The A/C compressor draws alot of power and this creates heat in the plug, especially if it has a long on cycle. Electricity moving through a wire creates heat, throw in a little bit of extra resistance/arcing due to a marginal connection and you get even more heat. If you replaced the wire plug in an attempt to cool off the wiring, try changing out the receptacle, it could be worn and the plug-in isn't making a good connection on the spades, but chances are everything is Ok and the heat is normal.
Last edited by jakewash; 06/25/1004:21 PM. Reason: I see Chris beat me to it!