In the prelude to the questions, we gave away the fact this is a flexible
conduit. What we didn't give away is the particular type. This is Liquidtight
Flexible Metal Conduit (LFMC), whereas in Article 348 the wiring method was
Flexible Metal Conduit. Many people confuse the two, because they are both
flexible and have a metal spiral as part of their construction. But they do not
look alike. LFMC has a nonmetallic outer covering that gives it the
characteristic of being liquidtight.
350 lists three permitted uses (in contrast to only one listed for Article 348) [350.10]. It can be used
where conditions require flexibility, as permitted by the Hazardous Location
Articles (501.10, 502.10, 503.10, 504.20), and for direct burial where listed
and marked for the purpose.
From an engineering standpoint, you want to avoid using LFMC except where you
need the flexibility. It's necessarily weaker than RMC or IMC and is more easily
broached due to its continuous joints. Don't use it for the reason of trying to
get around raceway support issues. It still must be supported, and it's going to
be more difficult to install than RMC or IMC simply because it is flexible.
348 lists seven prohibited uses, Article 350 lists only two [350.12]. There are
big advantages to that outer coating!
The first prohibition listed is the standard, "use your brain" one that is more
a disclaimer than an actual requirement. Don't use it where subject to physical
damage. On the surface, this is nonsense because the main reason for using
conduit is to protect the enclosed wires from physical damage. What is meant
here is that you should use common sense in how you install this conduit knowing
full well it's not invincible.
The second one addresses an issue people might not be inclined to consider.
Don't use it where the ambient temperature is too high for the rating of the
conduit. That outer coating is not as temperature-resilient as the steel
use an approved means [350.30]. The NEC doesn't provide detail on what exactly
this means for LFMC, but using "approved" anything is a general principle
throughout the Code. Also, see 110.2.
bushings, couplings, connectors, brackets, etc., listed for use with LFMC