I'm putting in a 100A subpanel next to the 200A main to add some more outlets for the garage. The walls are finished and the main is flush mounted in the wall so I plan to do the same with the subpanel.
Since it's for 100A and my breaker says it's for 75C wire I'm planning to use 3AWG CU for the hots and neutral. I couldn't find the NEC code for it, but I keep reading I can use 8AWG for the ground (would love some confirmation of that).
The wire distance will be about 32" plus excess for routing because there will just be one 16" OC stud bay in between the two panels. I think I can get away with 5 feet of wire, but plan to measure it out and may need 6.
My question is, if I want to run the wire in the wall, are my only options to get something like SER 3-3-3-5 cable and route that through the studs, or get separate THHN wires and ground and somehow put conduit in the studs (notching them and adding a nail plate)? Home Depot has both, but THHN is cheaper (assuming I have enough conduit already). Also, if I use THHN, it's okay if all 3 wires are black right (nothing in code says the neutral needs to be white)? That's all Home Depot carries for 3AWG copper.