Yes, I have used an extension cord since I purchased my car in late August 2013. The cord I am using has AWG 12 wire (purchased at Lowe's) and is 15 ft. in length. [I wanted an AWG 10 cord, but could not find a short enough cord in that gauge.] The wire gauge is the same as the in wall wiring (for up to 20 A service). I would recommend using the shortest length extension that will reach to minimize the series resistance (and accompanying voltage drop). The extension cord does not seem noticeably warmer when charging than the ambient temperature. Make sure that any cord you use is rated for at least the 13 A that the charger is rated at, a higher rating would probably be better. The cord I use is rated for 15 A.
