That is a great guide from Napit, Floody, thanks for the link. It is a little dated though.
I think the key phrase is included in the side bar, the 'correct method'. With experience one is able to assess and estimate with some precision, intuitively. However, if the 'correct method' is observed and applied as shown, it does
prove the selection beyond question. There may be occasions where one is required to substantiate their reasoning, particularly where cost is a prime factor or you are tendering for a particularly lucrative contract. In my earnest opinion, of course.
There is a thread of mine which should be of interest, particularly as you mention cable selection for a ring final which is a 'Standard circuit arrangement', and to my understanding is subject to certain requirements re. In / Iz. My original concern was with the order of application for correction factors, to which there has been some confusion.
http://www.electriciansforums.net/electrical-course-trainees-only/72256-application-diversity.html
You will cover the process in greater detail come level 3; I'm currently on the 2365 L3, and part of a recent assignment contributing to our final grade focused on this element. Having decided on a number of final circuits, we were asked determine the minimum cross-sectional area of live conductors for current-carrying capacity and voltage drop for one of the high-bay lighting circuits. Disconnection times and thermal constraints did not feature at all.
