THHN use with Blue Plastic Romex Connector Switch/Outlet Boxes

Question

I extended my patio, added a pool, and a pool half bath (CMU construction) and now I am getting ready to wire it all together.

I planed to use 4 blue plastic boxes (1x 4 gang, 1x 2 gang, & 2x single boxes) that have Romex connections openings on the back to push Romex through and the "grabber tabs" on the back. I would like to use THHN with these boxes because I have multiple circuits (ie. 4 gang) and have a GFCI with multiple outlets to wire in.

Questions:
#1. Given concrete block walls mostly filled with concrete and nothing combustable, can I use THHN through the below described electrical conduit WITHOUT a connector from the end of the pipe (inside the CMU wall) to/through the back of blue box through the Romex pass through opening and still meet code?

Or #2. Given built CMU walls and stucco formed around the blue plastic boxes and the laid-in conduit is also set in concrete, if I must keep the wires inside a pipe through a connector into the outlet/switch box and CANNOT simply span that very short distance (maybe 1/4 or 1/8 inch) than what type of connector should I use?

Or #3. Given electrical conduit through the CMU walls to the point of the outlet/switch boxes, can I simply CUT OPEN the back of the blue boxes to more easily accommodate THHN running from the open pipe to the outlet/switches inside the concreted CMU walls?

Further detail to #3: I have a PVC electrical piece male connector with a flat larger flange that fits inside a coupler. Considering drilling the appropriate size hole in the blue plastic box and then glueing that male connector on from inside the box to meet up with/connect with the conduit inside the wall....thoughts? (The conduit doesn't move due to concrete fill)

Or #4. Given the holes for the blue boxes are in the CMU walls with stucco on, pipe laid in and block cavities filled, should I consider using a different outlet/switch box other than the blue boxes?

The half bath is built on the existing patio with CMU walls, with rebar/pins in some of the block and cavities filled with concrete. I added electrical conduit to the CMU walls during construction so I could pass wires later and I had holes made in the block & stucco in order to fit the blue plastic remodelers boxes. Above the new walls I connected electrical conduit from the pipe sticking up and ran that pipe to the point where I plan a good size electrical junction box (maybe 8"x10") that will sit above the patio sub panel. All main power wires (#8 THHN) and the outlets/lights (min 12 ga) will pass through this junction box and go out to another point through conduit.

I laid in conduit & wire for a home-run to a sub panel on the side of my home for the pool equipment, a small sub panel in my shed in my back yard and a small sub panel on the new patio. The 1st or main sub has 3 legs of #3 copper to an 80 amp breaker in my main panel for my home ('08 build w/200 amps service). The 2 smaller subs in the shed and new patio have #8 copper leads and neutral each with a #6 ground wire to their own ground rod. No heavy equipment only LED lights and ceiling fans.

Answer 1

You cannot use THHN without running it in conduit, with suitable terminations, all the way into the boxes.

The blue plastic boxes are not suitable for this use. They are cheap junk for running NM/B only, and not suited to other wiring methods, and it's dubious if they are suited to that, IMHO. I used them as an ignorant kid. I don't use them now. They are chintzy and fragile at best.

Masonry boxes, from steel, with knockouts, exist, and don't even cost much money.

You should have fully completed the conduit paths when building the wall (ENT is the classic masonry wall conduit) all the way into their boxes. You may now need to partially demolish your work and redo it correctly, if you have "formed blue boxes in place without addressing the need to connect to conduit" which is what I think I'm reading.

Each and every sub-panel needs 4-wires (or 3-wires and continuous non-flexible metallic conduit, but with a pool involved, you may need the wire anyway) from the main, or the upstream sub-panel - hot, hot, neutral and ground. Connecting them to a local grounding rod is fine, but not a replacement for a grounding wire (separate and isolated from neutral) all the way back to the Main Panel feed and bond point.

Answer 2

• You have to use one of the wiring methods in NEC Chapter 3.

• You cannot modify boxes or fittings. NEC 110.2.

• You must use boxes and fittings in accordance with their instructions. NEC 110.3(B).

• Do not confuse the difference between conduit as a wiring method and merely using a random stick of conduit solely as a damage shield. Conduit as a wiring method must be 100% pullable, and wires cannot be installed until the conduit is complete.

Question 1, no, not a listed wiring method.

Question 2, you must use fittings that are UL-listed for both the box and conduit type. You're not allowed to wing-ding this kind of stuff.

If the parts you bought don't want to go together, it means you bought the wrong parts. Here, it really helps to quit buying at big-box and shop at real electrical supplies. Corporate management of big-box stores use computers to optimize $ per shelf-foot. That means only the fastest movers are a bargain; every other SKU is "marked up" to "pay its freight" for the shelf space it is using. If it can't, the SKU dropped altogether. As a result, big-box stores are overpriced on the little stuff. Real electrical supplies are not only a bargain, they are full of answers to questions like "I need to go from here to here".

Question 3, no, can't modify.

Question 4, very very much so. You must not get "tunnel vision" and insist on working only with products you're already familiar with, e.g. blue plastic boxes in your case. Any job you want to do in electrical, they make stuff for that.

Since you already laid conduit in the walls, you'll need to choose boxes made to dock up with that conduit.