Is it legal to run Romex inside EMT or PVC conduit?

Yes, indoors. NEC 334.15(B) even requires conduit protection for exposed NM cable subject to damage. The cable must meet conduit fill using its major diameter, and NM cannot enter wet locations including outdoor or underground raceways.

Can you sleeve Romex through PVC underground?

No. NEC 300.9 makes the inside of a raceway in a wet location a wet location, and 334.12(B)(4) prohibits NM cable in wet locations. Use THWN conductors or UF cable instead.

Can You Put NM Cable, Romex, Inside Conduit?

A user asked the app whether Romex can run inside conduit. Yes. Nothing in NEC 2023 prohibits nonmetallic sheathed cable inside a raceway. In fact 334.15(B) requires exactly that when NM cable in exposed work needs protection from physical damage, listing RMC, IMC, PVC, EMT, and other means. The myths come from three real limits that ride along.

Limit one: the conduit fill math is brutal

Chapter 9, Table 1 fill percentages apply to cables in raceways, and Note 9 says a multiconductor cable counts as a single conductor using its actual dimensions; for elliptical cables like NM, you use the major diameter as if it were a round conductor. A single cable in a raceway is limited to 53 percent fill. Flat 12/2 NM has a major diameter around 0.46 inches, which computes to a 0.166 square inch circle; a half inch EMT has only 0.304 square inches total, and 53 percent of that is 0.161. So 12/2 Romex technically overfills half inch EMT and needs three quarter inch. One reason sleeves feel roomy in the field is that short protective sleeves are widely treated as protection rather than a complete raceway system, but when the conduit is a real run, do the fill math with the major diameter and size up.

Limit two: no wet locations, and conduit outdoors is one

334.12(B)(4) forbids NM cable in wet or damp locations, and 300.9 declares the interior of a raceway in a wet location to be a wet location itself. That kills the popular move of sleeving Romex through PVC underground or up an exterior wall. Outdoors or underground, switch to THWN conductors or UF cable. Inside a garage or basement, sleeving NM in EMT up a wall to a panel is routine and compliant.

Limit three: ampacity does not improve

NM cable keeps its 60 degree ampacity limit from 334.80 no matter what raceway surrounds it, so 12 AWG NM is still a 20 amp conductor inside conduit. Derating for more than three current carrying conductors applies when multiple cables share a sleeve longer than 24 inches, using the 90 degree column as the starting point but landing no higher than the 60 degree value. Secure the cable within 12 inches of the box where it exits per 334.30, and use a fitting sized for cable, not a standard NM connector jammed with two cables.

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