I am running standard twin & earth PVC cable. Various cables will be run from the CU it as follows: clipped to the wall for a short space then under the downstairs floorboards in free space then up the wall buried in the plaster (no conduit) then run between the joists (between downstairs ceiling and upstairs floorboards, laying above the ceiling) At one point, two cables will be passing through the same hole in a standard size joist.
The cables will not be grouped together at any point other than passing through the joist - they will be side by side when going up the wall.
Does the thermal insulation factor come into this anywhere (e.g. when buried under the plaster?). Does the grouping factor come into account for where the two go through the joist together? Thanks
On Mon, 17 May 2004 21:10:24 +0000 (UTC), in uk.d-i-y snipped-for-privacy@cix.compulink.co.uk strung together this:
Well, if cables are briefly grouped, e.g. running through joists then no, grouping doesn't apply. In a domestic situation though, calculations are generally not required as 2.5mm is well in for sockets, as is 1.5mm for lighting etc... are you overthinking a job there?
Grouping can be ignored if the point at which the cables are running together is less than 1.5 mtrs. When two cables are popping side by side through a joist the grouping factor is totally negligible to the final circuit as the effect is so small that you'd need very accurate test gear to find it.
As others have said, grouping doesn't really come into the equation through a hole in a joist - but don't try to squeeze cables through a too-small hole, eh? Apart from anything else it'll be a pig of a job pulling them :-)
As no-one seems to have covered the point, buried in plaster counts as practically the same as clipped direct, so no derating needed there either.
One thing you haven't mentioned is Voltage drop, but again in general you'd have to have a very big house, or an extremely tortuous wiring route in order to hit problems with that domestically. You haven't mentioned thermal effects either - none of this route is to be alongside heating pipes is it?
As for the wider comment someone made about domestic circuits being generally over specified anyway, this is of course true (you wouldn't expect anything else on uk.d-i-y, would you? :-). 2.5mm2 cable for example is rated for a full-size ring (32A MCB, 20A cable) even when buried in conduit in a thermally insulating wall. This is installation method 6. Buried in plaster is method 2, and is to be treated as method
1- clipped direct. For completeness:
(From table 4D5A BS7671 or 6F in the OSG)
Cable Current capacity Size method 1 method 6 Comment (mine)
1mm2 16A 11.5A Generally used on 5A/6A lighting
1.5mm2 20A 14.5A 6A or 10A lighting
2.5mm2 27A 20A 20A radial or 32A ring
These next two are the ones you need to be careful about:
4mm2 37A 26A 20A or 32A radial
6mm2 47A 32A Showers and cookers up to 45A
No, for all the cases you've mentioned the 'clipped direct' (M1) ratings would apply without any derating for thermal insulation. Cables embedded in plaster count as 'clipped direct' unless the plaster is a special thermal insulating type (containing polystyrene beads).
If the cables in the wall are touching (side-by-side), the grouping factors (Cg) are 0.85, 0.79, 0.75, 0.73 for 2, 3, 4, 5 cables (falling to 0.70 for 9 cables). However if you space them out so as to have one cable width's clearance between them the Cg becomes 0.94 for 2 cables and 0.90 for 3 or more. With 2 widths clearance no grouping has to be applied.
I'll take issue slightly with what the others have said here since, if you are passing through several joists on a run, the cables will tend to remain bunched between the joists. The 'bunched' Cg for two cables is then 0.80.
For a 30/32 A ring circuit you need to achieve 20 A installed rating (or 27 A with a rewireable fuse). So the Cg can be as low as 0.74 except for the rewireable fuse case, where no grouping at all is allowable!
It sounds as if you ought to get yourself a copy of the On-Site Guide (new brown one).
Thanks for the comments on this. I am currently rewiring my house in stages. Last year I added an isolating switch and replaced the existing fused CU with two larger split ones, so the circuits are now protected with mcbs (and an RCD where necessary). I also replaced the existing storage radiator CU and relay arrangement for a new CU with MCBs and a modular contactor. The relay that had originally been fitted was an open relay bolted to a piece of asbestos sheet and contained in a perspex sandwich box with the lid held on by insulating tape! There were two relays like this, one for the immersion heater. However I wired the immersion cable direct to the mcb and the immersion is now controlled via an immersion timer in the airing cupboard.
The existing wiring was done by a previous owner about 28 years ago and although not really in need for renewal, I am not happy with it. Rewired by a previous owner (an electrician), the methods leave a lot to be desired by today's standards. One thing that really bugs me is that most of the lighting cables - about
16 - end up in a large adaptable box under the floorboards joined together in a spaghetti junction of chocolate blocks. Whilst there is nothing wrong with this, it is almost impossible to identify the cables. More worrying, although there are two two way circuits (one for the landing light and one for the hall light (which also has an intermediate switch)), only two core cable is used. L1 and L2 are on one cable and the common for the landing light (red) is on the same cable (black) as the hall light. To me this is downright dangerous as it means that the cable is effectively fed from two mcbs.
The only place 3core and earth was used was from the landing light switch to the landing light - the yellow connected to the switch, the blue connected via a chocolate block in the light switch box to a cable coming from the adaptable box neutral and the red connected via a chocolate block in the switch to another cable from the adaptable box that was live. I.e. the 3 core cable wasn't being used for it's correct purpose but, again, being used to share circuits. At the junction box for the ceiling rose, the yellow and blue were used as the supply for the light and the red was connected to the red of a T&E whose black was connected to the blue and this went to another box that had the upstairs lighting circuit supply. Now you see why I want to rewire :-)
There is an infra red heater in the bathroom which is currently wired into the upstairs light circuit, which I believe is / was common practice. However I am going to extend the up ring main into the loft (still as part of the ring and not a spur) to give a socket there for future use, and at the same time will wire the bathroom heater to this.
One of the jobs I have to do is to replace the cable feeding a brick built shed in the garden. The cable is, I suspect, 1.5mm T&E PVC and is taken from a fused spur on the down ring main. It terminated in a fused CU in the shed. I've replaced the CU with one that has mcbs and is RCD protected, I just need to replace the T&E with a much higher rated SWA cable and take it to the CU.
I have always done my own wiring and I have John Whitfield's guide to the IEE wiring regulations which I find useful. However I was rather stuck on the correction factors as I didn't know how they related to what I was doing. If unsure, I would have erred on the side of caution, however that may have meant using larger cable than necessary. The only place ambient temperature might be a problem is for the cables in the loft. The cables would be clipped to the surface, but it does get quite hot up there and I assume that I should allow for a 50 degree summer temperature and the necessary correcting factor. This might limit me extending the ring main as a ring to the loft.
Am I right in assuming that if a spur was taken to a single socket in the loft (the actual ring main would be at bedroom floor level), it would be quite alright to use 2.5mm as, even with the corrective factor for 50 degrees, the cable would not be carrying more than could be drawn from the socket?
Voltage drop shouldn't be a problem as there are no overlong runs of cable.
The shower cable size I'll bear in mind as I currently have a 8 or 9Kw shower in 6mm, but I want to rewire for the possibility of a 10Kw one - so I'll rewire using 10mm.
Sorry, just realise how long this post has got :-)
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