Hi. I know this is an old thread, but the topic is no less relevant today than it was back then so I don't see why not.
C-H was probably right about it coming down to people's unwillingness to question the authorities. Now, I'll examine the dimensions of some common flexible cords as specified by UL (I don't have access to the standards documents themselves - too expensive for anyone not in the trade
- but I managed to retrieve the dimensions of many of the cords from a few manufacturers' websites)...
Compare SJT to ST (or, indeed, their rubber or TPE-insulated counterparts), for example. The 18AWG and 16AWG versions of both have the same inner insulation thickness of 0.03" (0.76mm), with only the outer sheath being 0.06" (1.52mm) thick on ST (except for the 5-conductor variants which further increase it to 0.08" (2.03mm)) versus 0.03" thick on SJT. The category voltage is up to 300V for SJT (and SVT, SPT and NISPT), but 600V for ST. Then compare them to the European H05VV-F, which has a category voltage also up to 300V on any phase, but up to 500V between phases in the cord; its construction is comparable to SJT (or actually has a bit
thinner inner insulators in the 0.75/1.0/1.5 sq.mm variants). So H05VV-F is suitable for 231/400Y (or 277/480Y although I don't know of a region actually using the harmonised cables with that voltage) but SJT isn't? If you don't need the extra abrasion resistance and aren't worried about "by-the-book" inspectors then I guess you could use SJT for 277/480Y, save a little cash and be happy.
NISPT-2 is another oddity; what exactly is the point of making a cord with nice thick inner insulators, but a thinner (and therefore easier-to-break)
outer sheath than even that of European H03VVH2-F?
But it's SPT-3 that's the real case of "why bother?"; the 18AWG and 16AWG variants have an insulation thickness of 0.06" which is the same as the sum of the inner+outer insulation thicknesses for SJT (as already mentioned), so I think it's a fair comparison. But the 14AWG and 12AWG versions apparently had to increase the insulation thickness to 0.08" and 0.096" (2.41mm) respectively, so these "household use only" cords ended up using as much as or even slightly
more PVC than the "suitable for commercial use" SJT. So I can't say I'm surprised that it has no near-equivalent elsewhere in the world. (Mind you, that whole "residential/commerical" distinction was almost entirely stupid to begin with, and I can only say I'm glad it's more-or-less exclusive to North America. But from what I see of them, the UL never were any good at making
logical standards.)
Anyway, it's interesting to note that the (modern) Australia/New Zealand TPS cables insulate the earth conductor as well as the circuit conductors (avoiding the use of add-on sleeves, at the expense of a somewhat wider and costlier cable), and that their earth conductors also have at least 7 strands even when the circuit conductors are solid (most likely so that the earth is the last to break if the cable is abused by repeated bending).
