Wiring

[Rick]

I think I have read enough posts to have this one worked out, please can somebody confirm. There is a desprate shortage of local sparkies round my way, the nerrerst is an hours drive away - my first plan, of employing one as a consultant is not working too well. My father in law was qualifed till he retired, a few weeks before Part - P arrived, so he will help out, I just have to make sure I do the new bit correct. This mainly seems to be restrictions on where I can run the cables - and new colours.

For where to run my wires,

They can be 50mm below the surface, in which case I can put them where I like. Each room (except one, see below) will have at least one studd wall, 100mm thick. This means I will need to put them in the insulation layers, do I have to do anything special for this ?

In the other walls I can run the cables vertically from a "device", or horrizontally between 2 devices in the plasterwork, which some plastic protection - is this correct ?

I intend to do each room, with a seperate run to/from the fuse box, and two rings in the kitchen, one normal, one economey 7.

If this is OK, then my plans are fine, as I intend to finish a room in turn, rather than first fix the whole house, then 2nd fix the whole house. This is mainly because I am living in the house, and if I do a room in turn I can keep living in it.

For my main room, I will have no studd walls, and no clear 100mm space at the top, as that space if full of ceiling beams, and its too little anyway. The plan is to put a wooden "trench" into the floor insulation, and put the wires in this. I would then seal the top with steel plate, which would be earth bonded. Is this OK ? I'd run the "trench" round all the walls, and run the cables up to the devices.

Final problem, where can I put junction boxes ?

Can they go into the studd walls ?

I can live without them in the floor.

Thanks Rick

[Andy Wade]

You do need to ensure that cables don't become totally surrounded by insulation (unless you've done custom circuit designs taking that into account). If using the usual cable sizes you have two options: cables can either be clipped along the studs or run in PVC conduit fixed to the studs. For these methods and T&E cable the current ratings in columns A & B of the following table apply:

Size Ratings in amps (ambient temperature 30 deg.C mm^2 A B C conductor temperature 70 deg.C)

---- ---- -- --

1 11.5 12 16 1.5 14.5 15 20 2.5 20 21* 27 4 26 27 37 6 32 35 47 10 44 47 64 16 57 63 85

Key to columns: A - in conduit in insulated wall (ref. method 6) B - directly in insulated wall (ref. method 15) C - clipped direct (ref. method 1)

Yes, and no protection is required for cables in these zones. For more detail (illustrated) see

I intend to do each room, with a seperate run to/from the fuse box, If you mean having a separate power (sockets) circuit for each room, each with its own MCB, then yes, that's OK. If we're talking about normal sized rooms then you could use 20 A radial circuits rather than rings.

You mean one ring on a time-switched supply? That would be unusual, but I suppose you could. I take it you're using storage heaters too (otherwise the DNO or metering company wouldn't normally give you a switched supply).

I think that could be made to work, or you could use a proprietary floor trunking system. Isn't there a floor void or roof space above where you could run the cables and drop down to each point though?

You shouldn't need any if wiring anew from scratch. Make all your branches and spurs at wiring accessories, then the accessibility requirement is met.

Not normal screw terminal ones, if they're going to be plasterboarded over. Crimped or soldered joints in a suitable enclosure are needed for that.

HTH

[John Rumm]

I take it that when you say 100mm thick, that is the thickness of the studding? (i.e. with plasterboard and skim it is more like 125mm).

If it genuinely 100mm thick then you may have problems achieving the required depth from *both* sides.

Yup, but you don't need the protection (although oval conduit is handy for allowing wires to be replaced later). Remember also you can use the

150mm zone beside each corner, and down from the ceiling.

[Rick]

Andy

Do you have a link on custom cable sizes, where the cables are totally in the insulation, or can I simply use 2.5mm cable, and reduce the MCB size. If I have a ring for each room, this would be fine by me, most rooms will be bedrooms, studies, only the kitchen has lots of power requirment.

Thanks Rick

[Andy Wade]

That really is a non-preferred way of doing it. The cable rating should be taken as being half the clipped-direct rating. For a ring, where you need 20 A as-installed rating, you'd need 40 A clipped-direct rating, which would mean having to use 6 mm^2 cable. Since the terminal capacity of most BS 1363 accessories is 10 mm^2 you have an immediate problem in that it would be difficult to fit the two ring cables in the terminal, let alone any spurs. IOW forget it.

I'd advise strongly against such a non-standard design approach for power circuits. There may be circumstances where you can get away with cables totally embedded in insulation, e.g. parts of lighting circuits, but you still need to be careful - e.g. you may need 2.5 mm^2 cable on a

10 A lighting circuit and could run into problems with terminal sizes again.

[Rick]

Thanks Andy, I read some stuff, and was comming to the conclusion that I need to think again, for the reason that I don't like working with the thick cables, I never thought of the problems of getting the cables into the connectors, I guess I'd have found that out too late :-(

I will now look at trying to use the allowed zones, I guess a box of chalk is about to be used, by me drawing all the cables onto the walls. I'll run them between the insulation and the plasterboard.

I now see one reason Drivel insulates the outside of his house, I just ordered another lorry full of the stuff.

Thanks Rick

[Jim Ingram]

where you

rating,

Hi Andy,

This document I came across the other day would (if I'm reading it correctly) seem to disagree.

[Andy Wade]

totally surrounded in thermal insulation at all, so far as I can see.

[Jim Ingram]

The whole of the first two paragraphs??

"Consultants raised concerns that ring circuits protected by 32 amp circuit breakers and wired in 2.5 mm2 twin with earth cable installed in conduit in a thermally insulated wall might not meet the specific requirements of BS 7671 : 2001. The old Regulation

433-02-04 required that the "minimum current carrying capacity Iz of the cable be not less than 0.67 x the rated current setting In of the protective device".

For 32 amp devices this would require a cable rating of 21.44 amps. Table 4D2A in Appendix 4 reference method 4 (enclosed in an insulated wall etc) provides a rating of 18.5 amps.

Now, as contractors will know, this practice has been carried out for some time and if allowance is made for ambient temperature and the close protection provided by circuit breakers problems would not be anticipated. However, the issue would not go way so the National Committee responsible for BS 7671 looked into the matter. The Electrical Research Association were asked to measure the rating of the standard UK flat twin with earth insulated and sheathed cables. The cable ratings given in BS 7671 were not derived from the direct measurement of this particular type of cable. The rating tables in the International and Cenelec and British Standard (7671) are based on typical type tests, and in the case of table 4D2A, on round cables. The results of the ERA's work are published in a new table 4D5A. The rating of 2.5 mm2 flat twin with earth enclosed in conduit in an insulated wall (now installation method 6) was found to be 20 amps. The rating when installed directly into insulated wall (now installation method 15) is 21 amps."

It then goes on to say "After some debate the National Committee, being keen to maintain the use of the UK ring circuit, amended the basic requirement for the rating of the current carrying capacity of the cable to be no less than 20 amps."

Perhaps I'm simply confused by the terminology. If so what is the difference between being enclosed in a thermaly insulated wall and cables being surrounded by thermal insulation (I believe the original post related to walls as opposed to insulation in ceilings which might well be a different).

Jim

[Andy Wade]

'Fraid so.

It's what I pointed out at the start. The table I posted is basically Table 4D5A. It deals with two cable installation methods "installed directly in an insulated wall" (Method 15) and "enclosed in conduit in an insulated wall" (Method 6). In both cases one side of the cable or its conduit is not covered in thermal insulation, but is against studwork, allowing heat to escape. Wood has a thermal conductivity about four times higher than most insulation materials.

Where a cable is completely surrounded by insulation on all sides (and for a significant length) is a completely different situation. There's far more resistance to getting the heat out and, for any given current, the cable will run much hotter. The current rating is half the Method 1 (clipped direct) rating, pushing up the cable size required to an impractical extent in some cases, as I tried to explain.

[Jim Ingram]

Thanks Andy. Your original explanation was fine. I should have read your first post more carefully.

Jim