I guess you mean Pp 177 on brown on site guide.
That has the typical CCC for different methods. It is also present on page 84 as Table 7.1(iii) following the "standard circuits" that I was looking at.
Page 75 has the various RFC combinations of OCPD and cable size that seem to be considered "standard". If you look at the table then only 4mm is listed as acceptable for methods 101 or 103. Previously it was the longer length that I looked at when considering 4mm (e.g. 171m instead of 106m on VD limit).
Just to expand on balanced load viz RFC, I guess the most load would be the kitchen sockets at around 25% of the way around the ring circuit. This is a studio flat with 50 m² floor space. So washing machine, toaster, kettle etc. would be at that point. So you are saying (and I have seen it reference often that the load will mostly (or all?) go onto the run to the kitchen? and the long leg of the ring will take hardly any load. Is there any scientific basis for this or regs. Can't say I have seen any. I just wonder what science this is based on. If that is the case it defeats the whole reason for having a ring then? Might as well do radials (4mm)
With method 103 a 4mm radial is only going to be good for 17.5A so with socket outlets then 16A MCB.
You probably would need two 4mm radials for the kitchen if you have more than two ~3kW loads (e.g. washing machine and dish washer perhaps?) as they would really have to be on separate radials, and at that point you might as well do the whole flat as a RFC in 4mm! More so if AFDD are needed...
Also if doing 4mm radials it is no different to a RFC for wiring. Provided you avoid spurs as then most sockets (all in RFC) have only 2 * 4mm cables. Which you should, as they are the Devil's work.
Just to beat this to death, the asterisk on the methods points out the cable must be touching the plasterboard in order to conform to the ccc tabulated. I am not positive it will actually contact it very well.
There are various ways you might look to getting out of using 4mm cable. Going to LSZH version rated at 90C gets you something like a 22% increase in CCC and the accessories are OK as your use of CSA and working current is the same as for 70C PVC, just you are relying on hotter safe running in the thermally insulated region.
Dropping the RFC to 20A is another route to keeping a limit on possible cable heating, but you might find it is getting close to probably total load if the kitchen has a good few gadgets.
But all of those might just bite you if there is any trouble down the line. Sticking to the OSG combinations from Table 7.1(ii) (for RFC, lights, or radials) is by far the easiest route to having a safe and justifiable design for most domestic work!
