So it's 25% limit re roof repairs? Thanks for that information.


First cable derating...
- A cable has a quoted Current Carrying Capacity (CCC) in "free air"
clipped to a non-insulating surface. An example would be a cable
clipped to a wall stud in an otherwise insulated wall, or clipped to a
joist in an otherwise insulated roof. For 1.5mm FTE cable its CCC is
19.5Amps.

- A cable is derated to a lower Corrected-CCC when it is run in
conduit or surrounded by insulation. An example would be a cable
surrounded in insulation for more than 500mm, correction factor would
be 0.5 (IIRC). For 1.5mm FTE whilst free-air CCC is 19.5Amps the
Corrected-CCC is just 9.75Amps.

Second "fuse" type...
- A rewireable fuse takes longer to disconnect an Over Current (OC)
fault than a cartridge fuse or type-B circuit breaker (MCB, RCBO). For
rewireable fuses a second correction factor of 0.735 must be applied -
that is, Corrected-CCC needs to be 1/0.735 higher than the fuse rating

After applying cable factors the Corrected-CCC of the cable must STILL
exceed the CPD rating.
For 1.5mm FTE run 500mm through insulation with 0.5 factor creating C-
CCC of 9.75A protected by a 6A Type-B CPD there is no problem.
Conversely there would be for 0.7mm (if I recall) imperial Flat-Twin
(no Earth CPC), the figure would be somewhat less.


So you need to find out...
- 1 - what type of overcurrent protection you have on the upstairs
lighting final circuit
- 2 - what size & type of cable you have currently installed

In new build I think they are using BS7211 XLPE LSOH which is higher
CCC rated to 90oC not BS6004 PVC cable which is lower CCC to 70oC. The
argument is that whilst the wiring accessories can not withstand
70oC, the cable itself will only be running warmer for the length
surrounded by insulation. BS7211 is more expensive, about £45 vs £25
at Screwfix, and much harder to work with re stiffness & stripping
(forget the simple hand twist & pull tools).

If your upstairs lighting drops are in oval conduit, changing the
upstairs lighting circuit is trivial - about £25/45 in branded cable
and no more than £8-10 for new ceiling roses. Additionally you can rid
yourself of those pesky junction boxes which would otherwise be hidden
under insulation. If your lighting drops are not in conduit, things
get more tricky - you end up with "loop-in" junction boxes directly
above each light drop relying on the old cable (which runs in plaster)
for the lighting drop and thicker cable to each ceiling rose. You may
need Part L compliant ceiling roses. Other issues might be
supplementary bonding in bathrooms.

If a reroof 25% requires a Building Notice then you can add the
wiring onto that at no extra cost? Not looked.

There are a lot of houses now with 200mm insulation and very slim
cables over the top. In theory as long as the cable is touching an
uninsulating material such as plasterboard or clipped to a joist that
is ok. In practice I guess we will know in about 5-30yrs time - and I
bet they don't provide a second grant to fix it (perhaps that's Gordon
Brown's plan re downstream employment & tax creation device :-)

Do not have the 17th regs to hand so can't be more specific re
figures, do have imperial data from 13th if the house is old.