According to The Real Tom Tom @ www.WorkAtHomePlans.com:

But.... I've heard this so many times, to not put NM in conduit. When
right in ART 334 says you need to do it, when tranversing floors and
tells you to do it where physical protection is needed. Even a note
in Chapter 9 explains how to calculate size when using an
multiconnductor cable for conduit sizing.

It boils down to something like this:

- you have to consider "conduit fill" (possible overheat
issues) and potentially conductor derating.
- when the wiring system _needs_ to be in conduit (ie: Chicago codes,
or hazardous location rules), you must use unsheathed cable. You
can't get away with stripping the sheath off and running individual
conductors. It's no longer a "permitted" use of the wire...
- when you're using it for "spot mechanical protection" of
a system that's otherwise permissible as NM, inspectors will give
you a certain amount of leeway. This is partially codified
(IIRC a 1-2' limit on "sleeves" without having to worry about
conduit fill), and partially "inspector permitted".
- whatever you do, they'll be concerned that it's done without
damaging the cable (eg: cable pulling grease if necessary)

For example, I've had inspectors approve 3-6' "drops" of NM
sheathed cable in PVC electrical conduit[*]. Mostly 14/2 (wg for
the Americans ;-) in 1/2" PVC, but also several of 14/3 and
even one of 10/3 also in 1/2" PVC.

[Note that in no case is there more than 3 current carrying
conductors in the PVC - the sheath wouldn't let you...]

In Canada, we also have a provision by which we can bury NMW
(aka UF, looks just like ordinary NM, only heavier) in PVC
black irrigation tubing (as long as it's the CSA-approved grade),
to reduce burial depth requirements by 6".

In both cases, while the length of conduit is longer than the 1-2',
the inspector knows that a free-air conduit on a wall is going
to be okay heat-wise, and buried system will generally not
have trouble with heating either.
[*] wired garage NM on ceiling, since walls already finished, the
shop outlets and switches are on PVC conduit "drops" on the
inside wall face to provide mechanical protection agains the
wall.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.