Anyone understand cable grouping factors?
When 2 or more cables (BTW only standard UK domestic pvc sheathed FTE
concerns me) are close together,
IEE Regs (16th ed/yellow cover) page 197 Table 4B1 gives reduction
factors for cable rating.
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.
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*.
The reason stated for derating is that cable close together can't
loose enough heat at full load.
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? If so why? ISTM you have the same potential
over-heating problem as 2 separate circuits, possibly worse, as both
legs will heat up for certain at exactly the same time, whilst 2
circuits might Cox & Box.
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?
Q3. Are there many situations in normal domestic wiring where you need
to apply grouping factors? Do you need to near the consumer unit
You really should be using the blue cover versions. Some significant
changes affecting cable ratings were made in Amendment no. 1 to BS
- 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).
No, it refers to circuits of single core cables (i.e. 'wires' in conduit,
etc.) and to multicore cables.
That is _not_ correct; there are two multicore cables and the grouping
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.
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 Ib<In, 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.
yes, I have these amendments by download, but that raises the Q....
Q What is an 'insulated wall'? Are walls built with light(ish)weight
building blocks considered 'insulating'; or lightweight plasters such
as carlite browing?
but still over the 20A ring rating - so still OK unless there is some
other adverse factor in the ring?
Please can you clarify what is considered best (or standard) practice
when running ring cable up & down a wall? Separate the cables by the
required gap? Find a completely different routes for the 2 legs? Or
just run them up & down touching, just accepting the grouping
streuth, you need to be a lawyer for this. That heading needs reading
very slowly many times: Send for the plain English people...
Many thanks for this clarification & of 7.2.1.
been there, done that, obviously didn't get the t shirt. but will try
No to both of those, IMO. Stud or cavity walls with thermal insulation
materials in them, or "special thermally insulating plasters" which
incorporate polystyrene beads etc. count at insulated. I've never seen a
strict definition in terms of U or k values though. The diagrams in
Appendix 4 give a clue as to what they mean by insulated.
Side-by-side (touching so far as Appendix 4 is concerned) or bunched are
usual. They're perfectly OK unless the presence of other cables in the same
chase gives a problem, in which case re-routing the other cables may be the
I think the organisation and layout of the OSG could be much better than it
is. It may be logical (in terms of relationship to BS 7671) but it's not at
all user friendly -- esp. w.r.t. the division of material between the body
and the appendices.
Took me some time to get the hang of it too. You have to read it several
times. "Education" -- as one of the profs used to say when I was at
college, many years ago -- "is a process of diminishing deception."
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