wall chasing

[Sam]

Hello,

What is the best way to chase walls for cabling? I read the wiki FAQ and it talks about making two lines with an angle grinder and then a bolster chisel to knock out the bit between. That's what I have been doing. I am using a 115mm angle grinder which is cutting about an inch into the wall. That's as far as it will go. It's cutting about 10mm of plaster and 15mm of breeze block. Is that deep enough or do I need to use a chisel to go deeper?

The angle grinder is making some nice deep lines, but I am not getting a clean cut of the gap between the lines. The breeze block is very rough. What is the best way to get a smooth finish? Do I need to sharpen the chisel? Could it be blunt?

What is the best thing to drop the cable in? Wickes & Screwfix sell some channeling, which is a bit like a gutter. The problems I see with this are:

1. plaster could get round the back and onto the cables or block the channel
2. the wire could scrape against the rough surface of the wall

Wickes also sell the same in metal. I guess the metal offers more protection against drilling/nailing through the cable but does it have to be earthed? If it became live I cans see that could be very dangerous.

I see the wiki uses proper conduit, which would avoid both of my problems above, but if it is 20mm in diameter, it would only be 5mm beneath the surface. Would that be deep enough? How do you hold it in place whilst the plaster sets around it?

Thanks, Sam.

[George]

Buy a sall cheap circular saw and change the blade for a masonry one you can then set the depth what you like and at the same tie have a flat edge to glide up the wall and also connect a hoover to it.. ;-)

[John Rumm]

It works, but only if you like dust!

One of the proper wall chasing machines that uses two angle grinder blades and has a housing to control the dust is better.

Plenty deep enough. If you use oval conduit for the cables you only 10mm of depth or so for the conduit.

You don't really need a smooth finish - as long as the trunking can sit in the chase deep enough to get covered up later, you should be ok.

This is capping - only really any use for protecting cables prior to plastering. Not much use for your application.

The thin metal stuff offers little more protection against nailing etc than the plastic. The main benefit is that it stays where you bend it a little better.

problems above, but if it is 20mm in diameter, it would only be 5mm

If you use the oval 22mm conduit:

depth is only 11mm

You can hold it in place with a blob of bonding plaster or gripfil etc. Even just a couple of large head clout nails.

[Steven Campbell]

I just used normal trunking and screwed it into the chased wall. Inserted the cables and then clipped the lid onto the trunking. This was firm enough to plaster over.

Steven.

[Frank Erskine]

That's fine as long as you remember that the cable is effectively buried. Right angle bends in trunking will make it virtually impossible to 'cut and draw' to replace a cable.

[Sam]

Thanks. I have returned the channeling and have exchanged it for the oval conduit. How does this affect the rating of the T&E?

I looked at the URL you gave for the wiki and that's not the page I found. I must have found a different DIY site (via google).

I read some old posts about the Aldi wall chaser. Now that people have had time to play with them, what were your experiences?

[John Rumm]

It reduces it a little, but not much. Column C is the normal figure for "clipped direct" or a cable buried directly in plaster. Using conduit results in de-rating to the figures shown in column A. So 2.5mm T&E drops from 27A to 23A

I looked at the URL you gave for the wiki and that's not the page I

Thought that may be the case since our one probably answered your question! ;-)

Not tried the aldi one. I have a Sparky one, which is ok, and does a good job of eliminating the dust. Has slightly awkward handling though.

[<me9]

And then you stick two cables in and it drops to 19.5, just below what is necessary for a ring.

[John Rumm]

Two smaller conduits solve that problem, and are often easier to thread anyway.

[Sam]

Thanks for the link to the table. Where do you get the figure for running two or more cables in the same conduit? I was going to run two separate conduits anyway; partly because I hadn't seen wide conduit in the shops and partly because like you, I think it would be easier to push two wires down two separate conduits anyway. Do the regs. say that the two conduits must be a certain distance apart? Do I have to chase two separate channels or can I place them together in one wide channel (the latter would be less work!)

One final question: our house does not have a hall, so you have to walk from the stairs through the lounge through the dining room to the kitchen. There are is one light switch at the stair end of the lounge and one at the kitchen end of the dining room but nothing in between. I would like to fit two-way switches between the lounge and dining room. These would be either side of the same (breeze block) wall. I am concerned if I chased a channel on either side of the wall, I would effectively be removing twice the depth and would this weaken the wall? Even if I offset the two channels, I wonder whether the effect might be the same because they were so close? Is it that you are allowed to remove one third the depth safely?

I wondered whether I should chase one channel on one side of the wall to carry both wires and then drill a hole through the wall to take the second wire to the other switch. Is this acceptable?

Thanks, Sam.

[John Rumm]

The On Site Guide has some info.

Drop me an email, I can send you some tables.

One wide chase is ok IIUC.

If you want switches on both sides of a wall, then chase one side only, and run all the wires in that chase. Then drill though at the relevant point for the other switch. (placing the switches slight offset from each other)

;-) Should have read to the end of your post before writing the above!

[Stephen]

I wonder if you can help me? I have a socket that is halfway up the wall and I would like to lower it to a more sensible level. The wires to this socket are run in the metal capping you describe. I am concerned because both wires are run together and having read the above post it sounds as though the cables are rated at 19.5A "below what is necessary for a ring".

When I lower the socket would it be a good idea to remove the capping and replace it with two conduits? If this one socket is like this, I suppose the rest of the house is. Should I be alarmed?

When you say the cable rating drops from 27A to 19A, does this mean the rating of the whole ring drops, or just that section?

Thanks, Stephen.

[Stephen]

Probably too late for the OP but if you just removed the plaster, would this give you enough room to run capping on either side of the wall without chasing?

The problem would be that if the light switches were back to back, you would not be able to screw through the back of the box without hitting the other! I don't know how you could get round this without making one switch noticeably higher/lower/further from the door than the other.

[Andrew Gabriel]

You're worrying too much. Rings are always installed as yours is.

[John Rumm]

Not sure that is correct anyway...

From the on site guide - 16th edition:

"7.2.1 Grouping of Cables (i) In domestic premises, except for heating cables, the Table 4B1 conventional circuit design permits any number of single-layer circuits when the spacing between adjacent surfaces of the cables exceeds one cable diameter, and, for other than semi-enclosed fuses, (BS 3036) up to 5 touching, single-layer, circuits, when clipped to a non-metallic surface (Installation Method 1)"

also, metal capping buried in plaster is treated as reference method C in the 17th edition (i.e. clipped direct - as above). Hence the installation method has no addition de-rating to impart.

In fact there are a couple of notes in the 17th edition that also mention that mechanical protection over a cable buried in plaster (i.e. capping) may in fact increase it current carrying capacity (for cables with conductors of CSA

[<me9]

I think he was confused by my post referring to running together enclosed in conduit rather than under capping.

[Stephen]

Sorry I jo>> metal capping buried in plaster is treated as reference method C

What is the situation with plastic capping? Is that treated any different?

I haven't got an on-site guide to check because I am waiting for the

17th edition version to be in print; it seems silly to get what will soon be obsolete if a new version is due any day.

Thanks again.

[John Rumm]

In reality no. (Although it won't have any benefit from the point of view of heat dissipation in the way that metal might.

Supposedly release on the first of this month, but my Amazon order is still waiting so it seems. (there are PDF versions of the 16th floating about!)

[Stephen]

I'm afraid they are not stud walls, unfortunately. The problem is that I don't know what is under the plaster. I think the door frame is about 9 cm wide, so I guess it's a 9 cm block, and I know from having lifted floor boards that the joists start over it, so it is bearing the upstairs floor on it.

I have seen these but they would drain to the bottom of the radiator rather than the bottom of the pipe run, wouldn't they? I don't know whether this would be a major problem? I think I might be better tee-ing off the pipe.

Thanks, Stephen.

[John Rumm]

A "normal" block wall is about 4" (10cm) plus at least half inch of plaster either side. Yours may be a thinner block I suppose.

Yup. Depending on how the pipe runs that may or may not matter. (in fact even if the pipe has a little water left in it, it does not usually matter.

In the past, I have tee'd off, into a service valve, then straight through the wall into the gully the other side (obviously choosing the tee position to match the gully rather than the other way around!)