cable between floor joists

When electrical cable (normal FT&E) is laid between joists, can it rest on the ceiling plasterboard or ought it to be clipped to the joist sides with Tower-type clips?

TIA

Reply to
jim_in_sussex
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Surely if you wanted to do that you would have to lift every board along the route!

Reply to
Ric

In message , Ric writes

maybe it's in the loft?

Reply to
chris French

Maybe the loft is converted?

Reply to
Ric

Not floor joists, then. Unless it's boarded over. Then there's a hole in the bucket, dear Lisa...

Reply to
Dave Plowman (News)

sorry if question sounds stupid, but it is something that's always had a question mark over it in my mind.

a. to clarify please add 'or ceiling' before joists

b. if it is legit to just rest cable on the (ordinary) plasterboard, which IEE regs installation method is this?

also ISTM that there must be some danger of the plaster board being punctured by various sharp objects like nails.

just going over things carefully to make sure there's no glaring exceptions before Part P bites...

TIA

Reply to
jim_in_sussex

As you may have guessed it is almost always impractical to clip along a joist, of whatever sort. Leaving the cable unclipped can also make it easier to replace at some point in .

Method 1 (free air, basically) BUT beware of bunching - grouping - where several cables run together, particularly if they run together through holes in joists. Better, in my mind, several small holes with one or two cables in each than one large hole with many. Of course if we *are* talking about the attic then we may be talking about Method 15 where the cable is resting on the plasterboard ceiling (a "thermally conductive surface" - see notes to Method 4) but otherwise covered with insulation.

Possibly, though most people don't tend to hang much from their ceilings other than lights which are usually already there. If a cable is loose though, and reasonably free to move, there is every chance that a nail (or screw, or drillbit) pushing through the plasterboard will simply push the cable out of the way. If the drill bit is followed by one of the expanding-type plasterboard fixings then there is a small chance the cable could be trapped I suppose...

Hwyl!

M.

Reply to
Martin Angove

You'd think that'd send the pedants back into their box, but once they have a hold... ;-)

Kinda-depends what's resting on the plasterboard! If it's the common case of the ground-floor-ceiling/upper-floor-floor void we're talking about, there's usually no insulation, and this is pretty much our mate Reference Method 1, Clipped Direct. If you're laying in a loft, you should either derate by 50% compared to Reference Method 1 (for the lighting cables which typically run there, the 1mmsq with a 6A MCB meets this derating, by no coincidence at all ;-), or indeed go through the extra bother of running them clipped towards the top of the joists so that the effect of any insulation (which may not be laid at the moment, but which reg 523-04 enjoins us to anticipate - the OSG suggests that Reg's wording refers to cable "in a space to which thermal insulation is likely to be applied".

Pretty damn unlikely, really - once the PB is in place, nailed up to the joists, you're not going see many nails going up through it! And even when it's being installed, the nails/screws go up into the joist, unless you miss. Walls - yes, picture hooks/shelf-fixings and the like go into them, hence the practice of: no cables except in 6-in-wide Zones vertically & horizontally from visible lectrickle fittings, and in the upper and side (but not bottom) corners of the room, or mechanically protected, or at least 2inches deep. But few householders nail shelves and pictures to their ceilings. (Mirrors, maybe, in the bedrooms of those with particular lifestyles: but the straight-laced engineers who draft the Regs and write Guides to 'em seem to consider that a minoruty interest!)

HTH - Stefek

Reply to
Stefek Zaba

snip

snip

Yes thanks, but it's the words 'clipped direct' that throw me. Loosely resting on plasterboard seems a long way from being clipped every 9 inches to a timber joist.

I had the impression that one reason for clipped direct having a good current rating was that holding the cable firmly against timber gave you a reasonable heat sink (possible surrounding insulation being ignored or assumed not present for the purpose of this discussion). But I suppose resting on cold p'board might be as good.

Still slightly puzzled, but thanks all the same.

Reply to
jim_in_sussex

Well, Reference Method 1 isn't that specific about just what it is you're clipped to - joist, plaster, whatever. So you have to assume that in drawing up the tables, our friends at the IEE will have taken a pretty poor-case assumption - e.g. clipped to wood, whose thermal conductivity is pretty piss-poor, and whose specific heat is pretty low. So there's not that much contribution to cooling from the clipped-to side. Indeed, ratings for Reference Method 13 - Free Air - and its kissing-cousing, Reference Method 11 - On Perforated Cable Tray - are a little higher than the "standard" Reference Method 1: so (according to one widely-applicable table (6E1) reproduced in the OSG) ratings for 1,

1.5, 2.5, and 4mmsq are 15,19.5,27, and 36A under RefMeth 1, while for RefMeth11-or-13 they jump massively to 17,22,30, and 40A - an "uprating" of a little over 10%. So using RefMeth1 figures for cable which is in practice "resting loosely on cold p'board" gives you a slight underestimate of current-carrying capacity; but allows for those parts of the cable run where you do choose to clip. And it's RefMeth1 for which figures are most widely quoted...

Frankly, for lighting and ring circuits in ordinary domestic circumstances, it's pretty hard to route cable in such a way as to make it overheat - just about the only situation is if you run 2.5mmsq under loft insulation for a substantial length (1m+, though the Regs say 0.5m is the point at which you must derate to 50% of RefMeth1). Things get trickier with bigger cables carrying heftier currents (showers especially, cookers to a degree), and caution is smarter than assuming things'll be OK...

HTH - Stefek

Reply to
Stefek Zaba

They're probably less likely to be damaged by such an event than if they were clipped, so had something to push against.

Certainly my understanding is that loose laid is perfectly acceptable. It is certainly pretty universal!

Christian.

Reply to
Christian McArdle

The full description of Reference Method 1 (clipped direct) in Appendix

4 of BS 7671 is "sheathed cables clipped direct to or lying on a non-metallic surface."

You suppose correctly.

Returning to the original question, the 50mm separation requirement only applies where a cable passes through a timber joist. At other points along the run the requirement is only that it "shall be run in such a position that it is not liable to be damaged by contact with the floor or the ceiling or their fixings" - see Reg. 522-06-05.

Reply to
Andy Wade

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