[TowerTalk] Romex for Rotor cable?

[Steve Jones]

A while back on TowerTalk I saw a comment that a nice solution for long 
8-conductor rotor cable runs is a pair of "12-3 with a ground" Romex cables, 
which provides eight conductors.  I'm thinking of using this arrangement for 
a roughly 300' run in my underground conduits.  But I also spotted a comment 
that running Romex in underground conduits violates NEC code.  Is this a 
code formality or is there a real problem using Romex in conduits?  The 
voltage won't be more than around 30 VDC.

73,
Steve
N6SJ