Wayne,

Well, here's what my 2002 NEC says: Chapter 9 Table 1 says the
acceptable wire fill for one conductor (or one cable) in conduit is
53%. However, Note (2) says "Table 1 applies only to complete conduit
or tubing systems and is not intended to apply to sections of conduit
or tubing used to protect exposed wiring from physical damage."

That's the kind of thing I was looking for. Thanks.

Since the conduit is open above and below, I would think any heat buildup
could escape easily.

Anthony