The way I understand it is that in the original edition of the "16th
Edition" regulations, flat twin-and-earth cable (FTE) wasn't
specifically mentioned. Instead people used to make educated guesses
based on touching, single cables (table 4D1A) and multicore cables
(4D2A). In fact (and certainly in practice) FTE cable performs better
than either of these two, due to the way the cores are arranged inside
the cable (they don't actually touch, for example). So, in the latest
version of the regulations (BS7671:2001, blue cover) there is a
separate table - 4D5A - specifically for FTE cable under certain
installation methods.

The methods covered a

Method 6 - enclosed in conduit in an insulated wall
Method 15 - installed directly in an insulated wall
Method 1 - clipped direct

The methods are explained in detail in appendix 4, table 4A1 (from
p210) of BS7671 but briefly, 6 and 15 involve clipping the cable to a
(thermally) conductive wall, then completely surrounding it with
insulation. Method 1 is cable clipped directly to a conductive wall
with free air around it.

Now, in free air, 2.5mm cable is capable of carrying some 30A. Even
Method 1 isn't this good, but the way I understand it, if the cable is
installed according to one of the mentioned methods *no further
derating is neccessary*, unless for reasons of voltage drop (distance)
or high ambient temperature, for example.

Table 4D5A is reproduced in the OSG as table 6F (p126). Here it is for
those without an OSG handy:

Conductor CSA Method 6 Method 15 Method 1
2.5 20 21 27

The confusing factor seems to be table 6B in the OSG, but the point to
note here is that this applies to cables *completely* surrounded by
thermal insulation. This might be the case if you put rockwool down in
the attic, lay the cables over the top of it, and then put more
rockwool on top of the cables. The more normal situation - where the
cables are resting on the ceiling with insulation only above - is
*not* addressed by 6B, but falls under 4D5A/6F. In the case where the
cable *is* completely surrounded, yes, a 50% derating factor may have
to be applied, but remember that this is relative to the "free air"
value for the cable which in the case of 2.5mm is 30A.

This is all explained on page 117 of the OSG, under the heading
"Thermal Insulation".

I have not been in this business very long, but as far as I can see
this is all pretty clear.

As far as the OP is concerned, if he *hasn't* gone to the trouble of
actually attaching the cable to one or other leaf of the cavity and if
he is intending to fill the cavity with loose insulation such that the
cable will be completely surrounded then yes, he probably does need to
rethink, and the idea of a 20A "ring" although non-standard is
possible. If, on the other hand, he *has* attached the cable to one of
the leafs (leaves?) or is intending to use solid batts of insulation
which will ensure that the cable is pushed up against a leaf then he
can probably rest, safe in the knowledge that he can plug three 3kW
heaters into his ring and not run the risk of melting his UPVC.

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
walls together?


With regards to the electical side of this, I think that I have 3
options.

Bearing in mind that I have 4 double sockets and the items that I
intend to use in the conservatory a

2 x Lamps with 60W bulbs
1 x TV
1 x Oil Filled Radiator (2.5 or 3kW)

My choices a

1. Leave it as it is (a 2.5mm ring) but not add any insulation to the
cavity.

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

Jon