Erm... I've seen places with a 2.5(!)mm2 single phase supply! Fused 20A.
Sure it's easier to just replace a fuse... still, I don't like the idea of being forced to bury so much expensive copper I'm never going to use and even might have to replace it when it ever comes to using the extra capacity.
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Most of all, we're talking apartment here, that means it's maybe 5m, sometimes less of wire in conduit, no harm in putting in conduit large enough for 10 or even 16mm2 and just pulling larger wires whenever needed!
To put it in simple words: I'd feel pretty stupid if I put in 5x6mm2 for my single phase supply to be able to upgrade to 3 phase and when I want to upgrade to 3ph I only hear: "I'm sorry, you have to tear out all your nice extra wire and install 10mm2!" So, I don't go above code minumum in that regard.
