CAN ELECTRICIANS INSTALL CABLES IN A CAVITY WALL?

A cavity wall is often an easy route to run a cable. What should electricians consider if they are considering running a cable through or down a cavity wall?

This question was raised following an eFIXX video review of the Hamilton Elemento outdoor sockets.

Joe Robinson explores the issues raised in relation to UK wiring regulations and the potential problems relating to cable derating and polystyrene insulation.

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🕐 TIME STAMPS 🕕
00:00 Can I run cables in cavities
00:50 Will the cable be supported in the cavity
01:12 Derating with thermal insulation
01:40 Part C of the Building Regulations
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#efixx

[efixx]

This question was raised following an eFIXX video review of the Hamilton Elemento outdoor sockets.

[christastic]

Interesting subject
There are several points I would like to make on this subject.
First of all I have never ever seen a twin and earth cable other than very long inappropriately installed as overheads damaged by its own weight in thirty years in the trade ever . Saying that I would always fix cables where they can be and always on new insulations with fire clips as required.
Second I have pulled redundant cables out of cavities that have cavity insulation and other than slight discolouration to the surface sometimes as dots the cable looked and felt like new
Thirdly the insulation usually acts like a rudimentary support for short retrospective installed runs .
Fourth . I don’t think anyone would resort to this practice in an ideal world but sometimes it’s the only practicable option for shorter runs .
Finally regards breaching the damp barrier I would suggest a slight misunderstanding of the rules intention to not breach the gap with damp conducting materials otherwise you couldn’t put cavity insulation in .
Most trades seal around the holes they make .

[mrclive]

Ran about two metres of 2.5 T&E down a cavity in the last month, quite proud of the achievement. It's fully sleeved end to end in flexible conduit, so well protected. There were few options open to me, it's a basement conversion with membranes everywhere, couldn't peirce those, so down the cavity it went.

My deceased business partner did a few years for the council and dropping cables down cavities was the preferred method.

[jacobedmonds]

As with other comments. I’m interested to know about running cable across the cavity for outside lights and sockets etc.
Also really interested to know what we’re supposed to do with metre tails… every domestic install I’ve ever worked on, the metre tails come through and up the cavity to the CU. Great video as always though. Interesting to see what the regs say.

[brianpiddock]

This Is a crazy one, Plumbers always put Gas pipes through walls . I've always put a bit of conduit or oval trunking between fittings never had a problem up to now.

[johnavery]

What are the alternatives for outside lights or an outside socket - conduit down from a soffit or SWA up from the ground? Both seem overkill and the easiest, safest, cheapest way is through the cavity surely?

[carlf]

So how can the DNO run meter tails up a cavity if you’re not supposed to use it?

How do you get power from inside to outside if you can’t bridge the gap?

Seems to me that someone at building regs hasn’t thought this guidance through as that’s nigh on every property in the U.K. going against the regulations.

[peterdifolco]

I’ve come across cables run vertically in cavities many times, yes there maybe some disagreements whether this is a bad thing but it prevents people accidentally drilling or nailing though a cable .

[HeathenGeek]

i ws surprised to hear 'No' at the end of that.
My takeaway was 'Yes, but with certain considerations'.
So. . . 'Yes'.

[steveosshenanigans]

It’s a common practice in Australia when running cables between power points or outside lights

[mikeselectricstuff]

Hard to see how contact with a small area of non-moving polystyrene will cause significant damage - there's only a finite amount of chemical there to react. IMO the sole issue is de-rating the cable as appropriate to take into account the effect of the thermal insulation ( including the possibility that insulation may be added later if not there already).
Possibly the only other thing to bear in mind is that cavity insulation is sometimes added by drilling holes from outside, so perhaps worth leaving a notice on the CU of the location of any cavity runs.

[ragemachine]

Common sense will suffice in every case whether it be cables, pipes or anything else passing through a cavity, the fact is building design has always presented problems for services and cost will always be a consideration, whatever the method of installation it doesn't necessarily mean it's not acceptable? if the the building regs were that critical we wouldn't have running water, flushing toilets and drains passing through walls?

[Daniells]

The building regs are clearly referring to vertical runs through the cavity, not horizontal runs to outside accessories. Do people still actually run cables vertically through the cavity these days? It's not something I've done since starting my apprenticeship in '98, although have seen properties wired like that. Thought it was old hat and people stopped doing it for the reasons Joe mentioned, plus the fact the building regs clearly state not to which is legislation.

[ravi]

Was speaking to a technical advisor from my local DNO in Dec 21 and I was surprised to find he had no issue with meter tails being run inside a cavity. I have to run some to a relocated meter box and the only realistic route id via the cavity in some conduit (a..k.a waste pipe)...

[muzikman]

I get not running meter tails for more than 3 meters to inside, as it is hard to trace where they are, but i dont think a 300mm horizontal bit of cable to an outside light/socket is gonna be a problem. I also agree with the PVC reacting with polystyrene insulation is a problem with the chemical reaction. i suppose if its in a metal conduit or trinking its gonna be fine. dunno... 🙄

[ady]

If the building regs only allow for what you quoted, then we shouldn't even run tails through, or drill out from inside for any electrical accessory ?

[nocode]

Yeah I remember we used to drill a hole in to the cavity then try fish out a string with a weight for some outside lighting jobs I think if your only going down a few hundred mm for a pir as long as its not ramd full of glass wool or polystyrene beeds filling the cavity with insulation was the worst thing to help transfer damp between the brick skins...👍

[chrisf]

I just discovered that whoever built my 1950s home used this trick. There's a spur coming off the upstairs sockets circuit that vanishes through a hole in the first floor wall, runs vertically inside the cavity for about ten foot, and then re-emerges in the room downstairs. I was a bit surprised when I first saw this but apparently it used to be quite common. I'm planning to replace the cable because it must be 70 years old by now, and the insulation has started to crumble.

[sbwende]

In Denmark it is very normal practice to run cables in the cavity. The inner leaf is built first after which the cabling is run including clipping after which the outer leaf together with the cavity wall insulation is built.

[glenwoofit]

So every time you run a cable through a cavity to a socket or outside lights you are going against building regs by bridging the cavity. Well that's every property in the UK then.