I looked at the 106 meters and could not work out to start with how it was arrived at, but at that time was a member of the IET and attended many lecturers so I asked.
It seems it is based on drawing 20 amp centre of the ring and the 12 amp even spread, so design current for circuit Ib = 26 amp not 32 amp, once one enters 26 amp into the calculations then you get the 106 meters, however there is no regulations saying one should take Ib at 26 amp, so as the designer you could take other figures as Ib.
@an203888 is correct before the volt drop was increased to 5% in the days when it was at 4% 80 meter was the figure considered as maximum for a ring final with 2.5 mm² cable, and the rule of thumb latter called traditionally was 100² meters.
I found with the correction factor Ct the formal to work out volt drop becomes complex, so I first made a excel program and when excel was not longer included free was a phone redid the program in java script to work out volt drop, I was a little worried that if I give an installation a clean bill of health with an EICR and it had excessive volt drop it could come back and bit me when next EICR was don't and it was found I had missed it. However fact that Ib is not fixed by the regulations, and the error in measuring the impedance once I started to enter figures I realised it would need to be way over limits before anyone could say it was really OTT for volt drop.
However the 20 amp centre of ring is based on a single double socket is normally rated at 20 amp, yes seems strange would expect 26 amp, but it seems the specifications consider 20 amp average is acceptable, which means with a radial on a 20 amp MCB we still take Ib at 20 amp. This means with 2.5 mm² radial down to 32 meters of cable to stay within the volt drop, one reason why I like ring finals. But as said it is down to the designer to decide what Ib is, and oddly there is nothing on the installation certificate to say what Ib has been used, so it is near impossible to say the ring final has too much of a volt drop.
So in the main only the ELI is considered, that is laid out in the regulations, however even that is not as cut and dried as one may think, again told that 0.35Ω is considered as the pass mark for a TN-C-S supply again this is based on volt drop for a 100 amp supply, so with a 60 amp supply it could be higher, but if the supply is 0.25Ω and the ring final centre point is 1.36Ω it has passed, but the supply could change to 0.35Ω latter meaning that ring final centre point is now 1.46Ω so fails, without any changes in the home. One would think you should allow for the supply ELI to alter as often supplied using a ring circuit, which unlike the ring final is not maintained as a ring, it just allows a section to be isolated so it can be worked on, so incoming ELI can alter.
The guide to the regulations is designed as that, a guide, it is not a regulation, back in the days of the 14th edition much of what is now in the guide was in the regulations, but to try to make regulations which it seems people hunt for ways to do minimum yet comply, to give suggestions seem to be taken as rules, even today we have an appendix which to my mind is not the regulations, but since we are tested on our knowledge of the appendix is seems it is taken as the regulations. So we get things like fixed appliances over 2 kW should have a dedicated supply, and the immersion heater has always had a dedicated supply, but it should also include the washing machine, dish washer, oven and tumble drier, but although there may be a dedicated supply to oven rarely is there a dedicated supply to laundry equipment which for some reason seems to end up in the kitchen, why food and laundry should be mixed I have never worked out.
But we are looking at design, and does the guy who designs the house actually say what should be fitted in each room? But returning to the 100² meters if you split a 100² meters into 4 rooms with sockets on the dividing wall you will find you need around 100 meters of cable by time you include the drops and the 1/3 rule on drilling beams.
We see the lollipop design but again although used can't find a regulation about its use, we talk about "current carrying capacity of the cable" which even with the same 2.5 mm² cable can vary along its length due to installation methods, so using a 6 mm² cable to a cooker connection unit and then use a ring final from that can reduce to volt drop in the ring, but I struggle to work out if permitted or not, it makes good sense, and I know it is done, but not sure if it complies, what I am looking at is:-
434.2 Position of devices for protection against fault current
Except where Regulation 434.2.1. 434.2.2 or 434.3 applies, a device providing protection against fault current shall be installed at the point where a reduction in the cross-sectional area or other change causes a reduction in the current-carrying capacity of the conductors of the installation.
I think it complies but not sure, but this of course is the problem,
@an203888 is doing the design, and he is going to sign the installation certificate to say he has the skill and has designed the installation, and even when writing this reply I have been looking at the old BS7671:2008 to see how it is worded, and we are now on BS7671:2018 so a bit outdated. Most of our design is rule of thumb and tradition, we don't work it all out from scratch, but
@an203888 has a problem, some one from the LABC is going to look at his work, he may be a sensible fella and say yes that's fine, or he could draw air through his teeth and say those hated words "jobs worth".
So what we are trying to do is to second guess what he will or will not permit.
Like so much else, he believes it in his head. It’s what Orwell called Thought Crime in 1984.
