[?] Installing recessed ceiling lights with no easy access from floor above.

In article , Dave Chapman writes

Not quite as scary as you might think.

Best to map out the approximate positions of your joists with a stud finder and choose provisional positions for you downlighters.

Start cutting your holes and check with some probes (cable fishing rods or some fence wire) that there are no obstructions and that your chosen positions will work.

Working from your downlighter holes, probe to find the exact positions of your joists and mark the surface. Then cut a slot in the PB or lath/plaster just wide enough for 1.5mm2 T&E and just long enough to bridge the joist.

Simply fish your cable through the joist bridges instead of drilling joists. Then simply fill and finish, the T&E will be a few mm under the surface.

Alternatively, you could cut a small notch in the bottom of the now visible joist just big enough to accept the cable leaving a hole that a bit of PB could be used to flush patch but I wouldn't bother as the hole described above should be small enough for an easy fill with polyfila.

Have done this invisibly a number of times on ornate corniced ceilings.

I'll leave the recommendations on lamp types to others as I don't use these myself.

You do know that neither method is allowed by the regs don't you? If the OP wishes to meet the regs then you would need the cable to be more than 50mm from the bottom of the joists or use one of the approved cables with an earthed metallic covering

For a little extra work the OP could cut a hole at the joist that needs to be passed - say a 100mm diameter hole so that there is access to either side of the joist, use a right angled drill to put a hole through the joist at the correct height and then screw the piece of plasterboard back into place before making good the damage.

On Thursday, 3 April 2014 18:46:34 UTC+1, snipped-for-privacy@blueyonder.co.uk wrote :

Could you elaborate on what the regs are for ceilings Adam?

Reading from the wiki

Mathew

Could you elaborate on what the regs are for ceilings Adam?

Reading from the wiki

Will do a proper reply later but I am going out tonight.

The regs for cables passing through joists are very similar to cables in walls but have no safe zone. It's reg 522.6.5 that I shall copy and paste:-)

I've got a very similar problem for my dormer roof ceiling. I might just use steel conduit for that, buried in a layer of celotex as it will literally be 2 straight sections screwed into a 90 bend and a besa box on each end. The hardest part will be threading it - no bending.

The wiki does give details of how joists should be crossed, it's in the electrical installation section. A possible solution is also suggested, namely notching with the notch protected with a 3mm thick covering plate.

Richard

I'm a little curious about the whole ceilings thing. I'm not going to argue with Adam as he's a fine man who knows his area.

But - what about all those installations where lighting wiring is pulled though end to end and left sitting on the top of the plasterboard (because there's not enough access to clip it to the side of the joists), ie 12mm give or take from the surface of the ceiling. It's quite common...

You can do it from below...how easy it is depends a bit on the size of the cutout you need for the new lights.

If the cutout is large enough, then you can sometimes get an angle drill or other small drill into the hole.

e.g:

With smaller holes you may find a flat bit and a couple of flat bit extensions (each 12") will let you get in at a shallow angle and get a hole into the joist each side of the hole.

The remaining way is to use an oscillating tool (or less elegantly a widish spade bit to make some overlapping holes) to cut a rectangular section of ceiling out under the joist - say 75mm by 10mm - just enough to expose the full width of the joist and a small gap either side. The cable can then be threaded down through the gap, under, and back up the other side, before filling the hole.

If going halogen then definitely 12V. For LED, whatever works out most cost effective.

While not a perfect solution, something like:

can make a poor situation better.

From the 16th edition version:

(i) be at least 50 mm measured vertically from the top, or bottom as appropriate, of thejoist or batten, or

(ii) incorporate an earthed armour or metal sheath suitable for use as a protective conductor, or shall be of insulated concentric construction, or shall be protected by enclosure in earthed steel conduit securely supported, or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like (see also Regulation 471 -13-04(v)).

Its much harder to nail into something that is just resting on the top of the PB (it just moves) than it is to nail into a wire that is through a joist but too close to the surface. The latter case especially since you would probably be aiming to hit the joist.

Note that in the Reg Adam mentioned, there is a get out clause at the end: "or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails" that basically means if you can protect it mechanically, then closer than 50mm from the surface is OK.

Ah - I see. It really IS just the joist then, not being >50mm from the PB surface. Been trying to get my head around the thinking behind that one. Originally I thought the "50mm" was more to do with not weakening the joist.

My dormers are awkward:

The only sane way to get conduit in is perpendicular to the rafters, then smooth bend down the dormer wall until it intersects with the hipped roof - then it is in open attic.

Conduit will be fixed to rafters and celotex butted either side.

So I guess I could put flat steel plates over where the conduit crosses the rafter, spaced off the rafter with tube-spacers/small-blocks/whatever.

Obviously the wall can be dealt with with a dummy accessory plate to give a zone.

Hmm - that makes things much clearer... Ta.

Tim

My lack of understanding might've be along the same lines. I didn't realise the 50mm depth was specifically regarding *joists* and thought that it was being suggested to apply to the whole ceiling i.e. in between the joists a lso. I was scratching my head because of the amount of cabling I've seen si tting on top of the plasterboard and figured they can't all have been cowbo y jobs.

It makes sense that, as has been mentioned, it is the joist region that wil l be attracting screws/nails and that any such fixings between them would l ikely just push the cable to one side.

Mathew

In article , ARW writes

Not saying I would give flying foobar either way though.

Does that apply to RCD protected circuits too? If so, more onerous conditions than in a walls then? Quite funny as I suspect walls are far more likely to be pierced for fixings than ceilings.

For me that would involve bashing through lath and plaster, breaking through the ash deadening subfloor (getting showered in ash in the process) and then drilling through. No marks for guessing which way I'll be doing it but at least I wont be running on diagonals across the ceiling.

Now this is a slightly odd one... it would appear the short answer is Yes.

reg 522.6.6 basically restates the clauses of 522.6.5 but for cables in walls, and then adds on the the extra permitted "safe zones" (i.e. at wall corners, and ceiling intersection etc).

The bit about RCD protection is then in 522.6.7, that basically says if you are not compliant with 522.6.6, then you need additional protection from an RCD as specced in 415.1.1

However I can't see any equivalent section referring back to 522.6.5

(there is also 522.6.8 that applies to walls with metallic components - that basically says treat it as if the cable is less than 50mm depth regardless of how deep it actually is)

Such as external stud walls full of foil faced celotex?

Or do they mean structural metalwork, like girders and metal studwork?

It's interesting that most of this discussion has addressed the wiring prob lems, which are probably the greater as the question of mains versus 12v is spurious if you can't get the wires in! Only one contributor made comment on the supply and he I disagree with.

From an electronics engineer's point of view, the compressing of electronic components into the lamp base to drop 240vac to 12v or less at around 0.5A is an anethema (thought this was the right word and I see it is a "thing o f abhorrence") - this suits well what I want to say. The same applied to t he CFL's we all loved and cherished so dearly and like mains LED's, it wasn 't the light element that failed but the insy-winsy bits crammed into the b ase to make them go.

Proper mains to 12v transformers in a reasonable package size cost no more and often less than an LED bulb. and will have a far greater life than any 'on-board' voltage dropper system.

I recently re-lit my hall using 20W equivalent (3W @ 12v actual) COB type - that's Chip On Board - MR16 bulbs, and the household has been impressed by the light, the spread and the total lack of waste heat. Four of these in d ouble adjustable holders has given us far better illumination than the prev ious two 50W halogens.

Rob

The later I believe...

There might be a reason for that