MDF electrical trunking ??

[brugnospamsia]

Dear group,

The only formal training my limited attention span permitted was in electronics and not electrical installation. I also work as a technician on a site with all cables accessible and understand the difficulty of predicting future needs.

This explains my instinct to wire my house in an unconventional way avoiding cutting joists and running cables under the floor or buried in plaster.

My plan is to use vertical and horizontal cable ducts around the stairwell at the centre of my house in addition to the already useful duct formed by the gap between the first and second first upper floor joists which runs the whole length of the house and which will also carry gas and water pipes.

In the absence of or high price of trunking that is easy on the eye, and given that PVC trunking is permitted, I wonder if there would be any objection to my using bespoke MDF service ducts.

I have done a fair bit of Googling on the subject but to no avail.

thanks ...

Jeremy

[Bob Eager]

Ducts presumably count as trunking, so the cables will have to be derated as per the regs. So you'll need better than 2.5mm2 for a final ring circuit, for example, unless protected at (say) 20A.

[Tim]

Don't forget to apply any required grouping correction per the regs too if there's more than one cable in there too.

I can't honestly advise on the pro's and cons of MDF for this. The only question I would ask myself (and which I can't answer without checking) is would MDF constitutes a greater fire or fume risk if one of the cables got excessively hot due to a fault?

I suspect it doesn't matter as cables are routinely clipped to joists etc, but it's worth thinking about.

HTH

Timbo

[Tim]

I should have said *as well* as any derating due to enclosure as Bob mentioned, I meant to reply to his reply.

Butterfingers me...

Timbo

[brugnospamsia]

oh bum !

mind you at the moment I have a single ring for the whole house with an immersion heater on it fused at 30 amps .....

Anyone know where I can get the asbestos substitute board I once saw them use in my building and whether it's horribly expensive ?

Jeremy

[Alex]

The board you describe is called superboard available from Jewsons, a 8x4 sheet around £80,most likely available from all other merchants as well

[Andy Wade]

That's wrong though. The cable for a 32A ring only has to achieve 20A in-situ rating, which a single 2.5 T&E cable in conduit, trunking or duct easily does. You'd only have to go up to 4mm^2 if dictated by the application of grouping factors with cables of other relevant circuits, or unusually high ambient temperature.

Masterboard, from any builders' merchant?

[Bob Eager]

I was assuming that there would be grouping if he was making ducts - perhaps there isn't...!

[brugnospamsia]

thinking about it, the cable density is very low. The only problem I can see is that the wide vertical duct will carry central heating pipes too - but I'll insulate and keep them seperate. In essence my proposal isn't much different from running cables behind drylining.

Is it possible to obtain meter tails longer than 1 metre ?

I would like to move my CU ...

thanks...

Jeremy

[Tim S]

Believe so, as I heard of much longer runs. It's worth getting professional advice if you're going to go more than about 2m away. Something I read recently suggested >2m might require a fused isolator on your side of the meter. I'm thinking of doing the same thing. The electricity supply company might be able to advise.

Cheers

Timbo

[brugnospamsia]

I wish they'd put my meter outside for free instead of pestering me to have it changed :-(

[bigcat]

Si is it permissible to use wood as trunking? I'm thinking here of tweaked skirting board primarily.

NT

[Andy Wade]

Pretty-well any electrical wholesaler will sell you any length of 25mm^2

6181Y cable you like by the metre - limited only by the reel length!

That's essentially correct. The maximum length of meter tails is limited to either two or three metres, depending on who your supply distribution network operator is. This is because they will not allow their main fuse to be used to protect your wiring, other than over the short length of the tails.

If you want to move your consumer unit some distance away from the meter you need to install a 100A DP switch-fuse (assuming a normal single-phase domestic supply) within the 2m or 3m tails-reach limit of the meter. You can then run a distribution circuit of any length you like (in principle) to the CU - which should now strictly speaking be called a distribution board. Or you can feed multiple distribution circuits and boards using a suitably designed 'main switchboard' scheme by the meter.

The distribution circuit(s) will need to be properly designed to comply with all the relevant requirements of BS 7671: cable size/rating, voltage drop, earth fault loop impedance and so on. There are several valid choices for the type of cable that could be used - 'meter tail cable' (6181Y), conduit cable (6491X) in conduit or trunking, steel wire armoured (SWA), or mineral insulated (MICC) are probably the main ones.

A Google search will reveal lots of previous threads on this subject, but because of the need for proper design input I don't suggest tackling this as DIY unless you really know what you're doing.

[Bob Mannix]

This is all entirely correct. The new protected cable is often referred to as a "sub-main". Don't forget you may need to run a new earth cable as well, of suitable size. I have done it myself DIY.

[Andy Wade]

In common parlance it is indeed, but if you look at the definitions in BS 7671 'sub-main' specifically refers to the case of a distribution circuit that feeds a separate building or installation.