Drew,
Does this mean your air compressor is going to be sitting out in the weather? If it does, the NEMA rating of your plug is probably the least of your worries.
They make lots of weather proof plugs and receptacles, but they are pretty expensive. If your compressor is going to be in a shed, or under some shed roof, then you could put a conventional plug and receptacle (dryer) in a small NEMA -4x enclosure and call it good. The best bet would be to try and run the cord through a wall and put the receptacle on the inside...warm and dry.
Making a connection outside in the elements is usually the last resort...think "creative"!
A conventional single phase capacitor start motor isn't weather proof..you must have a shed over this thing.
Stuart
