I have put heavy duty plastic conduit up inside a new stud wall from floor to ceiling on the 1st floor, to supply power to the loft (2nd floor) when that is converted in the future. The wall is 3 inch timber with plasterboard 1 side, cement board the other. So the wall is 4 inches thick. But I just thought - do I need metal conduit ? There's not much point in putting this in if I can't use it (according to regs) in the future ! Cheers, Simon.
Since it seems likely your cable(s) won't be buried more than 50mm from the surface, you'd better fit a socket vertically in line with the conduit, thereby giving others a clue that cables are likely to run horizontally or vertically to reach it, and therefore you don't *need* to protect the cables at all to comply ...
What's the point of the plastic conduit inside the wall anyway? I can see there might be a case for protective, earthed metal conduit, but why plastic?
I'm never quite sure whether it's OK to have cables passing down the middle of a 100mm wall unannounced - given that nominally they are 50mm from either surface - so in practice I do as Andy B says and always make sure they run in line with an accessory, for the avoidance of any doubt.
David
Unless the conduit run is within 150 mm of a corner, in which case it's OK. Also if there is thermal insulation or sound deadening material in the stud wall the conduit should be firmly cleated to a stud, so as not to become totally surrounded in such material.
As to cable current ratings, installation ref. method 6 applies if there is thermal insulation, so (for example) the rating of 2.5 T&E is just on the 20 A limit for a ring circuit, before any grouping factors are applied for multiple cables. Is one conduit enough? What are the OP's intended circuit arrangements, or will this be a distribution circuit to a separate disboard?
I thought that "short" lengths of conductive mechanical protection need not be earthed.
Plastic conduit would at least allow cables to be threaded through in the future.
You may recall at one time the regs said all conductive mechanical protection must be earthed. Sadly all it meant was that plastic conduit and sheathing was used instead, one of the worst rules the IEE ever devised.
The IEE saw the light and the next regs said that short lengths of metal mechanical protection need not be earthed but not sure if this has been carried over to the recent regs.
'cos he wants to put the cable(s) in later.
That's not OK because they cannot be 50 mm or more from both surfaces.
Yes, sorry!
David
I put 2 x 25mm conduit to take wiring for ring main and lighting. It looks as if I might be advised to change it to metal. A socket there would be a strange place at the top of the stairs. ** Anyone know good suppliers of metal conduit ** - I only need 3 x 3m tubes, 25mm out side diameter. I'll put in an extra stud to make sure the conduits cannot be surrounded by insulation, since it would be a pain to drill more holes. Simon.
With 100mm studs, and 1/2" PB and skim each side (i.e. a 5 1/4" thickness wall) then you can just about manage it assuming your cables can pass through a 1/2" hole in the centre of the studs.
Yeah. But I guess with conduit the wires are slopping around inside, you cannot prove this. One side will be tiled so the overall thing is actually even thicker. Cheers, Simon.
Oh, OK - I was interpreting "100 mm wall" a bit too literally, I suppose. But the same construction except for 75 mm studs would give about 105 mm wall thickness and then going outside the safe zones wouldn't really be on.
Any electrical wholesaler will be able to supply steel conduit and fittings. You'll need to bush the ends to protect the cables. Earthing is needed - you can use ordinary BS 951 earth clamps, but these need to remain accessible for inspection and testing. Otherwise you could do something like terminating one or both ends of the conduit runs in steel adaptable boxes, and use an M4 or M5 screw, nut and lockwasher to make an earth stud inside the box, to which you connect using a crimped ring terminal. The earth needs to meet the requirements for a circuit protective conductor for the largest circuit involved, but in this case the size will be the minimum allowed for a separate non-mechanically protected run, i.e. 4 mm^2. This should run alongside the circuit cables and could pick-up the earth at the nearest wiring accessory (or go back to the consumer unit, if more convenient).
From what you've said there will only be three cables, 2 ring legs and lighting. The ring legs will need one conduit each, unless you use 4 mm^2 cable. I can see two options for the lighting cable: (i) a separate conduit, or (ii) put it in with one leg of the ring.
Option (ii) saves a length of conduit, but is it allowable? If the design current in the lighting cable is less then 30% of its grouped rating, then it can be ignored for the purpose of calculating the rating for the rest of the group - i.e. the ring cable in this case. Since no derating for grouping can be applied for the ring, as I said last time, this means that this option will only work if we can keep the lighting cable below the 30% limit. The "design current" means the highest normal load current that can flow in that cable, with all the lights on and with at least 100 W lamps in all the general-purpose lampholders. For fluorescent lighting take the current demand as 1.8 times circuit_watts divided by 230.
If the lighting cable is 1.0 T&E its method 6 rating is 11.5 amps, derated by a grouping factor of 0.8 (two cables, enclosed & bunched), i.e. 9.2 A. 30% of that is 2.76 A, i.e. just over 630 VA loading at
230 V. For 1.5 T&E cable the corresponding calculation is 14.5 A * 0.8A single run of 2.5 T&E will go through 20 mm conduit quite happily, especially if it's a dead straight run. For option (ii) I'd suggest using 25 mm for the leg which shares with the lighting cable.
HTH
Why can't both ring legs go into one 25mm conduit ?
I can see two options for the lighting cable: (i) a
I also wanted space for a 10mm^2 cable (fits in 25mm plastic conduit at least) for possible electric shower in loft. Sounds like if I put in 3 x 25mm conduits in, that should do. Cheers, Simon.
That is in the current regs. [471-13-4(v)] but it's not applicable here:
(a) it only applies to inaccessible lengths not exceeding 150 mm;
(b) reg. 522-06-06 dealing with cables concealed in walls specifically requires such protection to be earthed.
In general, metal conduit containing only insulated and sheathed (T&E) cables need not be earthed unless required by 522-06-06. (See 471-09-04.)
Because the as-installed cable rating for a ring must be 20 A or more at all points. For two 2.5 T&E cables together the grouped rating would only be 16 A, so no good. Even if there's no thermal insulation in the wall this will not work.
Yes, and the 10 mm^2 cable will only have 44 A rating in these conditions, so you'll be limited to a 40 A circuit (9.2 kW).
OK, I'm getting annoyed now ;-) Thinking I might forget about the conduit. With the insulation around it etc, I'll be able to get virtually nothing up into the loft ! What would be the recommended way of getting a 10.xkW electric shower cable from the consumer unit into the loft in a typical house. Surely it will be buried somehow ?
16mm^2 - I don't think so ! No previous house I have been aquainted with has followed the regs it would seem. And if I stick a dummy power socket in the vertical line with all this, I won't need conduit, ratings will be up, inspector will pass it - is this all true ? If so, I'll do that ! Simon.
Straight in the wall in a safe zone (corner), usually. Rating for 10 T&E with no thermal insulation is then 64 A.
A dummy socket might be frowned on...
Can you not make use of the corner safe zones?
Or two/three conduits for ring & lighting as discussed and a separate 10 mm^2 cable in a corner zone for the shower. 10 T&E clipped straight to a stud in an insulated wall gives 47 A rating, which would allow a 45 A circuit and a "10.8 kW" (at 240 V) shower. Or put a genuine working socket on the ring in line with the shower cable.
Another option for the shower would be to use LSF thermosetting cable (type 6242BH) - like ordinary T&E, but with a white sheath - and that gives 57 amps rating in the conduit. Then you're back to three conduits, one sharing lights and ring.
Are you having a bathroom extractor fan, which will need a 3-pole isolation switch - typically mounted high up on the wall out of the way: if so, would it be a good place to position that, in line with your conduit?
ISTR some discussion here recently about whether a simple blanking faceplate would count as a wiring accessory for this purpose: is that better or worse than a dummy socket?!? If so, that could also be positioned right up high out of the way.
David
David
Thanks guys. I've got 2 x 25mm galv conduit up through the wall, and I've got some spare 10mm TWE from the electric shower install in the last house, so I'll cleat this to a stud inside the 150mm safe zone for later use. I'll have to blag them wanting to see it exposed if that comes up later, when I do the loft. Simon.
Take some photos now just in case...
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