Last night, I ran the cables for my ring main into the cavity of the
dwarf wall in my conservatory (before the plastic went on).. Should I
do anything to protect the cable, as it seems a little 'exposed'.
I was recently reading about problems with "damp" when the cavity is
breached.. When looking at the 'bat ties' which run between the
cavity, I noticed that they have little kinks the shaft, to prevent
water from running down them.
Because the cable is obviously not 100% straight, it touches both of
the walls at various points.. Could this be a source of damp?
I will be putting some insulation into the cavity, but should be doing
anything else to protect the cables?
Any info on this would be apprecaited
You may well find it bridges the cavity. It is usually better to chase the
inside wall and plaster over. If you must run in there, you should use cable
clips to fix to the inner (dry) leaf. The bricks are over 50mm wide, so you
can run the cable at any height. I'd suggest near the top where you can
swing the hammer for the clips. You must derate the cable for the expected
insulation, which may require a ring main to be run in 4mm (or even 6mm in
some cases) after the calculations.
AIUI, 2.5mm cable run in contact a conductive surface with the rest of
the cable completely surrounded by insulation is rated at 21A which is
sufficient for a 32A MCB protected ring main. This is all in the OSG
and was discussed here fairly recently (don't have the OSG to hand so
can't look up the table number for you). The only other thing you
*might* have to take into account is the total length of the circuit,
but that is very rarely a problem in a "normal" house!
I don't have my OSG on me either. However, I seem to recall a 50% reduction
for being buried in insulation, which is what you would have to use if using
the OP's original idea of letting it float within the cavity, not clipped.
This would mean his original plan would be seriously non-compliant, quite
apart from the cavity bridging issue.
But it is reduced from 27A "clipped direct". This is the *new* table
4D5A in the regulations or 6F in the OSG. Note that 21A is given with
"method 15" - where the cable is partly surrounded by insulation.
Sorry, brief message. My experiment with NS6 on this G3/300MHz/64M/OS9
Mac using Google is rather sss-lll-ooo-www :-/
Martin Angove: http://www.tridwr.demon.co.uk /
(As a reader only): http://www.livtech.co.uk /
I was originally going to run the circuit as a 'radial spur'. As I
have 4 double sockets, I was going to install a 13A fuse spur off the
existing ring and then just run a radial circuit from the spur to the
But, considering that I want to put a 3kW heater in, the conservatory,
then thats 12.5A already..
So I decided on a ring.. I do have a fairly large house, and extending
it through the conservatory has added an extra 20m to this (10 meters
each way), so I hope that this won't be a problem!
It will, unfortunately. A 2.5mm cable buried in insulation is not suitable
for a 32A ring main. If you can't redo in thicker cable (i.e. the windows
have gone in), I'd run the new section back to the consumer unit and use a
20A MCB. That will be enough for your 3kW heater and a few other bits.
Alternatively, run it as 2 13A spurs, with the fan heater off an FCU
directly and the sockets off another fused 13A spur. You can easily do this
as you ran as a ring, so you've already got two supplies to the sockets,
provided the FCU for the heater can be on the first or last socket of the
I have looked it up in the regs and am not sure what you are referring
I have the "IEE Wireing Regulations" for 1991 to hand (which is out of
date, but I wouldn't have thought that things would change to much
regarding current capacity of a conductor).
Table 4D24 covers "Multicore P.V.C Insulated Cable, Non-Armored
(Copper)" and WORSE case for a 2.5mm cable is 18.5 amps (for a heat
As I have a ring, this will give me capability of 37amps and
protecting this by a 32MCB should be fine.
Now condidering that this cable is in a cool, cavity on an external
wall , I would imagine that it would be able to carry MORE than
18.5amps, but even if 18.5A was the limit, its still within spec.
In "Free Air", the same table shows that the cable will carry 30amps,
so I would im
I am not talking about packing insulation around the cable, but
instead, placing it fairly loosly in the cavity. As the cable is right
at the bottom of the cavity (below the DPC) there would still be a air
space around the cable.
But, back to your statement about a 2.5mm cable not being upto the
job. From the info that I have, even if packed tightly in an insulated
space, it would still be fine.
Please can you confirm where you got your info, as I am slightly
worried now and want to be sure that I am right before continuing.
Hopelessly out of date in many respects, I fear. Christian has posted
details relevant to ring circuts. It would be worth you buying the
up-to-date OSG (blue cover).
Serious misunderstanding there: a ring cable has to be rated at 20A
(formerly two-thirds of the fuse/MCB rating), and _not_ half of the fuse/MCB
rating. This is allow for the fact that the load won't be uniformly
distributed around the ring. Another amendment since 1991 requires that you
specifically consider the likely distribution of load on every ring to
ensure that the actual cable rating won't be exceed for prolonged periods.
A problem could arise if (say) you had all your kitchen appliances towards
one end of a long ring, leading to the need to consider a different circuit
layout, or ring sections in 4mm^2 cable.
That's not a terribly sensible place to run a cable. It's an area usually
full of spiky mortar snots and other brickies' debris. I'd be worried about
the risk of slight movement leading to cable damage.
Agreed -- in fact it's only just OK for the 13A fused spur.
But that will gain him absolutely nothing. You're replacing a ring which
requires a cable rated at 20A with a radial circuit also requiring a cable
rated at 20A, whilst the cable (as instlled) only has an effective rating of
That's what I was going to suggest.
The heater socket or FCU needs to be on a run of its own. In fact with a
single FCU or single 13A socket, it could be an unfused spur.
Sorry, I wasn't clear. It would become a 20A ring. A bit non-standard, I
know, but safer than running 32A.
Another misunderstanding. I'll try to explain better with some ASCII art.
OP's original plan:
---+-// original ring cut here //-----------+-----
original ring left intact
I suppose the FCU on the right could just be an unfused spur socket for the
heater. The left hand FCU could be on the ring as shown, or installed
adjacent to SKT1. In either case, it does not require laying more cable.
However, I'm now getting confused as to the rating of 2.5mm cable layed
within insulation. Not having regs with me doesn't help. What is the
definitive current capability of it?
Oh OK, but received wisdom is to avoid non-standard arrangement like this,
especially in domestic installations.
I think we're saying the same thing really, but got there by different
There's no installation Method number for "totally surrounded by thermal
insulation", because it's something that you're supposed to avoid. So (IMO)
you take the clipped direct rating (27A) and divide by 2, hence 13.5A.
I had precisely the same issue as the OP, viz. dwarf cavity walls and
wanting not to use plaster on the inside but to have brick.
I wanted to run a power ring circuit and also CAT5, CT100 and speaker
Clipping the power cables within the cavity to a leaf using convention
clips was not practical, so I devised a way to do this differently. I
also had in mind the rules about cables needing to be in a band of
150mm down from the top of a wall and at least 50 mm below a surface.
So, I began by fitting insulating batts up to a distance of about
250mm below the top level of the wall. I ran all of the non power
cables simply by laying them on the batts at this level and feeding
through. I then added strips of batt to take the insulation up to a
level 100mm below the top level of the wall.
I made special brackets from fairly stout galvanised strip about 8mm
wide and fashioned towards one end a U-shape designed to accept the
power cable. The other was bent to an L-shape, but actually slightly
acute of the 90 degree angle. The distance from the U part to this
bend was about 60mm, as was the remaining straight piece the other
side of the L. I covered the U part in two layers of heat shrink
sleeve and drilled a hole 25mm from the other end.
At each clipping position I had had the builder put in wooden blocks
between the bricks to a depth down from the top of 15mm, and coming
forward from the cavity side of the inner leaf by 50mm. Mortar was
placed in front of these blocks so that the inside face of the inner
leaf is normal. The same thing could have been done after building
by carefully drilling down from the top and removing mortar.
Each clip was installed by putting it into place holding the cable,
and temporarily holding with a clamp. Then the fixing hole position
was marked from above. The assembly was removed and the hole
drilled from above to receive a plug. The bracket was then refitted
with the cable, clamping again while the screw was inserted.
The effect is that the cable is held in place against the inner leaf
of the wall about 70mm down. I forget how frequently the clips were
placed, but within the horizontal clipping distance.
The space in the top of 100mm was left without any insulating batt.
So in effect, the cable is clipped to a masonry surface and is at
least 50mm away from any potential mechanical damage.
To email, substitute .nospam with .gl
Glad you said that... Following advise from earlier posters, I was
tempted to bring the cables to the top of the leaf.. This would have
bought them to around 20-30mm from the top of the wall, and once the
'window sills' had gone in, less than 50mm below a surface.
I spoke to our office 'electrician' and doesn't see a problem leaveing
the cable in the cavity as it is.. Sure, its quite sharp down there,
but there is going to be very little (if any) movement once its all
finished and therefore little risk of damage in the future.
Now that the base and wall is finished, I don't really have much
choice.. As I said above, I could quite easily raise them to the top
of the walls, but then I would break the regulations (150mm and 50mm)
that you mention..
I simply can't win!
Many thanks for the advice.
Many thanks for this info.. Am I right in thinking that this entire
debate has come about, because I mentioned "insulation"?
At the moment, the 2x2.5mm cables are simply lying in the cavity,
which does currently does not have any insulation in it... Based on
all of the info that I have read, I think that in this configuration
(i.e cable in free air), its more than up to the jobs. And i would
hope it is, as thats what the rest of my ring is wired with.
With regards to attaching the cable to the leaf.. Its no so much that
I "havn't" gone to the trouble, but more that I didn't realise that I
should. I too was concerned about the damage which could be done to
the cable in the cavity (due to sharp edges on the morter) and one of
the reasons why I originally asked this question. However, I was
realiable informed by sevearal of my (apparantly well qualified)
collagues that it was OK as once the cable was in, it wasn't going to
move and it was VERY unlikely that any damage could or would occur at
a later date.
However, this weekend I will look at the possibility of bringing the
cable to the top of the leaf and fixing it in some way.. It may be an
off the wall suggestion (no pun inteded), but what about fixing the
cables (using cable ties) to the metal 'bat ties' which link the 2
With regards to the electical side of this, I think that I have 3
Bearing in mind that I have 4 double sockets and the items that I
intend to use in the conservatory are:
2 x Lamps with 60W bulbs
1 x TV
1 x Oil Filled Radiator (2.5 or 3kW)
My choices are:
1. Leave it as it is (a 2.5mm ring) but not add any insulation to the
2. Disregard one of the cables and connect the other as a radial
circuit to a via a 13amp Fused Spur - This was my original plan, but
would restrict me to a total of 13amps in the conservatory.
3. Use the 2nd cable to create 2 radial circults protected by 2 x
13amp Fused spurs.. One for sockets 1 and 3, the other for 2 and 4. -
This means that in theory, I have access to a total 26amps (which is
completely over the top), as long as I am careful which devices to
plug in which sockets.
Your thoughts on this would be appreciated
I am SERIOUSLY confused now and to be honest, slightly worried.
As far as I know, a standard domestic ring is done with 2.5mm Twin and
Earth.. If this is the case, when whats the difference between that
and what I am doing now. I am simply breaking the ring at the back of
an existing socket and extending it through the conservatory.
Has all of this started because I mentioned 'insulation'.. If it is,
thats simple.. I won't use insulation, or I will bring the cable ABOVE
The ring mains in my house are wired in 2.5mm and protected by a 32A
MCB! Does this mean that they not up to the job and are consiquently
If what I have done in my conservatory really isn't up to the job,
then I will simply resort to plan B which was to go for a 'radial
circuit' protected by a 13A spur.
With regards to the 'heater' outlet, I am not 'hard-wiring' in.. I am
buying a DeLonghi oil filled radiator (2.5kW or 3kW) which AFAIK comes
fitted with a standard 13amp plug, which can be plugged into any
Can someone please explain, as I am a really confused now.
YES. If there's no thermal insultion, or if there is and the cable touches
the wall on one side there's no problem at all.
No, they sound perfectly normal.
The point there was that you were proposing to put in a fused spur, knowing
that you would connect a 3kW heater, so that the use of any other appliance
would overload the circuit. Hence our suggestion to split it into two spurs
with the heater socket on its own. But if you revert to Plan A (minus
thermal insulation) then this is no longer relevant.
What you have suggested is not possible in my case.. Running cables on
the inside is no good, as I am leaving the brickwork exposed and won't
be plastering it over.
I could bring the cables to the top of the internal wall, but I really
don't think that I will be able to clip it in place.. There just isn't
enough room in the cavity to swing a hammer, even from above.. Chances
are that I would end up chipping the tops of the bricks and making a
At the moment, I have pushed the cable down into the cavity as far as
they would go. Effectivly they are lying below the DPC level.
I really hope that this is OK as I have already shreaded my arms and
don't want to do anything more.
Due to the insulation, the work you've already done won't be compliant. The
insulation will cause the wiring to overheat. If you want to keep it, then
run the circuit back to its own MCB, rated at 20A, rather than 32A. This
should give better protection, in line with the 2.5mm's actual capability
buried in insulation. With bare brick in a conservatory, you might not care
about the bridged cavity anyway. There's no plaster to spoil.
One other technique you could use is to take thin battens and nail clips to
them every 40cm. Stick these to the top of the wall inside the cavity with
Gripfill, let it bond, and then push the cable under the clips, or use
plastic cable ties. You'll probably still have to derate the cable, as I'm
not sure if wood is a conductive surface (no OSG again!), but at least it
will be tidier, simple to do and won't bridge the cavity.
The final suggestion is to use metal conduit on the inside wall. This will
make it look a bit like a school or light industrial premises. However,
depending on the style of the house and conservatory, this might actually
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