House Rewire: Channeling/Wiring Design

[Organoman]

Hi,

A while back I asked for advice on a CU and circuit design. I've noe bought the CU and have decided on the number/location of sockets, switches etc.

I've been over both the Electricans Guide and the On-Site Guide to BS7671 and require a sanity check on my wiring design. I'd also like some advice on channeling, so without futher ado :-)

Wiring:

1) The ground floor has concrete floors so I'll have to do all my cabling for the ground floor from above. Lots of channeling. My question relates to how I route the first floor cables. Can I also route the cable for the socket ring under the first floor floorboards and create a smaller channel up to the socket? AIUI this seems to be acceptable as it will be in a vertical line from under the floor to the socket itself. Am I correct in this thinking.

2) In the living room I intend having five double sockets close together to support my AV system. Do I have to channel and cable up and down for each one or can I have the cable come down in a channel to the leftmost socket, pass through to the other sockets then have the cable return up in a channel at the rightmost socket?

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use to create 30mm wide channels. Has any one any experience of this or similar chisel? Axminster also do an SDS set for sinking electrical boxes for single/double sockets. Has anybody any experience of these. I have approx 45 double sockets to install and I get the feeling this will make my job a lot easier.

[Organoman]

Hi,

A while back I asked for advice on a CU and circuit design. I've noe bought the CU and have decided on the number/location of sockets, switches etc.

I've been over both the Electricans Guide and the On-Site Guide to BS7671 and require a sanity check on my wiring design. I'd also like some advice on channeling, so without futher ado :-)

Wiring:

1) The ground floor has concrete floors so I'll have to do all my cabling for the ground floor from above. Lots of channeling. My question relates to how I route the first floor cables. Can I also route the cable for the socket ring under the first floor floorboards and create a smaller channel up to the socket? AIUI this seems to be acceptable as it will be in a vertical line from under the floor to the socket itself. Am I correct in this thinking.

2) In the living room I intend having five double sockets close together to support my AV system. Do I have to channel and cable up and down for each one or can I have the cable come down in a channel to the leftmost socket, pass through to the other sockets then have the cable return up in a channel at the rightmost socket?

Channeling:

1) I have seen a channeling chisel at Axminster tools which you can use to create 30mm wide channels. Has any one any experience of this or similar chisel? Axminster also do an SDS set for sinking electrical boxes for single/double sockets. Has anybody any experience of these. I have approx 45 double sockets to install and I get the feeling this will make my job a lot easier.

[Chris Cowley]

Yes you are correct.

Yes, you can run the cable horizontally between sockets on the same wall and that's exactly what I'd do for a group of very closely-positioned sockets like that.

[Andy Burns]

Of course you can (indeed should) do the latter, always better to ask I suppose, but having to ask such a question doesn't bode well ...

[Tim S]

Hi

Not an electrician but...

Organoman wrote:

This is a perfectly normal way to do it. Watch the routes where you need to go through a joist - there is a correct range of positions that you should drill holes in the middle of a joist to avoid weakening it unnecessarily - certain fraction of the span to another fraction, from the supported end. But I cannot recall what the range is - someone else will be along no doubt to answer that.

Whitfield Electrician's Guide Ed. 7 Page 72 Fig 4.14 "Acceptable installation zones..." explains all.

Essentially, yes - cables may be run vertically or horizontally from the fitting, or within 150mm of an internal corner to ceiling or another wall.

Other routes require the cable to be protected, which may include using specialist types of cable. In my unqualified opinion, you are better to go horizontally between all sockets in a room (doors not withstanding) or you run the risk of excessive cable length with all the ups and downs and thus voltage drops going out of spec.

Don't take my word for it - but I think the diagram I cited is fairly clear.

If the plaster depth allows, have you considered PVC oval conduit? Cheap as chips and you'll get the cables out again if you ever need to.

HTH

Tim

[John White]

Yes. As long as the cables are running vertically above or below the sockets then you do not need to take any special precautions to protect them. You could also put the ground and first floor sockets on the same ring as this would reduce the amount of wiring you would need to thread through the joists.

If you need more than one ring then splitting the house vertically rather than horizontally can save a lot of extra work and materials.

It is also permitted to run cables in a horizontal line from the socket outlets, without any special protection. So your scheme meets with the regulations.

more than just a 30mm channel, especially where the plaster was a bit flaky anyway. My preferred technique on long channels is to cut two slots in the plaster with an old tenon saw, and then use a SDS drill and conventional chisel bit to remove the plaster between them.

I may however be somewhat old-fashioned in this as it's the same technique I used with a lump hammer and bolster chisel.

sockets went well in good plaster over block, but after that the tool became blunt rather quickly. It also had difficulties with hard or uneven walls and made a LOT of dust. Perhaps in block walls, (as in the illustration) they might last longer.

I find that a good chisel bit in a light-weight SDS drill gets the job done just as quickly.

John

[Andrew Gabriel]

Yes (need to be passed through joist centres, not immediately under the floorboards).

You lost me there.

Yes, providing they are at the same height.

wall chaser instead, such as

I haven't seen this particular make (I have a Ferm one). You can adjust slot width for different trunking sizes.

before going blunt IME. The square cutter is useless -- much easier to square up the circular hole afterwards with an SDS chisel bit. In thermal blocks, the tool will last much longer (and the square cutter might work -- I didn't try it).

[Andrew Gabriel]

Just to add -- you can hire them from tool hire places too.

[Ed Sirett]

I've not tried the circular cutter it looks like a recipe for making a lot of uncontrolled mess. I find the square cutter useful for marking and trimming the hole. The bulk of the 'meat' I remove with a chisel bit and then swap to the square chisel to clean the edges and then repeat with the chisel again until done. The square cutter simply goes nowhere into normal bricks let along engineering bricks but will clean up the edges.

[John Rumm]

Yup.

Yup, cables can be unprotected in plaster if the are in a vertical or horizontal line with a visible accessory. Note that this also extends to the reverse of a wall if it is easy to see if from the location (i.e. by a doorway). You can also use the 6" margin at the corners and tops of walls. Anywhere else and the cable needs to be buried more than 2" deep or protected by mechanical means.

On modern plaster they will cut a nice neat channel. On older flaky stuff they will tend to pull off large areas of skim in the area of the cut. They are quite handy for getting behind smaller covings and skirtings however. (an alternative can be a very long drill bit user almost parallel to the wall into the chase behind the obstruction).

For older walls I tend to simply use a 40mm wide chisel on the SDS to cut a couple of parallel slots and break out the middle. Make sure you have a drill where you can lock the chisel bit into a fixed position though else you will have a hard time cutting a straight line!

If you go for a wall chaser then make sure it has *very* good dust extraction. Don't even dream of using an angle grinder!

chisel bit in an SDS in about 5 to 10 mins anyway. (offer box to wall, draw round it, chisel a little inside the line all round the perimeter (mark the depth of the box on the chisel bit!), and finally chop out the middle with a few oblique cuts with the chisel.

[meow2222]

Assuming your sockets are in a horizontal row, channel down to the left one, wire horzontally to each, return the wire to the left inside the pattreses, then route it back up the original channel. No sense making

2 channels.

While youre doing it, good opportunity to put in cat 5 and coax. Cat5 has a slew of uses, and costs s all.

NT

[chris French]

In our old house (1930's semi) the skim layer was relatively hard but he underlaying plaster would come away from the wall quite easily, taking even a hammer and chisel to it would have great chunks falling off.

For their the trick was to score through the plaster at the sides of the channel with a Stanley knife, then you could remove most of the plaster v. easily with a chisel

[Ian White]

OTOH, if the area is already in a mess, two narrow slits with a thin disc won't add much.

For minimum dust, score two parallel lines through the skim coat, and then gently score/cut through the plaster by hand using a large hacksaw blade. Once you're down to brick, you can prise most of the plaster strip away in clean chunks.

[Organoman]

Why? Everybody has to start somewhere. Surely it's better to ask a silly question than to plough ahead and make a mistake. I figured it would be sensible to route them as I suggested but I was a bit worried about possibly overloading the cable.

Cheers.

[Organoman]

That sounds like a much better idea :-)

That was the plan, though I'm a bit concerned about having power, network and television cables all bundled together. Won't there be the capacity for lots of interference?

Thanks.

[Organoman]

Thanks for all your advice. Especially regards channeling. I'll try some of the suggestions before forking out on the box cutter I think.

Thanks again.

[Tim S]

They shouldn't be bundled together unless the data cables (etc) are rated to mains voltage too - just run a separate conduit or chase down for aerial, data, phones etc.

The interference may be a problem on the phones, but CAT5(e) carrying ethernet packets will generally be OK.

Cheers

Tim

[Christian McArdle]

As a further modification, I would also pass the ring cable back unbroken through the first socket and use just one wider channel back up.

Christian.

[John Rumm]

The only downside being that you then have to wire the return cable from the last socket back through all of the other socket back boxes. This can make it harder to fit the face plates of the sockets and makes the wiring take longer. So you may not gain any time doing it this way and you make adding further wires in future harder.

Only keep em away from the mains (unless you like hum)!

Yup, hence why 50mm separation between services is recommended.

[meow2222]

You can strip off the grey cover while its inside the pattresses if space gets tight. I've not found it a problem myself, I'd sooner do that than cut and plaster another channel, but I may be biased.

Cat 5 is twisted pair and uses differential input, so hum is a non-issue there. TV coax is screened plus the equipment is all C coupled and wouldnt notice any hum as its way out of band, less than a millionth the transmission frequency. Even if you connected mains direct to coax (dont of course) the tvs would still work fine. This has unintentionally happened before.

For possible as yet unknown future apps, we dont know what is and isnt hum sensitive. However if you've got the channels it makes a lot more sense to put wire in than not. Most apps are fine like this, including audio distribution, there are some that arent, but future apps designed to use round the house wiring are most likely to be humproofed.

NT