Well, it was obvious that running ELV all the way through a house would be rather inefficient, which is why I figured it would work better to place the PSUs inside the building, near(er) to the loads. Equally obvious was that less than 24V would be unwieldy, so that's why I picked 48V as the first choice in my post (for the same power delivered, each doubling of voltage reduces the wiring losses by a factor of 4, remember); as going much above that would bring us back into the "hazardous" range. With 48V at 15A (720W) you could just about heat a small room, even (though I'd stick with mains for that, so as not to "waste" heat in a PSU that may well be outside the room being heated). If the anticipated loads are sparsely placed, then you might opt for several smaller PSUs rather than a few big ones, so the wires don't have to run as far from each unit (and can also be of a smaller cross-section). You could of course also get a total span up to double the calculated length for a single cable, if you ran cables in opposing directions from the PSU...
The quick-and-dirty way to connect those 6mm2 wires to smaller terminals, presuming that they have 7 strands, would be to cut off 4 of the strands and then terminate the other 3 (2.57mm2 worth); while not recommended anywhere, provided the actual loading isn't above the (thermal) limit of 2.5mm2 you could get away with it pretty much forever. (Alternatively, use two strands for 1.71mm2, or just one for 0.86mm2.) On that topic, you might like to see
Aussie240's effort at wiring up a half-decently functional 12V supply for lights and electronics...
