I'd want to do this right even if I didn't have to have it
inspected under Part P!

I've slowly been replacing the wiring in my flat to a proper ring
final circuit (previously, there was a spur to a single socket in
each room, dating back to 1960). Because of the concrete floors I
have been chasing the walls and plastering over the cables.
However the final leg back to the consumer unit will involve both
cables running parallel along with a 3core+earth cable I have to
put in for a new light switch so I can rehang a bedroom door.

Because of limited space, these wires are going to be pretty close,
and I'm trying to understand page 41 of the On-site Guide (section
7.2.1 Grouping of cables). With twin+earth, for example, what is
"one cable diameter"? Is it the diameter of the conductive part,
the diameter of a single insulated core, or the width of the total
sheathed cable?

Alternatively, what are single-layer circuits? Does that simply
mean the cables don't overlap (in which case, I am covered by the
second part of 7.2.1(i) because I have less that 5 circuits for
this section) or is it something more complicated than that?

Many thanks

Stephen

Stephen Gower wrote:

[...]
However the final leg back to the consumer unit will involve both
cables running parallel along with a 3core+earth cable I have to
put in for a new light switch so I can rehang a bedroom door.

The light switch cable can be ignored if it's only carrying the current
drawn by one or two lamps, i.e. probably well under 1 amp. For the
purpose of grouping factors you only have two cables.

Because of limited space, these wires are going to be pretty close,
and I'm trying to understand page 41 of the On-site Guide (section
7.2.1 Grouping of cables). With twin+earth, for example, what is
"one cable diameter"? Is it the diameter of the conductive part,
the diameter of a single insulated core, or the width of the total
sheathed cable?

The last of those.

Alternatively, what are single-layer circuits? Does that simply
mean the cables don't overlap ...

Yes, it means a single layer of cables flat against the wall, either
just touching or spaced by one cable width, as appropriate. 'Single
layer' clearly has much better heat dissipation than 'bunched' or
'bundled' and the Cg factors are correspondingly higher. See also Table
6C in your OSG (pp118-119).

... (in which case, I am covered by the second part of 7.2.1(i)
because I have less that 5 circuits for this section) or is it
something more complicated than that?

Yes, you've nothing to worry about here (unless perhaps the protective
device is still a re-wireable fuse). A more contentious question would
be this: because they say "circuits" rather then "cables" can you
consider the cables for the two ends of a ring as one? A literal
reading would imply that you can, because the ring is one final circuit.
However I don't think that that's the intention, as you need to
achieve at least 20 A grouped rating for the cable of a ring at all
points along its length.

HTH
--
Andy