From reading some of the other threads, I'm the guy you guys love to hate: a homeowner who wants to add a socket outlet or two himself. By the way it's a single-phase installation, I remember a blurb in the standard about that being relevant.
I did, however, shell out for SANS 10142-whatever. I have my issues with some of the wording (ambiguity in places), but overall it's quite clear to me what I need to do to make my socket outlet(s) good.
In particular, wire size. By my reading, the correct wire size for an outlet on a 20A circuit has to be 4mm^2, not 2.5mm^2, because I'm using "installation method 3" (conductors in conduit in air, in ceiling space). That means I already can't use the 27A claimed for 2.5mm^2 wire on its packaging, and more importantly by the electrical goods sales staff, due to the method alone. Then I add some thermal derating, assuming a roof space temperature of 50C (I measured 10C higher than indoors temp, assuming worst-case 40C in Cape Town); I think the factor was about 0.65 or something - I don't have the spec in front of me. And then I find that the 4mm^2 wire is just *barely* adequate for such a 20A circuit.
It was actually quite funny, I'm normally quite a softly-softly type of guy, but there I was at the counter insisting that I wanted the thicker wire, and no, they can just give me what I asked for instead of the 2.5mm^2 they strongly suggested. In a room full of people who looked like they do this stuff all day.
Am I needlessly overengineering my new outlet? I don't think so. But what's with the supply shops claiming 2.5mm^2 is enough? When I looked at (some of!) my other pre-existing outlets, there seems to be a lot of the thinner wire around. Only a few with what looks to be a thicker wire - it might even be 4mm^2 wire from the quick look I gave it.
What's going on here? (And also, shouldn't supply shop staff be trying to get me to buy *more* copper than I need?