Running NM cable through wall to PVC Box

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I've read numerous posts on this as well as article 334 and think I have this worked out, but thought I'd post here before drilling holes.

I'm wiring a detached shop. I want to install a surface mounted PVC J box on the outside wall. I plan to run multiple NM cables to this box and then transition from there to THWN to run in sched 80 conduit along the exterior wall to a disconnect for a heat pump, dedicated outlet for an air compressor, and another outlet for general use.

If I understand the code correctly, I can run the nm cable for a short distance in conduit.

What I want to do is this:
  • drill holes in the back of the JBox and install a short amount of conduit (or perhaps just a coupler) that will be long enough to pass through holes drilled in the siding and sheathing.
  • Inside the building, attach pull elbows to the conduit.
  • Run the NM cable from the attic space, down the stud cavity, and into the elbow.
  • I'll install bushings where the cable enters the conduit and will stable the cable to the stud within 8" of where it enters the conduit.

I believe if I do this I will not have to have the cable clamped where it enters the J box. Putting a clamp on the J box will require larger holes to be drilled, which I'd like to avoid.

I can't find anything in the NEC that prohibits that.

The two other approaches I've considered are:
  1. build a small wood frame to sit between the j box and sheathing that will allow room for regular NM clamps to be mounted to the box.
  2. install a J box in the attic space and run THWN from there down stud cavity, through the wall, and into the J box (all in conduit). This is probably the most complete way, but seems like quite q bit of extra work just to get to the J Box.
 

Little Bill

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