"pickerel" wrote in message
om...
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.

So, Qs:

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
perhaps??



perplexed pickerel


http://www.lexcie.zetnet.co.uk/traction.htm


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BigWallop

http://basecuritysystems.no-ip.com

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