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?
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.

TIA

Phil

On Wed, 16 Jun 2010 10:46:46 +0100, "TheScullster"
wibbled:

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 Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

"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

On Wed, 16 Jun 2010 13:52:32 +0100, "TheScullster"
wibbled:


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?

Right and wrong ;-

The regulations require each leg of the ring to be capable of handling
20A continuously. It's an arbitrary, round, made up number that has some
leeway for current imbalance between the 2 legs. Meet 20A for any+every
section of cable and you're OK.

Might I suggest procuring a copy of:

http://www.amazon.co.uk/IEE-site-Gui...dp/0863418546/
ref=sr_1_1?ie=UTF8&s=books&qid=1276703557&sr=8-1

It's a condensed domestic summary of the regs with lots of diagrams, and
the more pertinent tables (though they forgot to mention reference method
B so you have to be a bit creative). You can do stuff that's not in there
that is still regs compliant, but if you do what the OnSite says, you're
guaranteed to be OK. Good section on bonding with minimum wire sizes too.

That is totally worth the 16 quid even if all you do is put in one
circuit.


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.

Yes. 32A is a pretty standard circuit. I'm running one, even though I
will have a gas cooker. You are allowed to have a 32A radial socket
circuit and that's what I will do - 13A socket on the bit behind the
cooker. Right place and the isolator switch next to the cooker still does
what is expected. If anyone wants an electric cooker, all they have to do
is switch the socket for a cooker terminal plate.

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.

That'll be me:

http://photos.dionic.net/v/public/bu..._0019.jpg.html

I did it to stop the conduit sliding out.

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?

One or the other is perfectly acceptable - as long as the cable is
protected from the sharp edge of the hole. You could even use a conduit
through-hole adaptor.

One other question - is your run of cable massively long? The other thing
that can go wrong is too high a cable impedance (Live-Earth loop) and/or
too much voltage drop.

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...


Phil





--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

"Tim Watts" wrote


The regulations require each leg of the ring to be capable of handling
20A continuously. It's an arbitrary, round, made up number that has some
leeway for current imbalance between the 2 legs. Meet 20A for any+every
section of cable and you're OK.

Might I suggest procuring a copy of:

http://www.amazon.co.uk/IEE-site-Gui...dp/0863418546/
ref=sr_1_1?ie=UTF8&s=books&qid=1276703557&sr=8-1

It's a condensed domestic summary of the regs with lots of diagrams, and
the more pertinent tables (though they forgot to mention reference method
B so you have to be a bit creative). You can do stuff that's not in there
that is still regs compliant, but if you do what the OnSite says, you're
guaranteed to be OK. Good section on bonding with minimum wire sizes too.

That is totally worth the 16 quid even if all you do is put in one
circuit.


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.

Yes. 32A is a pretty standard circuit. I'm running one, even though I
will have a gas cooker. You are allowed to have a 32A radial socket
circuit and that's what I will do - 13A socket on the bit behind the
cooker. Right place and the isolator switch next to the cooker still does
what is expected. If anyone wants an electric cooker, all they have to do
is switch the socket for a cooker terminal plate.


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?

One or the other is perfectly acceptable - as long as the cable is
protected from the sharp edge of the hole. You could even use a conduit
through-hole adaptor.

One other question - is your run of cable massively long? The other thing
that can go wrong is too high a cable impedance (Live-Earth loop) and/or
too much voltage drop.

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...



Thanks again Tim

The CU is at the other end of the building to the kitchen (not quite 10
bedroomed mansion though).
The cable runs to get to the boundary of the kitchen loft space are about
12m.
The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total cable
length of approx 50m.

I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from
our electrical department (don't think they knew they had it!).
It seems pretty similar to the 17th Edition OSG, so that's a bonus. The
problem with a lot of it though can be interpretation and application, which
is difficult I find when you are DIYing rather than immersed in this stuff
daily.

Phil

TheScullster
wibbled on Thursday 17 June 2010 09:28


"Tim Watts" wrote

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...



Thanks again Tim

The CU is at the other end of the building to the kitchen (not quite 10
bedroomed mansion though).
The cable runs to get to the boundary of the kitchen loft space are about
12m.
The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total
cable length of approx 50m.

I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from
our electrical department (don't think they knew they had it!).
It seems pretty similar to the 17th Edition OSG, so that's a bonus. The
problem with a lot of it though can be interpretation and application,
which is difficult I find when you are DIYing rather than immersed in this
stuff daily.

Phil

Hi,

OK - 50m...


OnSite Guide summarises your max cable lengths as follows:

6mm2 @ 32A = 49m
10mm2 @ 32A = 81m

That is with a BS88-2, BS88-6 fuse or Type B MCB *only*. What do you have?



For all earthing systems:

Your permitted L-E loop impedance including the supply for a Type B 32A
breaker is 1.16 Ohms for a TN* earthing system

You must either assume for TN-S, the supply impedance (called Ze) is 0.8
Ohms or TN-C-S (PEN) 0.35 Ohms *unless* you measure it[1] and prove it is
lower.


[1] With a proper tester not a multimeter(!)

So your max circuit loop resistance (it is a DC test) is TN-S = 1.16-0.8 =
0.36 Ohms, TN-C-S = 1.16-0.35= 0.81 Ohms

You could test that with a decent very low ranging multimeter (and allow the
worst case stated meter error to be safe). Connect L-E at the CU of your
cooker circuit (obviously connected to nothing else) with a chock block,
then measure the resistance between L-E at the cooker connection point. If
the resistance is lower than the stated values you are OK.

Note: for a TN-S earthing system, the L-E resistance limit (called R1+R2)
coincides more of less with the voltage drop limit. If you have a TN-C-S
system you might get away with a slightly longer cable run without danger[1]
but your voltage drops will go outside of tolerance.

[1] The safety factory of R1+R2, combined with your Ze is making sure the
MCB/fuse trips in =0.4 seconds on a dead L-E short at the furthest end of
the circuit (cooker end).

Even if you have a TT (rod) earth and master TT RCD, it's best to use these
limits on cable lengths (unless you do a more in depth calculation) as you
also need the breaker to trip correctly for a L-N fault.

Conclusion
==========

You're on the limit with 6mm2 cable for 50 odd metres. And you should
absolutely measure your circuit run IMO, for the resultant L-E resistance
after it's all wired in bar the CU connection.

Do you have access to a suitable meter?

I would do a more detailed length measurement. If it is 49m, and if you
don't have access to a low resolution ohmmeter, it might be necessary to
switch upto 10mm2 cable. You'll be pretty bombproof then.

HTH

Tim

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

"TheScullster" wrote in message
. uk...

"Tim Watts" wrote


The regulations require each leg of the ring to be capable of handling
20A continuously. It's an arbitrary, round, made up number that has some
leeway for current imbalance between the 2 legs. Meet 20A for any+every
section of cable and you're OK.

Might I suggest procuring a copy of:

http://www.amazon.co.uk/IEE-site-Gui...dp/0863418546/
ref=sr_1_1?ie=UTF8&s=books&qid=1276703557&sr=8-1

It's a condensed domestic summary of the regs with lots of diagrams, and
the more pertinent tables (though they forgot to mention reference method
B so you have to be a bit creative). You can do stuff that's not in there
that is still regs compliant, but if you do what the OnSite says, you're
guaranteed to be OK. Good section on bonding with minimum wire sizes too.

That is totally worth the 16 quid even if all you do is put in one
circuit.


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.

Yes. 32A is a pretty standard circuit. I'm running one, even though I
will have a gas cooker. You are allowed to have a 32A radial socket
circuit and that's what I will do - 13A socket on the bit behind the
cooker. Right place and the isolator switch next to the cooker still does
what is expected. If anyone wants an electric cooker, all they have to do
is switch the socket for a cooker terminal plate.


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?

One or the other is perfectly acceptable - as long as the cable is
protected from the sharp edge of the hole. You could even use a conduit
through-hole adaptor.

One other question - is your run of cable massively long? The other thing
that can go wrong is too high a cable impedance (Live-Earth loop) and/or
too much voltage drop.

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...



Thanks again Tim

The CU is at the other end of the building to the kitchen (not quite 10
bedroomed mansion though).
The cable runs to get to the boundary of the kitchen loft space are about
12m.
The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total
cable length of approx 50m.

I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from
our electrical department (don't think they knew they had it!).
It seems pretty similar to the 17th Edition OSG, so that's a bonus. The
problem with a lot of it though can be interpretation and application,
which is difficult I find when you are DIYing rather than immersed in this
stuff daily.

Phil


Hi Phil

That maximum cable for a 32A RCD protected ring is given as 106metres of
2.5T&E in the 17th edition. That is for the maximum 5% voltage drop.
For a 32A 6mm cooker supply the maximum length is 49m either with or without
RCD protection using a type B MCB.

Cheers
Adam

"TheScullster" wrote in message
. uk...

"Tim Watts" wrote


The regulations require each leg of the ring to be capable of handling
20A continuously. It's an arbitrary, round, made up number that has some
leeway for current imbalance between the 2 legs. Meet 20A for any+every
section of cable and you're OK.

Might I suggest procuring a copy of:

http://www.amazon.co.uk/IEE-site-Gui...dp/0863418546/
ref=sr_1_1?ie=UTF8&s=books&qid=1276703557&sr=8-1

It's a condensed domestic summary of the regs with lots of diagrams, and
the more pertinent tables (though they forgot to mention reference method
B so you have to be a bit creative). You can do stuff that's not in there
that is still regs compliant, but if you do what the OnSite says, you're
guaranteed to be OK. Good section on bonding with minimum wire sizes too.

That is totally worth the 16 quid even if all you do is put in one
circuit.


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.

Yes. 32A is a pretty standard circuit. I'm running one, even though I
will have a gas cooker. You are allowed to have a 32A radial socket
circuit and that's what I will do - 13A socket on the bit behind the
cooker. Right place and the isolator switch next to the cooker still does
what is expected. If anyone wants an electric cooker, all they have to do
is switch the socket for a cooker terminal plate.


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?

One or the other is perfectly acceptable - as long as the cable is
protected from the sharp edge of the hole. You could even use a conduit
through-hole adaptor.

One other question - is your run of cable massively long? The other thing
that can go wrong is too high a cable impedance (Live-Earth loop) and/or
too much voltage drop.

"Normal" runs are Ok, but it's worth asking if you live in a 10 bedroom
mansion and the main CU is in the garage beyond the stables(!!) You get
the idea...



Thanks again Tim

The CU is at the other end of the building to the kitchen (not quite 10
bedroomed mansion though).
The cable runs to get to the boundary of the kitchen loft space are about
12m.
The kitchen is 5m x 3.6m and, at a rough estimate, this gives a total
cable length of approx 50m.

I have managed to cadge a copy of Niceic's 17th Edition Toolbox Guide from
our electrical department (don't think they knew they had it!).
It seems pretty similar to the 17th Edition OSG, so that's a bonus. The
problem with a lot of it though can be interpretation and application,
which is difficult I find when you are DIYing rather than immersed in this
stuff daily.

Phil


Hi Phil

That maximum cable for a 32A RCD protected ring is given as 106metres of
2.5T&E in the 17th edition. That is for the maximum 5% voltage drop.
For a 32A 6mm cooker supply the maximum length is 49m either with or without
RCD protection using a type B MCB.

Cheers
Adam

On 16/06/2010 13:52, TheScullster wrote:
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?

Wouldn't it be great if someone made a book or website showing good
first fix practices? I'm working my way through ring mains and plumbing
at the moment and having to figure out everything from scratch is rather
tiresome.

On Wed, 16 Jun 2010 17:23:38 +0100, Jim wibbled:

On 16/06/2010 13:52, TheScullster wrote:
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?

Wouldn't it be great if someone made a book or website showing good
first fix practices? I'm working my way through ring mains and plumbing
at the moment and having to figure out everything from scratch is rather
tiresome.

Adam (and others):

Book opportunity!



--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

"Tim Watts" wrote in message
...
On Wed, 16 Jun 2010 17:23:38 +0100, Jim wibbled:

On 16/06/2010 13:52, TheScullster wrote:
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?

Wouldn't it be great if someone made a book or website showing good
first fix practices? I'm working my way through ring mains and plumbing
at the moment and having to figure out everything from scratch is rather
tiresome.

Adam (and others):

Book opportunity!



--
Tim Watts

I will happily take some photos for the wiki on a few of my jobs.

But first fix is all about dust on a rewire.

Adam