Is Romex in conduit illegal?

Is Romex in conduit illegal, no, Romex in conduit is not illegal in many situations, but it is restricted and must follow code, with NEC allowing Romex NM-B in conduit for protection where needed, but prohibiting in wet locations outdoors and with fill and derating rules. It is legal to run Romex in conduit indoors for short sections where protection from physical damage needed, such as where Romex passes through concrete or in unfinished basement or garage where protection required, with conduit providing protection. However, it is illegal to run Romex in conduit outdoors or underground because conduit outdoors is considered wet location and Romex not rated for wet, so not allowed there, must use UF or THWN. Also conduit fill limits apply, Romex being flat makes fill calculation complex, and derating may be required if multiple cables in conduit causing overheating. So is Romex in conduit illegal, no, not illegal if done per code for protection indoors in dry locations, with short runs allowed, but illegal in wet locations outdoors and underground, and must follow NEC support and fill rules, with many electricians preferring THHN individual wires in conduit rather than Romex because easier to pull and more code compliant, with local inspector may have interpretations, so generally allowed with conditions, not blanket illegal, but restricted.