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

In message ,
(pickerel) wrote:

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.



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

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?

If the wall is plaster on brick you may need to do some hollowing out,
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.


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?)

Sounds ok to me, but put protection over the cables to the sockets due
to their non-obvious run. No need for the box if you do that.


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?

AIUI the fuse can be either end of the spur, so long as electrically it
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.


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?

If this is for a ring or a spur from a ring, then 2.5mm cable is rated
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.


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?

I doubt you'll manage to completely fill round conduit with "flat" cable
:-) As for smoke and fire transmission I don't know. Sound transmission
will certainly be helped by sealing the ends.


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?

As for 4.


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.

No derating needed for standard circuit layouts. 2.5mm cable is good for
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).


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?

Don't see why not. Presumably there's some reason why it can't be run on
the same side as the socket?


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?

Not entirely sure about the layout, but regarding notching, the OSG
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.


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?
I think the 50mm guidance applies specifically to cables running through
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.

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?
Ummm... I'll let someone else answer this one.

Does the RSJ need earthing when cable is run so close?

Wouldn't have thought so.


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)?

Doesn't seem to be prohibited by the regulations, and the illustration
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.



Sorry its a long one, but many TIA


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
.... Salvation is only a Beer Bottle away...

Martin Angove wrote in message


Not sure what you mean by a "20
fuse spur". AIUI (OSG, p152/153 - not in the regs) fused spurs are
limited to 13A.

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


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.

No derating needed for standard circuit layouts.

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.



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?

Don't see why not. Presumably there's some reason why it can't be run on
the same side as the socket?

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





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?
Ummm... I'll let someone else answer this one.

please someone...come in


many thanks, almost there...


pickerel

In message ,
(pickerel) wrote:

Martin Angove wrote in message

(pickerel) wrote:
[...]
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.

No derating needed for standard circuit layouts.

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.

No, as I said several times, unless I have misinterpreted it, 2.5mm
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
.... If I save the whales, where do I keep them?