Is it ok to run THHN from a main panel, through PVC conduit in the attic, then transitioning to EMT into rooms?

Question

As the title suggests, I need to add a few plugs -- (1) 220v and (3) 120v -- in a total of 3 rooms in my workshop. However, everything is fully finished with drywall, insulation in between studs, and 20"+ of cellulose in the attic; in short, for at least one of the plugs it would be physically impossible to reach that wall and fish wire down, and very difficult for the rest. My thought was to maybe run a large PVC conduit (1.5" to 2", with the idea for plenty of future space for pulling) from directly above the panels, in the attic space, put tee's above each room, and then supported overhead at the top of the attic ridgeline. It would run in a perfectly straight line, except where it would tee off to go into X, Y, or Z room. When building, I screwed down a 2x8 walkway the entire middle length of the attic, with direct access all the way over to the panels, so I can move around easy enough up there.

However, since I don't want to have NM/Romex just popping through a hole in the ceiling, over and across to where the plug(s) need to be on the outside of the wall, I thought to transition to EMT once it comes through the ceiling. The only reason I'm saying EMT and not continuing with PVC is because I already have EMT in different rooms, and I feel the aesthetic would be thrown off with PVC just for these runs, as well as THHN being easier to work with/pull.

A couple of questions:

1. Should I use THHN to run through the PVC conduit, then transition to the EMT when I go through the ceiling and over to where the plug(s) need to be? The THHN would obviously be much easier to pull vs. NM, from my fairly limited experience.

• -OR- Since everything coming off the main panel is NM right now, should I just run NM up into the attic, then into the PVC conduit, and then create a junction box before going into each room, to transition from the Romex/PVC to the THHN/EMT?

2. If I use THHN wire and ground all the circuits to the main panels ground bar, do I need to ground every junction box separately in the EMT run, via the green screw? Or does the dedicated ground wire going back to the main panel forego the need for that?

I really appreciate any help!

Answer 1

You can do it that way, but there are potential reasons to choose other ways to do it.

The PVC section needs a ground wire. If it's all EMT you don't need one, as the properly assembled conduit itself is a listed grounding conductor.

If you have many circuits sharing "one big fat conduit" you have to derate the wires for each circuit, which means you have to use larger and more expensive wire for each circuit sharing the single big fat conduit.

If you use multiple smaller conduits, you can avoid being at a point where derating has a practical impact on your wire sizes.

You absolutely do not want NM in conduit, barring a few edge cases where it doesn't matter much. Conduit fill for "oval" cable is based on treating it as round cable of the larger axis diamter, and it's miserable to pull. I would use EMT/THHN the whole way, with separate run of EMT for each room.

It would also be code acceptable and quite common (but my rodent reality experience advises against) to run NM without conduit in the attic, and transition to EMT/THHN in a junction box, just deleting your large PVC conduit from the plan.