"pickerel" wrote in message
om...

IEE Regs (16th ed/yellow cover) page 197 Table 4B1 gives reduction
factors for cable rating.

You really should be using the blue cover versions. Some significant
changes affecting cable ratings were made in Amendment no. 1 to BS
7671:2001, notably:

- the introduction of Table 4D5A giving 'new improved' ratings for flat
T&E cables under some installation conditions, together with the
definition of a new reference installation mathod (method 6) for
cables in conduit in an insulated wall;

- clarification of the requirement for 30/32 A ring circuits, i.e. that
the cable rating _as_installed_ must be 20 A or more (also that you
must consider the likely distribution of load on the cct to ensure
that the cable rating won't be exceeded).

eg 2 x 2.5mm2 FTE cables (on wall under plaster say) touching would
reduce full load rating from 27A to 0.85 x 27A = 23A.

That's correct.

IEE On SIte Guide (yellow cover) section 7.2.1 (page 40) & table 6C
(page 116/7) also refer. The heading to Table 6C states the factors
(only) apply to *circuits*.

No, it refers to circuits of single core cables (i.e. 'wires' in conduit,
etc.) and to multicore cables.

Q1. If you run a cable (eg in a ring) down a wall to a socket box &
back up along the same route (ie touching the down leg) - is it
correct that group derating isn't applied because the legs are both
part of the 1 circuit?

That is _not_ correct; there are two multicore cables and the grouping
factor applies.

Q2. On Site Guide pg 40 7.2.1 states 'In domestic premises ....the
conventional circuit design permits...ifs & buts... up to 5 touching
...circuits....' - How does this fit in with the regs? What is it
about 'conventional circuit design' that allows this? Where does the
5 come from?

I think the basis of it is this: the conventional circuits (OSG Table 7.1)
will all apply for installation method 6 (M6), unless rewireable fuses are
used. If you divide the tablulated M6 ratings by the relevant Cg value for
5 cables (0.73) you'll find that it's within the tabulated M1 ratings[*].
Therefore if a circuit works for a single cable using M6, it will work for 5
touching cables installed using M1.
[*] For the 2.5mm^2 cable the M6 rating is 20 A, 20/0.73 is 27.4 A but the
M1 rating is 27 A. 'Favourable rounding' seems to have been applied here.

Q3. Are there many situations in normal domestic wiring where you
need to apply grouping factors? Do you need to near the consumer
unit perhaps??

Yes -- in principle, wherever you have more than one cable. With experience
you come to know situations where a cable size would actually have to be
increased, or an alternative route found. In practice be vigilant with
cables running in thermally insulated walls and roof spaces, and, as you
say, with the confluence of cables near a consumer unit or disboard.

Don't assume you always have to uprate by 1/Cg for all N cables. There's a
rule that says that if a cable is loaded to less than 30% of its grouped
rating, it can be ignored for the purpose of calculating the rest of the
group. This often applies to lighting circuits, or parts thereof. Also for
circuits where IbIn, the use of the "not liable to simultaneous overload"
formulas can help.

I recommend a thorough reading of the first few pages (headed 'Preface') of
Appendix 4 of BS 7671.

HTH
--
Andy