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I am running standard twin & earth PVC cable. Various cables will be
run from the CU it as follows:
clipped to the wall for a short space
then under the downstairs floorboards in free space
then up the wall buried in the plaster (no conduit)
then run between the joists (between downstairs ceiling and upstairs
floorboards, laying above the ceiling)
At one point, two cables will be passing through the same hole in a
standard size joist.

The cables will not be grouped together at any point other than passing
through the joist - they will be side by side when going up the wall.

Does the thermal insulation factor come into this anywhere (e.g. when
buried under the plaster?). Does the grouping factor come into account
for where the two go through the joist together?

Thanks

Roger

Thanks for the comments on this.
I am currently rewiring my house in stages.
Last year I added an isolating switch and replaced the existing fused CU
with two larger split ones, so the circuits are now protected with mcbs
(and an RCD where necessary). I also replaced the existing storage
radiator CU and relay arrangement for a new CU with MCBs and a modular
contactor. The relay that had originally been fitted was an open relay
bolted to a piece of asbestos sheet and contained in a perspex sandwich
box with the lid held on by insulating tape! There were two relays like
this, one for the immersion heater. However I wired the immersion cable
direct to the mcb and the immersion is now controlled via an immersion
timer in the airing cupboard.

The existing wiring was done by a previous owner about 28 years ago and
although not really in need for renewal, I am not happy with it. Rewired
by a previous owner (an electrician), the methods leave a lot to be
desired by today's standards.
One thing that really bugs me is that most of the lighting cables - about
16 - end up in a large adaptable box under the floorboards joined together
in a spaghetti junction of chocolate blocks. Whilst there is nothing wrong
with this, it is almost impossible to identify the cables. More worrying,
although there are two two way circuits (one for the landing light and one
for the hall light (which also has an intermediate switch)), only two core
cable is used. L1 and L2 are on one cable and the common for the landing
light (red) is on the same cable (black) as the hall light. To me this is
downright dangerous as it means that the cable is effectively fed from two
mcbs.

The only place 3core and earth was used was from the landing light switch
to the landing light - the yellow connected to the switch, the blue
connected via a chocolate block in the light switch box to a cable coming
from the adaptable box neutral and the red connected via a chocolate block
in the switch to another cable from the adaptable box that was live. I.e.
the 3 core cable wasn't being used for it's correct purpose but, again,
being used to share circuits. At the junction box for the ceiling rose,
the yellow and blue were used as the supply for the light and the red was
connected to the red of a T&E whose black was connected to the blue and
this went to another box that had the upstairs lighting circuit supply.
Now you see why I want to rewire :-)

There is an infra red heater in the bathroom which is currently wired into
the upstairs light circuit, which I believe is / was common practice.
However I am going to extend the up ring main into the loft (still as part
of the ring and not a spur) to give a socket there for future use, and at
the same time will wire the bathroom heater to this.

One of the jobs I have to do is to replace the cable feeding a brick built
shed in the garden. The cable is, I suspect, 1.5mm T&E PVC and is taken
from a fused spur on the down ring main. It terminated in a fused CU in
the shed. I've replaced the CU with one that has mcbs and is RCD
protected, I just need to replace the T&E with a much higher rated SWA
cable and take it to the CU.

I have always done my own wiring and I have John Whitfield's guide to the
IEE wiring regulations which I find useful. However I was rather stuck on
the correction factors as I didn't know how they related to what I was
doing. If unsure, I would have erred on the side of caution, however that
may have meant using larger cable than necessary. The only place ambient
temperature might be a problem is for the cables in the loft. The cables
would be clipped to the surface, but it does get quite hot up there and I
assume that I should allow for a 50 degree summer temperature and the
necessary correcting factor. This might limit me extending the ring main
as a ring to the loft.

Am I right in assuming that if a spur was taken to a single socket in the
loft (the actual ring main would be at bedroom floor level), it would be
quite alright to use 2.5mm as, even with the corrective factor for 50
degrees, the cable would not be carrying more than could be drawn from the
socket?

Voltage drop shouldn't be a problem as there are no overlong runs of
cable.

The shower cable size I'll bear in mind as I currently have a 8 or 9Kw
shower in 6mm, but I want to rewire for the possibility of a 10Kw one - so
I'll rewire using 10mm.

Sorry, just realise how long this post has got :-)

Roger