Domestic rewire, novel ideas required on cable management

[Tim S]

Hi,

The ongoing plans of the bungalow... Will be doing a rewire under LABC notice in a few months. Part of bungalow renovations include stripping the dormer conversion back to the skeleton, so an excellent time to get plumbing and cable management right. All existing services are to be stripped out first.

I have a triangular-cross-section void running horizontally right around the perimeter of the house, minimum size about 35-45cm horizontal width and

35-45cm height. In several places this opens out into relatively large areas.

It would be nice to run all the electrical circuits principally around the perimeter void, coming along the sides of joists for drops. This avoids butchering the already butchered joists any more (of course the myriad notches and holes won't be where I want them...)

For the joist runs, clipped direct in the conventional fashion is fine.

But, what about the perimeter run? This is where novel ideas are sought.

The best I've come up with so far is wire mesh basket tray. Trunking is out due to high cost, and the excessive derating and grouping factors I'll need to apply will pretty much kill any chance of using "standard" cable sizes. I'm really not up to getting into discussions with the inspector about circuit diversity as a way to reduce the cable sizing, so keeping things simple would be good.

Ideally, it should be possible to tie the cables to maintain gaps between each cable to allow neglible grouping factor (1.00 for a single tray, BS7671:2008 Table 4C4 method 32, max 6 spaced cables). In fact multiple trays would be OK, grouping factors not much worse.

Due to the void containing hot + cold water pipes I will probably have to apply ambient temperature derating - needs more consideration, though I can probably vent the void back into the living space.

A second idea: a poor mans cable tray consisting of a vertically oriented plank of wood running around with cables clipped in horizontal runs and spaced out vertically. Almost conventional then.

Don't really want to do anything too controversial, but - well, what would you do to get neat cabling?

Cheers

Tim

[meow2222]

why do you want neat cabling if its out of view? Just lay them down.

And dont forget...

NT

[John Rumm]

Yup, same here:

figured it would make a nice easy route for retrofitting all sorts of stuff. Not only that, but there were some quite sizeable areas voided off that would make useful storage space. So I chopped a new doorway into one of the walls in an upstairs room currently used as an office, to gain access to the storage space (probably about 6' square with headroom diminishing from 5' to nothing) and the void along the house.

First job was getting a new co-ax from the chimney to the distribution point. That was going to mean a 20 - 30 foot crawl through the triangular bits! Having negotiated the first five or so feet of the small void, I decided I did not really want to find out there was no turning room after a 30 foot crawl, and reversed my (not insubstantial) frame out of the gap, and waited until my mate who is almost equal in height, but only half the width paid a visit! ;-)

He confirmed that there is turning space down there...

I think you may be worrying a tad too much. If you need to keep any cables spaced, then the occasional cable clip when they traverse a joist will be fine. On most standard circuits you have a fair bit of margin to play with when installing stuff in a "traditional" way (i.e. not sandwiched between layers of celotex etc).

Depending on you ability to negotiate the small triangular section parts you may find that carefully clipping each one is going to take too much effort anyway and pushing stuff through with rods etc is the best you can do.

[dennis]

Remember to put in draw cords as shoving stuff along a basket tray can be a pain.

[Tim S]

Thanks for taking the time John,

John Rumm coughed up some electrons that declared:

Ow.

I've got one bonus - the those triangles exist in 3 sections, 2 where the dormers "front" walls intersect the roof, and one where the staircase goes.

The rest potentially potentially huge voids, which, like you, I want to reclaim for some storage (the fact the floor reinforcement stops and the floor thickness returns to 4" isn't an issue). I might pack the 4" upto 8" in places so the cupboard floor is level with the main floor, but I digress/

The bonus is: No way can I get into the triangle voids, but as I'll be stripping the plasterboard and glass wool out and relining, I'll make sure those sections have removable panels for access from the room.

Possibly worrying: I usually torture myself for weeks find all the theoretical difficulties, then it works out fine in the end. OTOH if I don;t think at all, it's a disaster! It just seems a shame not to be neat as the chance is there. I'm going to have a lot of services installed:

About 11 mains final circuits including "throughs" that go to shed and gate lights, though half will probably be able to go one way round the house and half the other way to cut down bulk.

Upto 48 comms cables, or at least the potential. I'll be putting the drops and back boxes in, but I may not pull more than 30% through initially, it's easy to lay in later.

Upto 5 water pipes (hot+cold, 2xrads, 1 header), 7 if solar panels ever get fitted. Only the rads + cold need to get everywhere, so less in some parts.

The idea of traying (DIY with plank or commercial style) seems like a good way to keep stuff apart. At least that was my thinking.

Cheers

Tim

[Tim S]

Hi - thanks for the response,

[e-mail] coughed up some electrons that declared:

Just seems a shame not to attempt to be neat. Also, the comment in reply to John's post: there's going to be quite a bit running through, hot pipes, LV and comms/SELV. All three should be seggregated. The pipes will probbaly get mounted hanging from the rafters, so them and most of their heat should be up out of the way.

Interesting. This one's useful too, if you don't have the wiring regs to hand for cable sizes:

[Tim S]

Hi Dennis, thanks for the reply :)

dennis@home coughed up some electrons that declared:

An excellent suggestion. Won't be needed here, see reply to John re: access via removable panels; but there will be a couple of bits of LV in ducting outside which it would most certainly be a good idea to leave cords in.

Cheers

Tim

[jgharston]

When I did mine I clipped the cables to the rear of the wall uprights, and while I had access, boarded out the floor as well.

I have central heating, mains power, lighting, telephone, networking and TV clipped along there, each about a hand's width apart from each other.

[Andy Wade]

Although using the "not liable to simultaneous overload" method (equations 3 and 4 on p. 257 of the red book) can certainly help.

Or even vertical battens spaced at, say, 300 mm to make a sort of cable ladder. That's what I did here under the floorboards on the ground floor arterial run from the CU. The method gives excellent cable ventilation.

[Tim S]

Andy Wade coughed up some electrons that declared:

I'll have to try to understand those. I have the book and the ECA guide fo rthe 17th next to me. I didn't realise how bad trunking would be until I looked up the numbers. Or how good basket and ladders are, if the cables are spaced.

Now there's another good idea. Cheers Andy.

[meow2222]

If you want tray effect without the spend, why not just nail either a

2" timber block or any other small insulating lump onto the top of the joist either side of where you want to confine the wires. Cost sod all. If you dont want to hammer in, screws with a short projection, as barely any strength is needed.

(I'm getting visions of some wag using rubber door stops making it look like knob & tube.)

NT