The user manual says to use a nema 6-15R with a 15A two pole Circuit breaker(240V). But my existing setup is a nema 6-20R with a 20A two pole CB(240V). I realize a 6-15P plug will fit into a 6-20R receptacle but I am worried about over-loading the new A/C. Does anyone know if this new A/C has built in "Overcurrent protection"? Also this A/C is the new "Dual inverter technology". This unit draws 10.5/11.6 amps.I read the NEC article Table 210.21(B)(2) that says the maximum load on a given circuit is 80% of the receptacle rating and circuit rating. The maximum load on a 20A OCPD is 16A. Is this OK to do? I don't want to damage my A/C or house?LG-LW2217IVSM 
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Electrical – plug the new LG-LW2217IVSM A/C nema 6-15P into a 6-20R with a 20A-240V CB
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The answer is it depends on the appliance. As long as the appliance does not expressly forbid using it on a 30A circuit you should be fine. You'll be able to change the plug and put on a 14-30P plug, just ignore the silver terminal (neutral).
Hard "no". The only receptacle you can install on a 30A circuit is a 30A receptacle. NEC 210.21B3.
You mentioned the EV industry's tendency to put 14-50 on every charger. That creates a real mess. You cannot install a 50A receptacle unless you have 50A wire and breaker. As you note in the above table, there's an exception where it's also allowed on 40A circuits, but the reason for the exception is there's no such thing as a 40A receptacle, and that exception is bullseyed at the special case of range/ovens. For a new -50 in a garage, where anything could be plugged into it from a better EV supply to a welder, best to use 50A cable so people get exactly what is on the tin.
Contemplating a situation where replacing the 10AWG cable is not practicable... I would get a "generator transfer switch" that takes the form of a small subpanel with an interlock so two specific 240V breakers can't be on at once, and takes readily available breakers (because I'd change them). Normally, those panels backfeed; I'd not. The 30A supply would go to its buses. I'd change one breaker to a 30A and feed the 14-30. The other to 20A and feed the 20A receptacle.
If forced to plug into 30A service, I would use a 30A receptacle and a cheater cable. This is not quite legal, but since it's not part of the building, it's out-of-scope for the permitting and inspection process. I'd specifically aim to make it as obvious as possible that it is a cheater cable -- to warn "the next guy" that this is not 50A as advertised. A receptacle installed in a building should always be what it looks like. I shudder to think of people solving this problem with a neat 14-50 installation onto 10AWG wire; later they sell the house to a guy motivated by that sweet 50A service in the garage.
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Best Answer
This unit requires a 15A breaker
The manual for this particular machine specifically calls out a 15A circuit breaker. That is the final word on the subject; NEC 110.3b requires you obey the labeling and instructions. (the reason is that the UL listing is contingent on installation according to the labeling and instructions; UL hasn't tested this unit on a 20A breaker and does not vouch for its reliability.)
Which means you must change the receptacle also
Since you must install a 15A circuit breaker, that will also necessitate installing a NEMA 6-15R receptacle, since you can't have a 20A receptacle on a 15A circuit.
However, the wire in the walls can remain the same. Presumably it is 12 AWG (or possibly 10 AWG; too-large wire is always allowed).
In some point in the future, when no longer using an air conditioner calling out a 15A breaker, you could roll it forward to a 20A breaker, since the wire is adequate. Note that a 20A circuit can have 15A receptacles, but only if there are 2 or more of them. (a single receptacle is for an appliance on a dedicated circuit; breakers must match there). I note that nothing in the documentation of this appliance calls for a dedicated circuit, so a dual NEMA 6-15 receptacle would be perfectly acceptable with this A/C, and possibly cheaper than a solo, and that dual receptacle would be acceptable on a 20A breaker (if not for this A/C unit's restriction).
NEC Code couldn't care less, except for 110.3(b)
If it hadn't been for the labeling and instructions, Code would have been fine with this. The whole point of the 20A/15A exception in 210.21(B)(3) is that you're allowed to load up 20A circuits with the common 15A receptacles, which take loads far smaller than 15A -- cell phone chargers, laptops, Roku boxes, clocks, LED lights, nightlights, etc. The electrical system is built constant-voltage, which means each device is responsible for its own current draw and must self-regulate.
That is why NEMA 5-20 and 6-20 receptacles have T-shaped pins - to allow NEMA x-15 plugs to fit. As discussed in 210.21(B)(3), this is the only exception -- the 40/50A quasi-exception is because NEMA refuses to define a standard for a 40A plug/socket... so 40A circuits are forced to use 50A receptacles.
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