"Tim Watts" wrote

Hi all

OK so I know I should have looked this up last night, but it's bugging
me now!
Running power and cooker cable around the kitchen loft and I would
prefer to run most in flat (single cable size) trunking. The reason is
that I will be running this across walls and along roof timbers (to keep
it out of insulation) and this will give a degree of protection against
knocks, or any critters that get in there. Also, I don't trust myself
100% when trying to clip cable to tough brick, not to welt the cable
occasionally.

But what is the impact on the cable capacity when run in trunking
against clipped?

Not enough to upset most assumptions most of the time (except one).

Specifically:

Enclosed in trunking or conduit *on* a masonry/wooden wall is Ref Method B
Ditto, but buried in plaster is also Method B

Open cable clipped to surface is Method C
unless it is clipped to the underside of a ceiling,then it is Method B

Ratings (Amps) for normal PVC T+E:

mm2 B C
1.0 13 15
1.5 16.5 19.5
2.5 23 27
4 30 36
6 38 46

That does make the difference between your (say 32A) cooker cable being
run in 6mm2 rather than 4mm2 but 6mm2 is a common choice anyway.

I would consider just using clips when running cable along roof timbers,
as the hammer velocity doesn't need to be quite as serious. The drops
down the walls will be in oval conduit chased and plastered.

Watch the size of the oval - 6mm2 is quite lumpy. Also, unless you can
get the right choice of oval to go through the 25mm knockout in the box,
don't forget to grommet the hole

Clip lid trunking is a good choice for fiddly bits in the roof space as
you can lay the cable in - 6mm2 will not pull very easily through long
bendy bits of conduit - though you'll be OK with the 1-2m drop down your
wall if you make sure the cable is nice and flat before you stuff it down.


Tim

Thanks for comprehensive response!
Can you clarify a couple of points please?
For 2.5mm cable, each leg of the ring should only be pulling 16A presumably
to cover the 32A total breaker capacity.
ISTR that 2.5mm is nominally rated at 20A as a spur.
So in both/all cases, the 23A rating will not be an issue - right?

The cooker cable is run in 6mm, currently serving a hob and double oven and
run in clip-top trunking, so that should serve the future purpose.

Some time ago, another poster gave links to images of his first fix work.
Can't remember the culprit, but he showed conduit being used between boxes,
with the conduit passing through grommets into each box.
From your suggestions above, it appears that as long as the conduit
protrudes into the box to protect the cable from the sharp box hole edge, a
grommet is not required - is this correct?

Phil