I concur that is not a 240 volt circuit but a 120 volt circuit. As you said if it is a dedicated home run it can easily be converted to 240 volts as long as there is room for a double pole beaker to be added to the panel.
You need to absolutely verify no other outlets on that circuit. Since it is a single outlet there is a great chance for that to be true. It is then a simple matter to convert it to a 240 volt circuit that will match the manufacturers required branch circuit requirements for the a/c.
Jon
The NEC specifically forbids any listed 15 or 20 amp rated cord and plug appliance like your window a/c to be operated on a 30 amp circuit. Below is a manual telling you as much. You are requied by the electrical codes to fololow the manufacturers instructions. Lots of things 'work' but not necessarily safely.
http://manuals.frigidaire.com/prodin...219a0951en.pdf
What Dan said earlier is true you can operate 15 amp appliances on a 20 amp circuit but his reasoning is incorrect. But that is where it ends.
If this is a new unit the manufacturer will tell you the same in the installation instructions. You risk fire at the A/c if the right conditions would suddenly exist.
Again you cannot connect to a 30 amp branch circuit with appliances that have 15 or 20 amp cords and plugs. This is a direct violation of the NEC and local codes.
The explanation will take some time and code references but it is not a matter of whether or not the circuit has more than enough power to operate the window a/c. I'll provide that if you like.
Simply put if the manufacturer wanted to allow you to put the window a/c on a 30 amp circuit they would have put a 30 amp plug on it..... It's not about voltage but the available amperage. The manufacturer is required to protect the compressor with an overload device and he protects the wiring of the unit (power cord) by sticking that 20 amp or 15 amp plug on the unit to force you to use a 15 or 20 amp branch circuit.