Routeing Electrical FTE cable

Am planning to improve existing wiring & also to add some extra circuits. Would very much appreciate guidance on the right way to route standard UK PVC FTE cable around various obstacles etc.
1. Running a horizontal cable between 2 sockets (on same level) on walls either side of a 90 deg corner: For 2.5mm^2 minimum bend radius is a quite large 30mm - what is usual - hollowed out the corner to allow the cable to form an even bend? Avoid this route altogether? - Or some alternative?
2. Kitchen end wall has a 1.8m wide window & 0.6m wall at side. A double socket + 2 x single unswitched sockets (with separate fused switches) are needed - the single sockets to be below the window (behind wash m/c & dishwasher) & switches at side of window above the work surface: is this scheme OK?: Route the ring cable down wall to 13A double - continue to switch fuse(1) then to switch fuse(2) & then back up the wall. Spur cables from each switch fuse to go down and then left or right to respective unswitched 13A sockets. (Also - is a blank box required at the point where a spur cable changes from vertical to horizontal run?)
3. Wish to add a spur straight up a wall from a 13A socket into the attic for 2 separate convenience sockets on a daisy chain. Regs specify a 20 fuse spur (SFAIUI this can be with or without a switch?). Must the spur fuse box be adjacent to the source socket or can it be put in the attic?
4. 2.5mm cable through a single brick or block wall (plastered both sides): is 25mm plastic conduit a sufficiently good sleeve or is metal conduit better on basis it will transmit heat away better? What if there are 2 cables - do they need separate conduit?
Is an air gap needed in the conduit? Sealing at ends seems prudent to reduce potential smoke & sound transmission, but is that the correct procedure?
5. 2.5 mm cable through a cavity wall (formerly external but now internal) with 50mm cavity filled with mineral wool insulation. Is similar sleeving to 4. (above) correct or does something need to be done to remove surrounding mineral woool insulation & if so what/how?
6. SFAIUI FTE can be laid immediately above insulation, but what should the clearance be, sideways & below, from insulation before derating factors apply? ISTR a 100mm clearance is recommended, but cannot trace anything in the regs etc.
7. Need a socket on one side of a cavity wall. Is it OK to run the cable through the wall into a blank box (ie on the other cavity leaf behind the socket) and then straight up the wall into the joist space?
8. In one room the joists T into an RSJ which supports the dividing (block) wall. On other side the first joist runs lengthways (parallel) with the RSJ. What is best way to feed a 2.5mm cable around this obstacle? Thoughts are to bring cable up level with the top of the RSJ, feed through a 25mm conduit in wall, and run cable over the far joist in a notch to allow floor boards to lie flat. Is a steel plate or other protection needed over the cable in the notch?
9. An RSJ runs across the middle of a room supporting timber joists. There is 50mm clearance between the top of the RSJ & the underside of the floorboards. I need to get ring & lighting cables through here. As the finished clearance will be less than 50mm, does the FTE need to go into earthed conduit? If so what is the correct method of earthing this conduit? ie what type of clips, size of earth cable & to which earth point should it be connected - is a ring earth terminal OK? Does the RSJ need earthing when cable is run so close?
10. It is quite common to see FTE lying on top of plasterboard ceilings unclipped & unsupported between joists - is this still permitted by the regs (if it ever was)?
Sorry its a long one, but many TIA
pickerel
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@hotmail.com (pickerel) wrote:

A quickie before bedtime... where I say "OSG" I mean the IEE On Site Guide to the regulations.

but I wouldn't have thought it'd be *that* bad. With plasterboard on studs it should be quite easy to accomodate. If it's a problem, take the cable down or up as appropriate and under the floor/over the ceiling.

to their non-obvious run. No need for the box if you do that.

is before the load (obvious, really). Not sure what you mean by a "20 fuse spur". AIUI (OSG, p152/153 - not in the regs) fused spurs are limited to 13A. In fact you will find that the units (yes, it's ok to use one without a switch) take standard "mains plug" type fuses which are only available up to 13A anyway.

20A "enclosed in conduit in an insulated wall" (table 4D5A, ammendment at back of blue regs book - not in OSG). 20A is what you need for a 30/32A ring. How many cables you can fit into the conduit is another matter. Table 4B1 (6C in OSG) would seem to imply that you would need to derate the cables to 0.80 if you are running two in the same conduit. This brings them down to 16A each which is barely acceptable for an MCB-protected ring main.
If your protection is with rewireable fuses then you need to derate the cable further, though this doesn't make much difference for standard installations.
On the other hand, I don't think you need the conduit in the above example and if you make the hole big enough...
I am, however, eager to be corrected.

:-) As for smoke and fire transmission I don't know. Sound transmission will certainly be helped by sealing the ends.

27A "clipped direct" but if you need that amount of current then you will know how to design for it. If you are talking about standard rings and spurs from them, then all you need to know is that 2.5mm cable is rated for 20A under the most arduous conditions normally encountered. Table 4D5A is your friend. (The figures for 1mm cable, for reference, are 11.5A in conduit in insulation and 16A clipped direct).

the same side as the socket?

recommends running the cable *through* joists at least 50mm from the top and the bottom of the joist (p52), or enclosed in earthed steel conduit run in notches. It also says:
"Alternatively, the cables can be provided with mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like. (Note, the requirement to prevent penetration is difficult to meet.)"
Then it goes on to say *where* in a joist you can drill holes and cut notches.

joists because of the likelyhood of fixings being screwed/hammered *into* the joists. Assuming no-one is likely to be screwing up into your RSJ, no-one is likely to be screwing through a floorboard with that length of screw either, unless they are terminally daft. I'd say it was ok without, but you can install it if you like.

in the OSG (fig 7.3.1 p52) shows this. The only way to stop doing this as far as I can see would be to force everything into conduit.

Hope my answers have been sensible. It's too late to go back and check them all now, and I'm sure others will do that anyway :-)
Hwyl!
M.
--
Martin Angove (it's Cornish for "Smith") - ARM/Digital SA110 RPC
See the Aber Valley -- http://www.tridwr.demon.co.uk/abervalley.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Not sure what you mean by a "20

yes - slip of the fingers, I'll have word with the typist.

fear the Q may have been misinterpreted... what was bothering me was derating for 'cables surrunded by thermal insulation' (table 16b - which can be a massive 50%): what is bothering me is how close is 'surrounded'? Somewhere in the back of my mind I recall some statement that a 100mm clearance airgap is needed or recommended. Some of my best 'choice runs' come close to thermal insulation - both fibre glass and pipe insulation sleeving.

roof on one side, ceiling/joists on other don't line up

please someone...come in
many thanks, almost there...
pickerel
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@hotmail.com (pickerel) wrote:

FTE cable is still rated at 20A even when enclosed in a conduit which is itself buried in an insulating material (material only without the conduit allows 21A) so long as one side is attached to (say) a wall.
These are installation methods 6 and 15 and are explained in appendix 4, table 4A1 of the full regulations. The figures of 20A and 21A come from table 4D5A which is a new ammendment and has been included in the On Site Guide as Table 6F. The fact is that this is already a derating (2.5mm is 27A when "clipped direct"), and no further derating is neccessary for the installation methods you describe. 20A is fine for each leg of a 32A MCB protected ring.
Only if the cable is completely surrounded (which seems to mean "touching on all sides") with insulation do you need to apply more derating. Under those circumstances you would apply it to the "free air" rating of the cable, so 2.5mm T&E derated 50% is capable of 13.75A which isn't much good for a ring. If you are planning on running cables *under* the insulation, resting on the ceiling you are fine. Likewise if you are planning on running the cables lying on top of the insulation but otherwise open to the air.
The only time you may come a cropper is with >1 cable in conduit OR if those lagged pipes you are talking about are hot.
Don't panic! It's probably ok!
Hwyl!
M.
--
Martin Angove (it's Cornish for "Smith") - ARM/Digital SA110 RPC
See the Aber Valley -- http://www.tridwr.demon.co.uk/abervalley.html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.