I have an attached drywalled garage w/ a subpanel that feeds several outlets in the garage. All wiring is in surface mount PVC conduit using THHN wiring, and all are serviced via GFCI breakers. I want to install an outdoor outlet / in-use box on a wall about 10' from where one of the outlets is. What is the code requirement for getting from a surface mount box inside the garage to a box mounted outside? Can I just run conduit & THHN wiring from the existing surface box to a conduit body that punches into the wall, and run THHN wiring from out of the conduit body and into the outdoor box? This would mean there would be a small length where the THHN wiring was "exposed" ( in the wall between the conduit body and the box). Is this OK, or is there a better way?
Is there a reason you can't put a short length of conduit between the hub fitting on the outdoor box and the hub on the conduit body?– ThreePhaseEelJan 4, 2021 at 1:59
Rather than a conduit body I would probably just add another surface mount box there. You would then have the option of also adding another recep at that location, for very little extra work or cost.– StayOnTargetJan 4, 2021 at 2:20
THHN must be entirely contained within conduit...
Your approach won't work because NEC 310.10 requires that you use insulated conductors as part of a Chapter 3 wiring method, instead of standing on their own.
but, you can use a stub length of conduit from the body to the box
However, outdoor-rated boxes generally have hubs built into them that accept conduits of an appropriate size. As a result, you can fit a stub length of PVC conduit (often called a "nipple") between the conduit body and the box, provided you're using a plastic weatherproof box. (These are often called "FS" or "FD" boxes.)