Numpty question: are there standard ways to maintain the minimum bend
radius when cable turns from the vertical through a right angle to run
along a flat surface (and vice versa) please?

I ask as I am tidying up after running fresh cable for some lights in
lofts. There are various places where the cable runs vertically and
then along rafters; and others where cable to a fitting runs to a
junction box which is not screwed down[1]. In all these it seems to me
that a badly placed box, foot, knee or whatever could bend the cable
more tightly than allowed by Table 4E.

It occurred to me that I could guard against this by passing the cable
round short lengths of 40mm[2] uPVC waste pipe. But I'm bothered 'cos
(i) I've never seen that done; (ii) I can't find anything about how it
should be done in practice; and (iii) none of the usual suspects seem to
sell cable guides for mains cable like the ones I have seen/used to
maintain the bend radius for fibre optic/network cables. So I suspect
I'm missing something

[1] These are Ashley J501 junction boxes with strain relief designed so
as to allow access from below which (I very much hope) don't need to be
screwed down.
[2] It's mostly 1.5mm with some older 1.0mm so this would do for most.
For the 1.5mm T&3 for smoke/heat alarms I have some old 60mm downpipe.
--
R

On 14/12/2009 13:57, neverwas wrote:
Numpty question: are there standard ways to maintain the minimum bend
radius when cable turns from the vertical through a right angle to run
along a flat surface (and vice versa) please?

I ask as I am tidying up after running fresh cable for some lights in
lofts. There are various places where the cable runs vertically and
then along rafters; and others where cable to a fitting runs to a
junction box which is not screwed down[1]. In all these it seems to me
that a badly placed box, foot, knee or whatever could bend the cable
more tightly than allowed by Table 4E.

It occurred to me that I could guard against this by passing the cable
round short lengths of 40mm[2] uPVC waste pipe. But I'm bothered 'cos
(i) I've never seen that done; (ii) I can't find anything about how it
should be done in practice; and (iii) none of the usual suspects seem to
sell cable guides for mains cable like the ones I have seen/used to
maintain the bend radius for fibre optic/network cables. So I suspect
I'm missing something

[1] These are Ashley J501 junction boxes with strain relief designed so
as to allow access from below which (I very much hope) don't need to be
screwed down.
[2] It's mostly 1.5mm with some older 1.0mm so this would do for most.
For the 1.5mm T&3 for smoke/heat alarms I have some old 60mm downpipe.

Nice to see someone who takes the time and effort to do things properly.
The wiring for the upstairs lights in my loft is like a spiders web with
wires and junction boxes lying on top of the insulation!

On 14/12/2009 15:28, Graham Jones wrote:
On 14/12/2009 13:57, neverwas wrote:
Numpty question: are there standard ways to maintain the minimum bend
radius when cable turns from the vertical through a right angle to run
along a flat surface (and vice versa) please?

I ask as I am tidying up after running fresh cable for some lights in
lofts. There are various places where the cable runs vertically and
then along rafters; and others where cable to a fitting runs to a
junction box which is not screwed down[1]. In all these it seems to me
that a badly placed box, foot, knee or whatever could bend the cable
more tightly than allowed by Table 4E.

It occurred to me that I could guard against this by passing the cable
round short lengths of 40mm[2] uPVC waste pipe. But I'm bothered 'cos
(i) I've never seen that done; (ii) I can't find anything about how it
should be done in practice; and (iii) none of the usual suspects seem to
sell cable guides for mains cable like the ones I have seen/used to
maintain the bend radius for fibre optic/network cables. So I suspect
I'm missing something

[1] These are Ashley J501 junction boxes with strain relief designed so
as to allow access from below which (I very much hope) don't need to be
screwed down.
[2] It's mostly 1.5mm with some older 1.0mm so this would do for most.
For the 1.5mm T&3 for smoke/heat alarms I have some old 60mm downpipe.

Nice to see someone who takes the time and effort to do things properly.
The wiring for the upstairs lights in my loft is like a spiders web with
wires and junction boxes lying on top of the insulation!

and also all the cables in the junction boxes have their earth's taken
outside the box and twisted together! So all earth's exposed and
twisted! (There is a joke there somewhere).

Graham Jones wrote:
On 14/12/2009 15:28, Graham Jones wrote:
Nice to see someone who takes the time and effort to do things properly.
The wiring for the upstairs lights in my loft is like a spiders web with
wires and junction boxes lying on top of the insulation!

and also all the cables in the junction boxes have their earth's taken
outside the box and twisted together! So all earth's exposed and
twisted! (There is a joke there somewhere).

Yeah, that guy did our loft too.

On Dec 14, 7:43*pm, "Steve Walker" wrote:
Graham Jones wrote:
On 14/12/2009 15:28, Graham Jones wrote:
Nice to see someone who takes the time and effort to do things properly.
The wiring for the upstairs lights in my loft is like a spiders web with
wires and junction boxes lying on top of the insulation!

and also all the cables in the junction boxes have their earth's taken
outside the box and twisted together! So all earth's exposed and
twisted! (There is a joke there somewhere).

Yeah, that guy did our loft too.

When I was doing the kitchen at my previous flat, I discovered a light
had no earth. So I popped up to the loft, and found that the cable
actually terminated in a chocbox, with a short extension to the actual
light.

Yes, the chocbox was completely uninsulated. And submerged in
insulation.

thanks to all those who humoured me

. The wiring for the upstairs lights in my loft is like a
spiders web with wires and junction boxes lying on top of the
insulation!

snip

Yeah, that guy did our loft too.

I can do that! gizza' job!

(I am sadly very much from the Yosser Hughes school of DIY.)

--
R

neverwas
wibbled on Monday 14 December 2009 13:57

Numpty question: are there standard ways to maintain the minimum bend
radius when cable turns from the vertical through a right angle to run
along a flat surface (and vice versa) please?

I ask as I am tidying up after running fresh cable for some lights in
lofts. There are various places where the cable runs vertically and
then along rafters; and others where cable to a fitting runs to a
junction box which is not screwed down[1]. In all these it seems to me
that a badly placed box, foot, knee or whatever could bend the cable
more tightly than allowed by Table 4E.

I don't think anyone worries too much, but it's nice that you care. The
bending radius guides are there for a reason, namely that over bending a
cable can cause the wires to cut through the insulation over time
(cheesewire effect).

Looking at it a different way, the T+E is very good at maintaining its shape
if supported with a modicum of clipping. Perhaps the issue to address is how
to prevent a foot or a knee clouting it - as a well misplaced boot could do
damage to the cable even on the flat. (Neighbour recently had this very
problem - booted woodworm contractor trod on a cable and caused a short
internally.)

JB's would be best screwed down, but there are plenty that aren't - reason
being most of the older style don't have cable clamps, so strain on the
conductors can be prevented by fixing the box and clipping the cable near
the entry points.

That's one thing I like about the Ashley JBs - clamps on all the entries.

--
Tim Watts

This space intentionally left blank...

For PVC FTE (grey 6242YH)...
- Do not kink it
- Do not twist it (unroll from the reel)
- Any twist-n-pull (CK) tool will strip it ok

For BS7211 (white) 6242BH 90oC XLPE LSOH...
- Do not pull a cable physically across another
- The sheath whilst hard will abrade themselves rapidly
- Sheath stripping requires a slice tool (very tough)

For FP200 & BS8436...
- Bend radius is critical here to avoid collapse
- General FP200 cables use silicone insulation
- Silicone insulation is a pain as it is easily nicked
- Prysmian FP200 Gold uses insudite insulation
- BS8436 & FP200 have quite high bend radius

For SWA...
- More tolerant of abuse
- Pipe cutter works well (there is a special tool)
- Leave bedding on insulated cores just beyond the gland

The "special tool" like a pipe cutter is about £7 from Discount
Electrical (no connection, just recall stumbling across it - beware
postage is about £7.50 whereas TLC is much cheaper for small orders).

For HiTuff or NYY-J...
- Bend radius is 12x Diameter (IIRC)
- Warm the cable before use
- Do not leave outside in cold as it goes completely rigid

All cables have a min install temperature (which also affects
stripping too).

Most people will not buy BS7211 as is hard to work with and more
expensive, but it is useful where insulation is concerned - for
example in a loft or internally insulated walls and so on. The very
high CCC is quoted at 90oC, you can only rely on that for running
through insulation - not at wiring accessories (so no you can not do
2.5mm radials with lots of sockets protected by a 32A MCB/RCBO).