Feed wire size

I use #10 Cu for 3.6/3.8 kW inverters, where #12 would meet code. Otherwise I'll use #6 ACWU, or #8 Cu if the client prefers the cable run inside PVC conduit.
For DC I only use #10 Cu RPVU, because that's all my suppliers stock.

p.s. I ignore power factor since I've never measured anything less than 99% above 50% load.

The calculations are easier if you notice that each step in AWG changes the resistance by 1.26x. So 12 AWG has 58.74% higher resistance than 10.

In my system I measured a power factor of 0.83. In particular 20.8A * 247V yielded only 4300W. Sunpower updated the FW, perhaps to correct the power factor. I need to measure again, but on my way to do that my car broke down <sigh> so it will have to wait until next weekend.

The economics of oversize wiring depends on the value of the generated electricity, the incremental cost of the wire, constraints on wire size (code, physically fitting it), the current and special constraints such as wire being too large to utilize. The value of the generated electricity is the big wild card in that equation; mine is about $0.23/kWh. In my case 8ga is the largest wire size that will fit in my conduit and also the largest wire size that will fit in the wire clamps of my 30A circuit breakers, so 6ga was never a consideration.

Also as Bruce Roe ( bcroe ) pointed out in extreme cases you may be forced to select bigger than economic size wire just to make the system work properly, particularly if you have a really long run.