40mm corrugated conduit for cables in cavity wall?

[clowes_ian]

I'd like to shroud the mains cables (about a dozen mixed 2.5mm and

1.0mm) passing through a cavity wall prior to the installation of blown/pumped cavity insulation. I sense this may be an awkward task in itself , but want to simplify the upcoming rewire by having a clearer path for the cables, and not have to worry so much about derating them all.

The stuff that will be used is apparently some kind of silicone/silica coated fibre that will 'pack'. Would it be better to let them do the job and then just pull it out from that section that the cables pass through?

Plan A is to demount the cables from the CU, push a suitable size of conduit over 3-4 cables at a time, and feed this through the first leaf, then the cavity (about 300mm vertical offset :-() and then the hole in the other brick leaf. May then pop a bit of expanding foam around the outsides to finalise filling and secure all in place. And finally reconnect the cables to the CU.

But I can't find a supplier of small amounts (say 5m) of suitably sized flexible conduit (I have 35-40mm in mind).

Can anyone propose an off-the-shelf supplier, or a different plan?

TIA IanC

[js.b1]

Can you not run the cables up the wall in trunking?

As in 25x16mm & 40x16mm trunking extending across the full width of the consumer unit to the upstairs floorboard area. Feed cables up through that - use 1 trunking per cable if 6.0mm or 4.0mm, or combine a 1.5mm which if loaded to 30% of grouped rating can be disregarded re grouping factors. Plaster skim, done, wallpaper/paint.

If you must go down the flexible conduit in external cavity route, you can find 32mm flexible conduit on Ebay quite often. You would need to use 2-3-4 pieces to get sufficient space to draw in a few cables. Failing that there is RS and perhaps an electrical factor although round here they would have a fit if you told them what it was for :-)

I assume your cables are PVC (BS6004 70oC) and not say LSOH (BS7211

90oC) because the sheath on LSOH does not like being drawn across another cable - it abrades like a hard cheese. PVC is more tolerant of such handling, LSOH (eg, white sheath since about 2005) is not.

[clowes_ian]

The CU is in the garage, the roof of which is below the 1st floor level, hence the stagger of the holes across the cavity.

There are two phases to this:

1) Insulation will be installed. I could just leave everything as it is for now, as long as the cable outer is OK with the insulation material and I ignore the possible thermal derating issue (the only moderately heavily loaded cables are the 2.5mm supply to immersion and kitchen for dishwasher). 2) Install new wiring and CU: At this point I can try a bit harder to find a new route, although this is tricky due to positioning of soil pipes on the same wall, solid floor and finish surfaces on opposite side of wall. Only 'obvious' improvement is to go through the garage roof, and straight back in through the wall at 1st floor cavity level. I'm starting to suspect I'll try the 'pull insulation out of wall' approach...

[<me9]

I think you'll find it almost impossible to get 35mm flexible conduit up a foot through an average width cavity unless you remove several bricks on each side. Unless the cables are well overrated the grouping factor will be likely to derate the cables to below protection level.

[js.b1]

Heh-heh, I know this problem.

What is on the other side of the wall?

- Go straight through the wall through an off-the-shelf oblong brick- sized ventilation duct. You would need to add an intumescent "letter- box" pillow, quite expensive but for garage-to-house regs will probably require it anyway.

- Then go up the wall by nail-clip/screw-p-clip to the wall

- Cover with a 2mm stainless or 3mm galvanised steel sheet re

526-06-06 - earth with 4mm 6491X back to the CU

The earth strictly needs to be accessible, this can be achieved by cutting the steel sheet so as to leave a protruding tab which enters an architrave box or socket. You need to file the edge of the box to the thickness of the sheet plus a little so it just enters. Drill the tab for M4 and bolt a 4mm ring crimp to it, earth accordingly, done. BS7671 correct, inspectable, maintainable :-)

Cut 20mm conduit to make spaces for the steel sheet, screw into place, cover in glass-fibre mesh tape, plaster over. It will ring like a bell if someone whacks it, so pretty obvious what it is.

If you lack a crimp, for a one-off-job just cut the yellow plastic off a ring crimp and solder with a soldering iron or blowtorch carefully (180-230oC). It's a mechanical job and once done you have a service panel for the future - stainless will never corrode, if you use galvanised it is cheaper but requires edge-treatment or will bleed rust through plaster. Check Ebay for sheet metal prices, many will cut to size, as will some local fabricators - just depends where you are.

Forget going up the cavity, it is a non-starter. I would not replace the CU (unless damaged, you can replace any enclosure). I would not replace the cables (unless damaged, you can replace a circuit).

[clowes_ian]

In that case I suspect I'll just let the insulation installation crew do what they think is right. They seem to believe that there is no problem, and that my proposals are OTT.

I'll endeavour to address any issues during the rewire.

IanC