Wiring regs query

[Bolted]

I'm doing some wiring in an extension and connected rooms which, due to dreaded Prat P, will be inspected by someone of unknown through-and/or-pickyness. Anyway, it's better to do things by the book but in this case, the book I have (OSG) doesn't seem to give the detail I need.

The query I have relates to a run of a group of cables (serving much of the rest of the house) in my cellar. When wired, the cellar had lath and plaster below the joists, but this was drooping, so has been removed and to give a bit more headroom and storage space in what is my workshop, I put insulation and plasterboard up in between the rafters against the flooring above, so the joists are exposed. This has left this run of cables exposed between a pair of joists, where previously they just lay on the top of the lath and plaster. I temporarily supported them on lengths of plywood as a quick fix, but as they run directly to the CU I'd like to sort them out properly so whoever the council sends out as an inspector doesn't object to the way they are supported and start worrying about higher grouping factors etc for the new cables I am running alongside the same route.

The question is what the spacing should be for ladder supports for horizontal runs of cable. App 6 mentions this as 'reference method 13' but App 4A doesn't state any spacing rules. It would seem logical that the spacing of such supports should be the same spacing as for cable clips for horizontal runs, but there is nothing which expressly says so that I can see in the OSG. Does anyone familiar with the BS itself know if that is correct?

[Andy Wade]

Ref. method 13 is for "sheathed multicore cables on ladder or brackets, separation > 2*De [or] sheathed multicore in free air, distance between wall and cable surface not < 0.3*De [and] any supporting metalwork under the cables occupying < 10% of the plan area." This method would only normally be applied to big power cables, such as a group of distribution circuits going off from a switchboard to separate dis-boards.

For your situation I'd use method 11: "Sheathed cables on a perforated cable tray, bunched and unenclosed. A perforated cable tray is a ventilated tray in which the holes occupy 30% or more of the surface area." Grouping factors for method 11 are given in Table 6C in the OSG, and if you can space by > 2*De the factor is unity.

De (effective diameter) for T&E cables is the larger dimension, i.e. the width.

For support spacing use Table 4A in the OSG, plus common sense. The BS itself has gives no further advice and the regs simply require support to be adequate. There might be more info in GN1 (selection and erection) but I don't have a copy of that.

In a workshop the inspector will be looking to find that cables are adequately protected from any mechanical damage that could occur as a result of the activities carried out. If they are overhead as described and you do a tidy-looking job I very much doubt that you'll have any problems.

[Owain]

I wouldn't bother with ladder supports but just use some cable tray. And as Andy Wade has said, there's a Reference Method for that.

Owain

[Bolted]

Thanks, not far from what I was proposing, but I probably wouldn't make the 10% of plan area. I have a load of 50mm wide scrap wood, which I was planning to put across the battens which hold up the plasterboard every [dimension I am trying to find out]. So 10% is out of reach unless I rip it all in half.

As a grouping purpose analogy I'd noted that and am happy with it, the calcs work fine, but I don't want to buy and install trays, obviously.

Just about what I thought, thanks for the confirmation. I always overdo things anyway.

There might be more info in GN1 (selection and

Oh well, let's hope mr inspector doesn't pull that one out.

That's a given, I've no present desire to kill myself or set the house alight, and long may that last.

Thanks for the response, Andy, very helpful and much appreciated.

[Bolted]

You prompted me to look at it, and the wide stuff is much cheaper than I thought (only ever bought a couple of bits of 50mm). So I may as well, as it'll give good mechanical protection as well. Thanks to both.

[boltmail]

OK, I have some cable tray on its way. I have some follow-up questions on earthing if anyone could be so kind (otherwise, heaven forbid, I might have to rely on common sense):

Should it be earthed direct to the main protective earth bonding block or is it enough to bond it to some water pipes it will run parallel to?

Should it be bonded with 10mm^2 like main services, or 6mm^2 or whatever it is for equipotential bonding like in a bathroom?

I assume radiator type earth clamps will do?

Is it enough to earth one end of the run and rely on the nuts and bolts holding it together to provide a path, or does each section need a clamp, with the earth wire running the length of the tray?

Thanks in advance.

[Andy Wade]

Question one is does it need earthing or bonding at all? If it's only carrying sheathed cables such as twin & earth it won't need earthing [see reg. 471-09-04], and if it's not entering from outside the building it won't need bonding. You can earth it though if you wish as a precaution against damaged cables making it live - connect it to the main earth terminal, not to water pipes.

For earthing the minimum size will be the same as the largest CPC size required for any of the circuits being carried, for bonding you'd treat it as main bonding (so 10 mm^2, usually).

Ezy-earth, yes, or just a screw, nuts, washers and a crimp ring terminal.

Yes, provided you're confident that said nuts & bolts will ensure good (and lasting) electrical continuity.

[Dave Liquorice]

Hum, not so sure all the metal trunking and open trays I've seen recently have had bonds across all joins.

[Andy Wade]

Some people do like their belts and braces...

[Dave Plowman (News)]

Dave might well be talking about a good TV studio installation. Where they do.

[boltmail]

I nearly put 'I assume that' it needs to be earthed, but then thought it must have to be. I think I'd be happier if it was for the reason you mention. Hadn't really grasped the difference between bonding and earthing, that makes more sense now.

Thanks for other advice.

[Andy Wade]

Yes, that thought did occur to me. And I wasn't trying to argue that belt and braces is a bad thing.