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.

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    There is something that says neutral must be white, but it is only applicable to #6 and smaller wire. Also, don't buy this stuff at Home Depot, form a relationship with a local electrical supply house. And buy your wire as the very last thing you do. – Harper - Reinstate Monica Nov 3 '19 at 18:36
  • Is there a specific reason you're using copper here? Also, why are you leaving a whole stud bay between the two panels? – ThreePhaseEel Nov 3 '19 at 20:08
  • Initially I thought copper would be better and easier to work with because the gauge would be smaller, but now I'm not sure if aluminum is better. As for the position of the panel, it was just easier that way because there is some low voltage phone wiring in the other stud bay I didn't want to mess with. I will go to an electrical supply store this week, just have to make a trip during business hours, which is less convenient. – Matt Nov 4 '19 at 4:05
  • How many spaces do you have open in your existing panel? Do you already have the subpanel on hand, or have you yet to buy that as well? – ThreePhaseEel Nov 4 '19 at 4:52
  • I would use FMC ( flexible metallic conduit) panel to panel, it is cheap flexible and will allow the use of thhn smirf tube would be easier but I don’t know if I have seen it that big. – Ed Beal Nov 6 '19 at 14:54

Your ground wire qualifies as an EGC, use table 250.122, #8 is good for 100A. Conduit would not require nail plate protection (300.4(A)(2) ex.1). You can phase tape wires #4 and larger (200.6).

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    Are you sure about using the 60°column for SER if the wall will be insulated? I see "...For SE cable with ungrounded conductor sizes 10 AWG and smaller, where installed in thermal insulation, the ampacity shall be in accordance with 60°C...". So my interpretation is that's not going to apply to 3-3-3-5 SER. The walls aren't insulated, but I did want to blow in insulation at some point, so I will need to take that into consideration. – Matt Nov 4 '19 at 4:08
  • Yeah, 338.4 doesn't apply to SER cables fatter than 10AWG. – ThreePhaseEel Nov 4 '19 at 4:51
  • Sorry, you're right, missed that change in 2017 code – NoSparksPlease Nov 4 '19 at 6:04

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