zanydroid
Solar Wizard
Pretty sure it is more than 3 (not including ground), not 2 (I’m using two 3-conductor + ground metal clad conduit for my 3 6 AWG home runs for precisely that reason).
It is 3. But 3 very rarely happens in split-phase residential, it only occurs in 3-phase systems. L1/L2/N does not count as 3 CCCs, it counts as 2 b/c I_L1+I_N = I_L2 is always true.
3 may even be impossible in code compliant split phase systems since all conductors from a circuit need to be in the same conduit / cable, which forces a multiple of 2.
You start derating from the max temperature of the wire type as allowed by code (sometimes it's lower than the certification on the jacket, sometimes it's derated when wet. I haven't designed with PV wire so I don't know the ins/outs of that wire type. 690.31 is where the details of PV wiring method are in NEC). And then the resulting ampacity must be lower than the ampacity allowed for that AWG / metal type / terminal temperature combination, factoring in any special case restrictions in code for the wire type.(Terminal temperature) This is the area I’m still confused about.
The conduit fill derating is certainly irrelevant by the time the terminals are involved, since the terminals are not sitting in the conduit.
I believe this is to give sufficient margin for the device to operate correctly. I'm not sure how the device termination temperature need to be derated for ambient temperature though.