Wiring cables *through* a cavity wall

[Lobster]

I need to put a ring main in a small extension to the main house (currently it has no power). The original outside wall to the house is a cavity wall, and the flat roof of the extension meets the house wall below the level of the ground floor ceiling. Floor in both main house and extension are solid, and all the wiring is above the ceiling therefore.

So how should I get the cables across the cavity and into the extension? There is no way through other than through the cavity. Should I pass it through a conduit of some sort to protect it within the cavity? Any thoughts (including what the regs say on this) much appreciated.

Thanks David

[Dark Angel]

IIRC when passing cables through a cavity, you should always pass them through a conduit of some sort to protect them in the event something should drop down the cavity and damage them.

Incidentally, I'm sure you're aware that if you're extending your ring main, or putting in a complete new circuit you have to get it checked by your local council's building control office now?

[andrewpreece]

There was a thread not so long ago that implied that cables run through cavities did not need to be within 6" of the sides or ceiling, as with buried cable. Also, with buried cables, they need to be 50mm deep in a wall if unprotected, I'm not sure how this relates to cavity walls, if you can keep the unprotected cables >=50mm from the surface this might be OK???

Andy.

[Lobster]

Hmm, not sure. Just to clarify, what I'm suggesting has the cables passing through the cavity perpendicularly; ie, through one leaf, across the gap at 90 degrees, then through the second leaf.

David

[AlexW]

I have had 2 conservatories built, 1 this month. "Pro" sparks wired both.

First the cable bridged the cavity unprotected to get the feed into the conservatory and was run inside the cavity unprotected.

Second, similar although the walls are rubble filled (more likely for things to drop eh?).

I am no expert and have no clue about the regs, but realistically whats going to fall on the cable? insulation? wall tie? snot of mortar?

AIUI most hazardous is the insulation (if EPS ... posted thread a while back on EPS & cables) ... I asked a spark I know about this and his opinion was that it would not cause a problem unless the circuit was seriously loaded causing the cables to heat.

HTH,

Alex.

[AlexW]

Also same spark I know has just had a conservatory too and put the the feed through the cavity in that manner (as described in the same conversation about EPS). He is part peed up with the NCA and all that. We had had several pints so this info may me err blurred.

[nightjar]

The easiest way is to drill right though with a 20mm x 300mm masonry drill, then push a bit of 20mm conduit through the holes. The conduit, being rigid, will go into the far hole more readily and then you only have one long hole to feed the cable through.

Colin Bignell

[Lobster]

Sounds good, thanks.

Thinking further on this: presumably this must arise everytime someone installs an external light fitting where there's a cavity wall. I wonder how many sparks do anything other than poke the cable straight through, as others have suggested.

I suppose one issue to consider is what happens if someone wants to install cavity wall insulation at a later date... if the cable is unprotected within the cavity, it will likely come into contact with polystyrene insulation, causing instant disintegration(?) of the outer sheath...

David

[dennis]

The plasticizer causes the polystyrene to shrink away from the cable so it breaks contact. Assuming your polystyrene is bonded together like it should be.

Mines been OK for the last 25 years, I'll let you know if anything happens.

[PC Paul]

Isn't PVC wiring only supposed to be 25 year life anyway?

[manatbandq]

Insulation can be small polystyrene beads. They might move to take up the gap, causing continuous contact.

MBQ

[dennis]

That's what I have. It should be bonded using PVA when its blown into the wall.

[dave stanton]

Where did that come from ?

Dave

[AlexW]

Ooopss...

ECA not NCA, slip of the fingers, step dad involved with caving you see....