I've got a detached shop with a 100 amp subpanel. Behind that shop is an older 480 sq ft garage that languished in disrepair for a number of years. I have now structurally rebuilt and re-roofed the older garage, and I plan to use it for both both storage and secondary working space.
I'd like to add a 60-amp sub panel to this building, as it currently has no electrical service. To go from the 100 amp sub panel in the main shop, I'll need to run through the main shop's walls about 25 feet, and then outside about 6 ft, and then of course up and into the 60 amp sub panel.
My plan is to run 6/3 Romex inside, through the walls (which will soon be insulated), and then switch to individual THWN wires (6-6-6-10) inside a metal "pull box" enclosure on the outside of the main shop. The THWN wires will continue in 1" conduit underground and into the second building. I'm aware that the second shop will need its own ground rod, and that the neutral and ground must be kept separate on the sub panel.
My questions are 1) is there any reason you can't or wouldn't want to run 6/3 Romex inside an insulated 2x4 wall, and 2) what is the approved way to connect the Romex to THWN inside the pull box. Do I just use split bolts, heavily wrapped with plastic tape and then electrical tape? Or do I have use something like a "Polaris" connector ($$$)?