Why is there such a strong argument against running Romex inside EMT conduit?

The inspector is likely following local amendments that are stricter than the national standard, but the technical reasons usally come down to conduit fill and heat dissipation. While the NEC technically allows Romex in conduit for physical protection under Article 334.15(B), your local AHJ has the final word. If they've decided NM cable isn't permitted in certain types of construction, putting it in a pipe doesn't magically change the wire type to somethig they accept.

When you pull a flat cable like 12/2 Romex into a pipe, the math for **conduit fill** gets messy. You have to treat that flat cable as if it were a solid round wire with a diameter equal to its widest point. In a 3/4 inch pipe, you might think you have plenty of room, but that cable takes up a massive percentage of the allowable 40% fill compared to individual THHN leads. That extra space isn't just for ease of pulling; it's there to ensure air can circulate so the conductors don't overheat when they're under a heavy load. From a practical standpoint, pulling Romex trough EMT is a total nightmare compared to using stranded THHN. The outer jacket on Romex has alot of surface area and a high-friction texture that grips the inside of the conduit. If you have even two 90-degree bends in that run, you'll be sweating and cursing by the time you get it through. Stranded THHN slides through like butter and allows you to fit way more capacity in the same pipe if you ever need to add a circuit later.

Most pros just mount a junction box where the Romex comes down the stud, then transition to individual THHN wires for the conduit run. It results in a much cleaner look and satisfies the inspector's concerns about heat and fill. It might feel like extra steps now, but it's the standard way to do it for a reason. You'll thank yourself later if you ever have to troubleshoot the circuit or pull another wire through that same pipe.

That part about the friction makes a ton of sense. I tried pulling a scrap piece thru a short bend just to see, and it was surprisingly tough. One thing I'm wondering—if I go the transition box route, do I need a specific type of connector to join the Romex to the box before it turns into the THHN? I’m worried about the Romex jacket getting sliced on the edge of the knockout. Also, is it okay to just wire nut them together in that box, or does that add too much bulk? I'm trying to keep the footprint small on my garage wall and don't want a massive junction box if I can help it.

You definatly need a standard NM connector where that cable enters the transition box. It clamps the wire and keeps it from rubbing against the sharp metal edge of the knockout.

Another thing to watch for in a garage is moisture. Even if your EMT looks dry, any conduit run on an exterior wall or cold concrete slab is often treated as a damp location. Romex has paper filler inside that acts like a wick for condensation. THHN is usally dual-rated as THWN-2, wich handles moisture way better. Just one more reason to stick with the transition box method instead of forcing the NM through.