ECN Forum Forums Code Related Discussion NEC & other Code issues Male plug on NM Cable

When UL tests a cord cap, they test it using the sizes and types of cord intended by the manufacturer. One of the major tests is for strain relief- the cord has a weight pulling on it for a specified period (70# for 10 min., I THINK). This, by the way, is about double the test performed on Romex connectors, if I remember right.
So- there is no way of knowing if a cord cap would successfully hold onto romex. Therefore, using a cord cap on romex violates the listing of the cord cap. 110.3B would be the NEC cite.
That said, there ARE mc/romex connectors out there that are also listed for flexible cord use; this is printed on the box. (Bridgeport comes to mind).

I can't say I've never done this, but if your connection is subject to being flexed at all, romex being made of solid conductors will eventually break.

Romex is designed to be put in place and not moved around, hence all the code rules on staples, etc. Cords, on the other hand, being stranded wire, are designed to be moved to a certain extent, depending on design.