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...... 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??
TIA
perplexed pickerel