The thing you need to remember JACCD is that BS7671 is a guide, not an instruction book. So whilst table 4D5 does specifically mention 'insulated and sheathed flat cable with a protective conductor', however, that more specific description did not appear in earlier regulations, although PVC flat T&E was in existence for a very long time. In some earlier regs you would have been using tables referencing something called 'multi core PVC non armored copper conductors' And the notes under 4D5 do refer to the full methods list 4A2, where the more general description Multi core Sheathed Cable is used. But 4A2 only mentions Flat T&E in thermal installation (100,101,102,103). Not 'A' or 'C'. However if you look at numbers 8 & 9, method B, there is mention of multi core PVC cable in conduit on a wooden or masonry wall. Including specific note to the thermal effects if the run is vertical or horizontal.
So I would suggest you have two options; The first is to actually contact the manufacturer of your specific cable to ask them for their cable capacity data for your method of installation. Or option two, make a bit of a sensible judgment call using data from the tables for PVC multi core cables, non-armored, copper conductors, installation method B, for the conductor size of your cables. and do your calculations on those figures. So don't forget BS7671 is a useful guide to your design, installation, certification and testing. It is not a comprehensive every possible permutation of answer will be found in here book. And departures and deviations are permitted, providing you can prove your deviation does not increase any dangers to people property or livestock than would not have been there if your deviation had not been implemented. To answer your "am I missing something" question. Don't become blind to a bit of common sense when required, is the thing you are missing I would guess.
Doc H.
