried at first, the comb was far better, as the "teeth" dig into the wall meaning the bit doesn't skate about all over the place.

I also used this tool during my gas meter box installation (see other post!), again, excellent for finishing off and tidying up.

Okay, onto the second part of my post, and probably a pretty dim question, but I now have to run chases to the floor for my new socket boxes. I will be fitting oval conduit to carry the 2.5mm2 T+E cable to the floor void. Is it normal to use two parrallel chases or one thick one carrying both cables? And depending on the choice, what size oval conduit is recommended? I see it comes in 13, 16, 22 and 29mm sizes.

I have baffled myself with grouping factors from the OSG and have come to the (hopefully correct!) conclusion that I can pretty much disregard this as the grouped cables will only be 6" lengths.

Thanks in advance, sorry for the long post!

Luke

[ARWadsworth]

I am glad you used enjoyed the Armeg Scutch comb.

As for your plan to drop the 2.5 t&E through conduit to under the floorboards then I see no problem. You can use either one conduit or two in parrallel, use whichever method you want or can easily get the cables through. Getting 2x2.5 T&E cables through 22mm is possible.

There is no grouping factor to apply as there is only one circuit in volved. There are cable capacities to consider for long lengths of conduit runs but not for the job you are considering.

Cheers

Adam

[Andy Dingley]

What would your rule of thumb be for round plastic conduit? 20mm or

25mm? Is it either possible, or practical (i.e. not a stupid amount of hassle) to pull 2=D72.5mm^2 for a ring circuit through 20mm, or is it better to go to 25mm.

[ARWadsworth]

I would not attempt to pull 2x 2.5T&Es through 20mm round conduit apart from on short straight lengths. Anything longer than a couple of metres and it becomes a hassle. If all the ring is in conduit then I would use singles not T&E.

Cheers

Adam

[Andy Burns]

I found getting a 2.5mm^2 2&E together with a 1.5mm^2 3&E through 20mm round plastic was "fun".

[Lobster]

This answers a question I have concerning a job I have coming up where I'd like to use conduit! Is it possible to buy single-core cable in lengths of less than 100m? I probably only need 10-20m, and am unlikely to ever need it to use it again after this...

(wonder if I could just take a Stanley knife to the roll of 2.5mm T&E I have in the garage!!)

David

[Andy Burns]

Cut lengths from a wholesaler?

But that would only be single insulated, unlike proper BS6491X cable.

[ARWadsworth]

BS6491X cable is the single insulated cable (and is stranded to allow it to be pulled through conduit). The double insulated cable is 6181Y.

Cheers

Adam

[Andy Burns]

Thanks for the correction.

[ARWadsworth]

You could ask Tim Watts about his experiences of mixing T&E with conduit. He used conduit for the vertical drops behind the plaster but I am sure that he used T&E as the cables went on a cable tray in the loft. I think he used

25mm conduit and he never tried to squeeze two cables down the same piece of conduit.

[Lobster]

Ah OK, I wasn't entirely serious but good to know!

Incidentally, if you're running 2.5mm singles, is 2.5mm green/yellow sleeved cable the correct one to use for the CPC (bearing in mind that in ordinary T&E the CPC is smaller than the L/N cables)?

Thanks David

[Bob Minchin]

You could do the Stanley knife trick but it would yield solid copper cable and so less flexible than the proper stranded conduit cable. Depends on how many bends to have to contend with or if you can thread the cables through as you add the bends/constrictions which would sound feasible for 10-20m total run.

Bob

[Cicero]

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You can *probably* get single core & Earth (double insulated - blue/ neutral + Earth) in 2.5mm and then add a single core live.

From previous information supplied here I bought double insulated 1.5mm single & Earth (6241Y), so other sizes are probably available too, which would solve your earth problem. Still solid rather than stranded.

Cic.

[js.b1]

If you can go around the room horizontally you can use 20mm conduit with 6491X singles. Cut lengths of Br Bl Gr/Ye 6491X can be bought from

for 20p (2.5mm) & 12p (1.5mm) per metre, but they do not list 4mm which is annoying re 30A radial & 4mm Gr/Ye is useful for "no- mechanical protection" re bonding.

If you have a wooden floor I would just use FTE and go under the floor, around the room or drop from the ceiling void above as necessary.

If you are using vertical cable entry (top/bottom) then I would use one for cable entry and one for cable exit. It only means a little more work, but makes things more flexible in future. The ideal is always 20mm round conduit if your plaster depth is already at the 20mm level so little excavation required - although 28mm oval with a 25mm hole is "spacious" too.

Use 35mm backboxes, that way you can have a good loop of cable inside for repairs if you do not oval and general wiring space for any future changes. Can be worth adding 1-2 other boxes with separate oval/ conduit even with blank plates for BT or network or anything.

[ARWadsworth]

There is no reason to run a 2.5mm green/yellow in most cases.

You will not need one for a 20m long ring. 1.5mm cpc will be OK.

[Tim Watts]

ARWadsworth wibbled on Sunday 01 August 2010 15:04

I don't know about round - I would expect to get 2 x 2.5mm2 or smaller through a short straight length of 20mm though, or even 3x 1.0mm2 at a push (better be short and dead straight mind).

I used nearly all oval - and 2.5 goes through ceiling->floor drops with no hassle - you can push it entirely from one end if it's clean off the reel.

I used 25mm round for the cooker point though - and that I expect to take back to the consumer unit as a continuous length, dropping to 20mm for the last few cm due to the knockouts. There I will be using 6mm2 singles direct from CU to cooker as there are bends in the wall (legal bends in zone I might add).

I would have dropped round conduit more, but it would have meant cutting into the brick, where the oval sat mostly in the plaster layer.

One thing I do know, is that 2.5mm2 can be got through a tortuous 20mm round conduit that goes 2m up the wall, bends (hand bent, not fittings) round over the bay ceiling and similarly down the other side. Needs a fish wire to pull it and relies on the conduit joints being clean.

Cheers

Tim