I used an extension cord for several years until I got a level II EVSE. I found that the quality of the build was as important as the gauge of wire used. I got a 12 gauge extension cord from a big name Chinese tool importer that melted down at the female end where the EVSE was plugged into it, where as a 14 gauge cord from a famous big box home improvement store did fine. I would check that connection after a half hour and then an hour to make sure that it wasn't getting warmer than what I could comfortably hold on to. I then ran across a 10 gauge cord from a warehouse club that ran so cool that I stopped checking up on it.