Circuit sizing for HVAC units is not like sizing for resistive loads.
The Mr Cool product brochure at
https://images.homedepot-static.com/catalog/pdfImages/3a/3aa74c83-da32-4ef7-9258-212eee3c335e.pdf states the minimum circuit ampacity (MCA) is 15A, and the max fuse (or breaker) is 25A.
MCA includes the 25% allowance, so you don't use it as a full load amperage and then upsize from there.
14 ga nm-b derates to 71% if any part of the cable runs thru an attic. The ampacity used for derating purposes is 25A, so the derated capacity is 17.75A. So, 14 ga nm-b is suitable for the circuit. But the max breaker for this is 25A and not 30A. In reality, a 20A breaker will hold the circuit without nuisance trips since all 2 pole breakers are now the HACR type.
I'd still run the circuit with 12 ga and use a 20A breaker to avoid arguments with electrical inspectors, most of whom aren't sufficiently familiar with the code provisions that apply to HVAC power circuits in my experience.
Just offhandedly stating "run a 30A circuit" which would imply 10 ga wire and a 30A breaker would result in grossly oversized wire and circuit protection in excess of the stated 25A maximum. The code clearly states that you comply with ALL of the manufacturer's installation instructions.
