Why are you asking the question? Is this a question concerning saving money? The cost of #14 MTM for a small job is insignificantly more than using #16. Often times projects are speced with #14 minimum even though smaller would be allowed by the code.
Please refer to NFPA 79 Art 15 Conductors, and 15.3 Conductor Sizing. a) Power Circuits, #14, b) Lighting and control circuits on the machine and in raceways, #16, c) Control circuits with control enclosures or operating stationed #18, and d) Electronic PLCs I/O #24 and #26.
Of course these are minimums as ampavcities are based upon table 9 for single conductor characteristics.
I you are asking the question please get a copy of NFPA 79 for your reference to have on file.