I think assuming method C with no grouping factors for the end sections of a ring is optimistic.
OK, although I wouldn't mind betting that such is precisely what many (or most) usually do assume in this and many other situations.
In any event, even if one considers the 'worst (compliant) case' of a ring final whose cable has a CCC of 20A, then one can guarantee that a 'fat spur' (or, indeed, a group of close-to-one-another sockets 'on the ring') will not overload any of the circuits cable if it is more than about 37.5% of the way around the ring - and that extreme case will not all that often be encountered, so the 'safe zone' will usually be larger than that.
The way I see it the guidelines in the appendix are designed to be simple rules that lead to good outcomes the majority of the time even when applied by those without the skill/time/inclination to do detailed calculations and risk assessments or keep detailed records. I don't think endorsing fat spurs would be helpful in that regard.
I totally agree that such 'idiot proof guidelines' are valuable for those who are unable or unwilling to think for themselves, but I don't think that should prevent people who
are able and willing to think from designing other regs-compliant arrangements.
In any event, the guidelines in Appendix 15 are far from a guarantee against those who cannot or will not think. For example, there is no explicit warning against having several (in fact, in only needs two) sockets close to the end of a ring final, so a non-thinker is as able to create a situation in which overloading of the short leg of a ring is a theoretical possibility by doing that as they could by installing a 'fat spur'.
Does *anyone* do a detailed assessment of the risk of ring imbalance every time they install a ring circuit? or every time they add a socket to an existing ring?
Presumably not, and it's not even possible without a crystal ball (in relation to 'what might be plugged in where'), unless all sockets and spurs are kept well away from the ends of a ring. As above, it theoretically only takes two double sockets close to one end to make it theoretically possible to overload the short leg, even with Method C and no de-rating factors - and even one double socket close to an end of the ring would be enough with a cable whose CCC was de-rated to 20A.
However, I think the saving grace is that people probably get unnecessarily excited/concerned about this 'ring imbalance' issue, particularly in domestic installations. There are lots of 'safety margins' built into the design CCC figures we work with, and (presumably reflecting that fact) even BS7671 only requires (for a ring final) that "
... the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable". I would suggest that it is extremely
unlikely in most domestic environments that one could even approach 'fully loading' a 32A circuit "for a long period" (unless one has, and uses, multiple fan heaters
), so it would seem very 'unlikely' that any part of the ring circuit cable would be overloaded "for a long period", even if all loads were connected close to one end of the ring.
Perhaps, but the reality is that most of the time, at least in domestic but even in many commercial settings, noone is preparing and maintaining accurate and detailed documentation of installations. At best you get a certificate that documents the main cable type an installation method for a circuit, but do you really expect people to record whether an upsized cable on one segment is due to an adverse installation method on that segment or due to being provisioned as a "fat spur" to support future expansion. At worst the certificate never existed at all or was lost a decade or two ago.
All true - although as I said, by no means unique to 'fat spurs', and it's a risk that one can't really sensibly do much about.
Imagine you were asked to upgrade a shower from a 7.5 kW to 10.5 kW one, and found that both ends of the supply cable to the 7.5 kW shower was, say, 10mm² (protected by a 32A MCB), with most of the cable not accessible for inspection. Would you assume that such a cable had been installed as 'future proofing' (such that you could use it for the more powerful shower, with an appropriate MCB) or would you consider the possibility that the apparently 'large' cable had been installed because, by virtue of the installation method, it was only adequate for a 7.5 kW shower?
Kind Regards, John
