Re: ENT
It depends on what edition you are refering to. In the 2002 NEC, 362.22 states cables can be installed where permitted in the cable article. Type MC specifically mentions raceways, type AC does not. In the 2005, the "permitted" has been changed to "where such use is not prohibited". I don't believe there is a single cable article that prohibits the installation of cable in a raceway, including types AC and MC.