Thin JB/backbox behind bathroom cabinet?

[Donkmeister]

We're building a new bathroom and as part of that bought a mirror cupboard with demister, lights etc in it. Stupidly, I didn't unbox and inspect it before getting the walls tiled.

The instructions show the cupboard's fixed flex being wired in to a flex outlet behind the cupboard. That makes sense to me, apart from the flex on the cupboard is about 40 cm long and at the top of the cupboard hence it would be a bit precarious to wire it in before fitting it.

The problem I have is that cutting a square hole to fit a backbox would be a bugger as the wall is tiled. Not impossible, but not ideal. I am having this inspected by BCO as part of a building notice, so compliance is important also.

I have 12mm of clearance in the recess in the back of the cupboard for any surface-mounted solutions.

Any thoughts?

 

[JohnW2]

The problem I have is that cutting a square hole to fit a backbox would be a bugger as the wall is tiled. Not impossible, but not ideal.

If the wall is already titled, how would you get the supply cable to the backbox even if you could fit it? - or, more generally, how is the supply cable getting to the vicinity of the cabinet?

Kind Regards, John

 

[Donkmeister]

If the wall is already titled, how would you get the supply cable to the backbox even if you could fit it? - or, more generally, how is the supply cable getting to the vicinity of the cabinet?

Kind Regards, John

Fortunately, it's a stud wall and the other side has not yet been skinned. So I have full access to the space behind the plasterboard.

 

[flameport]

The problem I have is that cutting a square hole to fit a backbox would be a bugger as the wall is tiled.

Not really - a few minutes with a small angle grinder and diamond cutting disc.

 

[Donkmeister]

Not really - a few minutes with a small angle grinder and diamond cutting disc.

Heck of a mess though - either wet slurry or shedloads of dust. I can do it but I'm trying to avoid it.
Drilling a hole is much more contained, so if there is a connection method that will be suitable, within 12mm of thickness, requiring just a hole for a cable then that is preferred

ETA I'm trying to avoid stitch-drilling a square hole also.

 

[JohnW2]

Heck of a mess though - either wet slurry or shedloads of dust. I can do it but I'm trying to avoid it.

Thinking a little laterally ... if this is all going to be concealed behind the cabinet, could you not remove an entire tile, complete with the plasterboard it is fixed to, then stick a piece of plasterboard with a hole br a backbox in it, onto the back of the plasterboard where you had created the hole?

That would also have the advantage that the surface of that new bit of plasterboard would then be below the level of the tiled surface by an amount equal to the thickness of plasterboard + thickness of tile + thickness of adhesive - thereby probably more than doubling the 12mm behind the cabinet you currently have for an accessory?

Kind Regards, John

 

[sparkwright]

Since the op doesn't want to disturb the tiling, one option could be to line crimp the cables together, with heat shrink (NOT tape) sleeved over the connections.

By 'staggering' the crimps, the joint will be slimmer and should fit in the 12mm gap.

 

[JohnW2]

Since the op doesn't want to disturb the tiling, one option could be to line crimp the cables together, with heat shrink (NOT tape) sleeved over the connections.

He could. However, as you will realise, my thought was that the OP might not be too concerned about 'disturbing the tiling', if it is to be hidden behind the cabinet, his main concern about the mess that he would create by cutting a square hole in a tile. What I suggested would avoid most of the miss.

What you suggest would, of course, avoid any doubts about whether the screwed connections in an accessory would be sufficiently 'accessible' to be compliant if they were behind a permanently installed cabinet.

Kind Regards, John

 

[ban-all-sheds]

It would be OK if they were accessible form the other side. A blanking plate there would also create a zone for the cable to run in. Nobody seems to have considered that if it's going to be wired from the other side, a cable might end up <50mm from the surface on that side.

 

[JohnW2]

I imagine that the wiring probably won't be accessible from the other side once the plasterboard and skim/whatever is installed. As for wiring being <50m from 'the other side', that is always a risk with thin stud walls, and something that is rarely considered. One might note that 18th-compliant lighting circuits will have to have RCD protection, and I suspect that installing a new illuminated cabinet would probably invoke that requirement for the whole circuit.

 

[Donkmeister]

The crimps + heatshrink idea is a good one - I have proper ratchet crimpers, heatshrink etc.

I'm happy to have a blanking plate on the other side of the wall, and that would actually make life really easy as I wouldn't have to find a way of supporting the cabinet 4 feet off the ground whilst making connections before hanging it - just hang it to the wall, then pop into the bedroom next door to terminate and connect. I think BAS's idea is the one I'm going to run with.

Thanks all!

 

[ban-all-sheds]

The 18th doesn't apply yet. But bathroom circuits needing RCD protection has been around for a while

 

[ban-all-sheds]

just hang it to the wall, then pop into the bedroom next door to terminate and connect.

Surely, (with a helper if necessary), feed the cable through a hole in a tile, hang the cabinet, and then go to the open stud wall side and connect it there?

 

[JohnW2]

The crimps + heatshrink idea is a good one - I have proper ratchet crimpers, heatshrink etc. ... I'm happy to have a blanking plate on the other side of the wall, and that would actually make life really easy as I wouldn't have to find a way of supporting the cabinet 4 feet off the ground whilst making connections before hanging it - just hang it to the wall, then pop into the bedroom next door to terminate and connect.

If you put a backbox and blank plate (or even an 'isolating switch', if you want) on the other side of the wall, then there would be no need to have crimped connections. The cable from the cabinet could then just go through the wall and be connected to the supply cable using some sort of conventional connector block or Wagos (or even switch, as above) behind the blank plate.

Kind Regards, John

 

[EFLImpudence]

I wouldn't have to find a way of supporting the cabinet 4 feet off the ground whilst making connections before hanging it

How, then, is it supposed to be connected?