Heater Install in Finished Garage

[GrantP]

Hello everyone. I am trying to install a new 240v 7500w heater in my finished garage. The main panel is in the garage and I have access to the attic above. I have fished 8/3 NM-B up from the panel in the wall into the attic and over to where I want the heater. I want to have a J box in the corner of the ceiling and wall and then go with 1/2" FMC containing 8 THHN from there to the heater. I have searched the interwebs and can't really find a good solution as to how to properly do this transition while securing the NM-B at the box. I thought about using a box with integrated cable clamps, but all the ones I could find did not look like they would work with round 8/3. I then thought about turning a 3/4 knockout clamp around and having the cable clamp inside the box but I wasn't sure if this was to-code. I could just use one of those NM plastic clips in the 3/4 knockout but was concerned about shoving the rather large 8/3 cable in there. The problem is that I want the box to be tight in the drywall corner and not have any NM/B showing above it. Any advice on this would be appreciated. Also, what is the best way to splice 8 AWG copper in the box? Just twist with pliers and use properly sized wire nuts? Thanks!

 

[Stuff]

First questions: Why 8/3? Most heaters don't need a neutral so 8/2 is enough. Why FMC? Why not continue NM inside a wall cavity? For exposed to physical damage you need IMC/RMC/RNC as FMC follows mostly the same rules as NM. Some places will allow EMT. Why not put box in attic?

Do not reverse NM clamp as that violates code as not intended to be installed that way. If you don't want it showing then chip out the drywall. The clamp does not need to be accessible; you just need to be able to remove the box cover to get to the spliced wires.

Yes, just twist wires together using the appropriate wire nut - they make ones for 8 gauge wire. Alternatively you can use much more expensive butt connectors.