Wa2ise,
I'd suggest taking a look at table 310.16 and 310.17. If the question is 'what is the current which can be safely carried without overheating the wire', then this NEC section allows 24A for 14ga THHN wire (with appropriate terminations, etc. etc.), and 35A for 'single conductors in free air'... even the worst case for 14ga wire, 60C insulation in a raceway allows 20A.
There is also a note referencing 240.4(D). This restricts the overcurrent protection used on 'small conductors' to 15A for 14ga wire, 20A for 12ga wire and 30A for 10ga wire. If I recall correctly, this is not to protect the wire from overload during normal operation, but instead to protect the wire during overload and short circuit conditions.
The 'unused ampacity' is available if you are in situations which require derating, say because of a large number of current carrying conductors in a single conduit, or high ambient temperatures.
-Jon
