[dreamcompiler]

What? 12AWG Romex is rated for 20A in normal residences in the US and it's already in a lot of homes, so the retrofit would be easy. And for LED lighting you wouldn't need nearly that much current at 24V, so voltage losses would be negligible.

[Dagger2]

"Rated for 20A" means that putting 20A through the cable will heat it to the maximum rated temperature, which is ~90C for that type of cable.

Heating all of your house wiring to 90C is a) dangerous, and b) expensive. Even if you're doing less than 20A, decreasing the voltage by a factor of 5 still increases energy loss in cabling by a factor of 25.