Mindful of Part P and doing everything by the book...
In the past, when running wiring inside stud partition, I've always drilled a hole through the middle of the ceiling and/or foot plates of the wall, and just dropped the cable down to middle of the partition to the switch or socket; traversing any necessary studs or noggins via similar holes in the middle, all cable-runs being at 90 deg of course. This is certainly how any pro-installed wiring I've ever seen has been done.
However, a read of Whitfield would indicate that this is unsatisfactory because the cable is vulnerable to damage from nails etc the other side of the wall, being
Well let's not get into the rights and wrongs of what thickness a stud wall should be, but you must agree a lot of people, me included, build them with 3"x2" (ie 75mm studs). CLS would make them even narrower.
Where there is any risk of cable damage/contact with live conductors a in this case the wiring should be run in metal conduit which is bonde to earth, most houses built in the late 50s and after with stud wall have the cables run this way although not all were earthed
The cable where vulnerable as in this case should be run in eart bonded metal conduit. The metal plates you refer to were usuall conduit nispection box lids or better short pieces 3 - 4" of condui nailed into the joist. The regulations state that due consideration t the cable being vulnerable from the other side of stud walls should b taken into consideration when running cables, as it may not be obviou that there is a cable on the other side to that where the switch o outlet is situated, hence the conduit. Under Part "P" any new circui must be installed by an approved electrical contractor or you mus notify the local buliding inspectorate before the work is carried ou and the circuit must not be energised until tested and inspected by th inspectorate or their designated agent
Official clarification on this issue has arrived fairly recently in Amendment no. 2 to BS 7671 (this is the amendment that's mainly concerned with the new wiring colours). Basically, if the location of switches and sockets etc. can be determined from the other side of the wall then there's no need for special precautions - the common-sense assumption is that before you attack a thin wall with your drill, etc., you should check _both_ sides for probable cable routes. If, however, the other side of the wall is not readily accessible for checking then any wiring must be more than 50mm deep (measured from the 'far' side) or else must be protected, unless in the usual corner/ceiling zones. NB the 2004 amendment applies specifically to walls of 100mm thickness or less.
There's an IEE /Wiring Matters/ article on the subject here:
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and verse is as follows:
*522-06-06* A cable concealed in a wall or partition at a depth of less than 50 mm from the surfaces of the wall or partition shall:
(i) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or
(ii) be of insulated concentric construction complying with BS 4553-1, BS 4553-2 or BS 4553-3, or
(iii) be enclosed in earthed conduit, trunking or ducting satisfying the requirements of these Regulations for a protective conductor, or be mechanically protected sufficient to prevent penetration of the cable by nails, screws and the like, or
(iv) be installed in a zone within 150 mm from the top of the wall or partition or within 150 mm of an angle formed by two adjoining walls or partitions. Where the cable is connected to a point, accessory or switchgear on any surface of the wall or partition, the cable may be installed in a zone either horizontally or vertically, to the point, accessory or switchgear. Where the location of the accessory, point or switchgear can be determined from the reverse side, a zone formed on one side of a wall of 100 mm thickness or less or partition of 100 mm thickness or less extends to the reverse side.
(Former Regulation 522-06-07 is deleted, since its meaning is now incorporated in the above.)
No, I don't think they are often used, and can't see where you'd usually need them for new work. They could be useful though for adding protection to old wiring (pre 14th Ed.) where cables under floorboards were often run through shallow notches in the joists.
Great, thanks for that - nice to see a 'common sense' approach prevailing!
When you say 'new work' does that include rewiring? Just that whenever I've undertaken any rewiring, I always come across plenty of notched joists (without protective plates!) and when running new cable I always make use of whatever notches or holes are there already... it doesn't seem a good idea to potentially weaken joists by drilling holes through them, where notches are already present. Incorrect strategy?
Yes. It's definitely incorrect to use shallow notches, unless you're adding mechanical protection - which requirement the OSG helpfully states is "difficult to meet" - or wiring in steel conduit. See Reg.
522-06-05 or page 52 of the OSG.
You can make use of existing holes that are more than 50mm away from the top or bottom of the joist, otherwise drill new holes on the centre-line of the joist (the neutral axis). A cable size hole will have negligible effect on the strength of a joist, but don't drill within 100mm (vertically) of any existing notches and keep adjacent holes at least 3 diameters apart. If in doubt stick to the usual guidelines and drill only in the prescribed zone between 25% and 40% of the joist's span.
[When you say 'new work' does that include rewiring? Just tha
whenever I've undertaken any rewiring, I always come across plenty of notched joists (without protective plates!) and when running new cable always make use of whatever notches or holes are there already... it doesn't seem a good idea to potentially weaken joists by drilling hole through them, where notches are already present. Incorrect strategy?
David Yes incorrect strategy for sure. the drilling of a 20mm hole in th centre of the joist does nothing to weaken it but a notch is a stres raising feature. the joists are well rated for strength as well. the hol should be 50mm down from the top in a joist larger than 100mm deep or i the middle if 100mm. Replacing of an existing wire is not new work bu you should use holes for safety if you are doing it. Likwe anything wit the no protective plates, two wrongs don't make a right. In the stu walls if its a light switch near a door opening I tend to take the vie that it is in a defined cable zone and visible from both sides. If in th middle to a socket and not readily visible from either side ( i.e. straight wall with doorway) I would at least dorp a length of condui down , drill the holes 22mm and it goes in easily at least close to you socket back box, this will prevent accidental nailing or screwing throug the cable. Hope this helps
Just to be clear, then, does that mean that if I come across notches, or badly positioned holes, in joists which were made back in 1953 (since which time the house has stayed standing!) then not only can I not make use of these notches/holes, but I also have to (potentially) further weaken said joists by drilling new holes through them(!)
If replacing any cable so as not to fall foul of Part P, you must use the same cable route and not modify it. Therefore drilling new holes etc in joists is a no no. I believe the IEE and NICEIC call it progress.
At the centre of the joist I might agree with you where the joist top and bottom edges are in compression and tension respectively and the centre merely hold the top and bottom edges apart. However at the joist ends the principle stress is a shear stress where any removal of timber will increase stress irrespective of whether it is a notch or a hole in the centre.
That's a nice enterta SCHEDULE 2B Regulation 12(5)
DESCRIPTIONS OF WORK WHERE NO BUILDING NOTICE OR DEPOSIT OF FULL PLANS REQUIRED
Work consisting of -
(a) replacing any socket-outlet, control switch or ceiling rose;
(b) replacing a damaged cable for a single circuit only;
(c) re-fixing or replacing enclosures of existing installation components, where the circuit protective measures are unaffected;
(d) providing mechanical protection to an existing fixed installation, where the circuit protective measures and current carrying capacity of conductors are unaffected by the increased thermal insulation.
1(b) doesn't say anything about the route that the replacement cable may or may not take, and 1(d) does allow you to improve mechanical protection without having to submit a building notice. IMHO installing a new cable in old shallow joist notches would be negligent.
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Just to be clear, then, does that mean that if I come across notches or badly positioned holes, in joists which were made back in 1953 (since which time the house has stayed standing!) then not only can I no make use of these notches/holes, but I also have to (potentially) further weaken said joists by drilling new holes through them(!)
David No you cant use the notches unless you make them deep enough to contain
4inch long piece of metal conduit which is fixed with a saddle, whic would mean more notching out. As I already said a 20mm or smaller hol will not in any way significntly weaken the joist. Because it is a roun hole and it is located centrally in the joist which is not a linea stress area! The reason for not using notches has as much to do wit electrical safety as weakening of the joist. Both Andy and I have given you good and correct advice why argue with it
Appreciated; and I'm not arguing, just seeking clarification (and if you look back, another poster actually specifically said I should *not* drill holes).
This work is probably going to be checked by the Part P police (now that I've missed the 1 April deadline ;-( ) so you can be sure I want to do it by the book; but I don't want to make extra unnecessary work for myself either.
Having taken on board what everyone's said in this thread, I think my strategy is going to be to drill holes for any new cable routes (as I originally intended), but where there are *existing* notches I'll probably make use of them, but protecting the new cables using these things:
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which fulfils the regs perfectly AFAICS. If I was to drill new holes instead, I would not want to put them too close to existing notches - common sense says that's a bad idea
- so would have to be pulling up more T&G floorboards to access new bits of joist; all of which sounds like extra unnecessary work to me.
Andy, many thanks for the post. I live and learn and was going by "expert" opinion rather than what the law specifies. I had believed the cable had to follow the same route presumably to have the same thermal rating as the original. I can't recall where this was.
It appears you may add a socket to an existing ring or a spur. You may also replace a single circuit whatever that means mind you may have to damage it first - LOL. All in all apart from working in a kitchen or a "special location", is there any reason why part P should stop any work.
It's also interesting that the act specifically uses the 16th edition of the wiring reg. I wonder what happens when the IEE comes out with another edition to bump up their book sales.
I still find it amazing I can no longer fit an RCD to my own wiring but have to put it off until I have sufficient work to warrant the expense.
[Great! that's good news as I thought I was going to source these
specially from TLC.
[Still never seen them used in practice though! :-) ]
David I agree you dont see many. I hadn't noticed screwfix selling the either. As far as the holes Fred, they actually specify the position of the hole relative to the run of the joist taking into account the loads. But eve close to the wall it is still a bending moment not a shear stress, yo would have to impose all the floor load at a point in a vertical plan right against the wall, which doesnt happen in reality. You wouldn' normally be trying to run cables that close to the wall. Interesting though and I hope our friend is now happy to proceed with hi wire replacemen
If we were talking of a cantilever then yes there would be a bending moment, but for a timber resting on a wall plate or masonry there just can't be one and hence there's just a shear stress. As you move towards the centre of the room then the bending moment becomes the dominant cause of stress with a reducing shear stress.
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