The heat the NEC is primarily concerned with is INTERNAL, secondary is ambient (air temp). The internal heat is generated by current flow through the wire's resistance and will add to the ambient. (larger wire = lower resistance = less heating and in wire, 14 is smaller than 12 or 10)
The NEC has tables to guide the installer on proper size of wire and conduit to reduce heat and dissipate properly.
The NEC is not an instruction manual but an installation statute (where adopted). If you are planning a DIY project do not use the code for the "how-to". There are many publications at HD, Lowes, and the bookstore that can guide you on DIY electrical projects and likely keep you within the code for those home projects while providing some of the necessary NEC information. Any project not covered in these self-help books should be left to a licensed electrician. Keep in mind the code is the minimum requirement to be followed. Also, someone with "electrician" on the side of the truck does not mean they are licensed or truly knowledgeable.
Conduit may not be required in the code for a wire type or wiring method but it might be desirable for a sense of security against physical damage in your particular installation. Oversizing wire or conduit guarantees that you will allow for heat. NEC is published by the NFPA, National Fire Prevention Assn, Much of the code is intended to prevent electrical fires.
(Bold below is emphasis added by me)
300.4 Protection Against Physical Damage. Where subject to physical damage, conductors, raceways, and cables shall be protected.
(A) Cables and Raceways Through Wood Members.
(1) Bored Holes. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed through bored holes in joists, rafters, or wood members, holes shall be bored so that the edge of the hole is not less
than 32 mm (1 1⁄4 in.) from the nearest edge of the wood member. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by screws or nails by a steel plate(s) or bushing(s), at least
1.6 mm (1⁄16 in.) thick, and of appropriate length and width
installed to cover the area of the wiring.
Exception No. 1: Steel plates shall not be required to protect rigid metal conduit, intermediate metal conduit,rigid nonmetallic conduit, or electrical metallic tubing.
With all that, yes, nm and nmc can be run in conduit but it is not a typical practice since it is designed and permitted to be run exposed with some exceptions relating to PROTECTION, remember in all code questions the AHJ (local inspector) and NFPA have final authority of interpretation of correct application. PLEASE, SAFETY FIRST! google for how electricity kills/