Need Help With Number of Wires in Conduit

I am just really confused with derating. I am trying to run a few simple circuits in a metal building/shop I built. It has a 200 amp single phase main. I want all circuits to have a separate neutral. I need 2, 20 amp, 110v circuits for my lights which will be on 4 switches, 2, 20 amp, 110v circuits for receptacles and 1 double 20 amp 220v circuit for my water well pump. I want to use THHN stranded #12 wire. I know if you go over 3 CCC you must derate the wire. My questions are:

1. Do i need a separate ground for the 110 and the 220 circuits or can I use 1 #12 as the equipment ground for the 220 and for the 110 circuits ground?

2. If my calculations are correct, that would be a total of 10 CCC and a ground plus the extra runs from the light switches back to the lights which is within the 15 limit for #10 or #12 wire in a 3/4 PVC. Since I believe that 10-20 CCC require 50% derate, would I need to use #10 wire rated at 40 amps and derate to 20 amps or can I still use #12 since they are not continuous load circuits?

Can anyone please offer some advice? I have no problem running cables and electric but want to be sure it is not a fire hazard. Thanks


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