I have been reading this thread and by now am pretty confused.
Start with the original post:
- a single 120v 15A breaker feed to the garage. To me that means the feed to the garage is probably 14ga.
- About 40' to the garage. A would assume closer to 60' to total wire distance
Can it work if there is no other draw on a 12A EVSE? Yes, if the plugs are good, connections tight, no other draw, etc. However...
For a 120' total wire length (there and back) you have a voltage drop of 3.6v for the wire alone. Not sure what code calls for but I thought it was 2% max, or 2.4v.
There has been talk about doubling the voltage. If you converted the wiring to 240v you could still do 12A and that would give you 2.88KW. You would just need a level 2 EVSE that would not draw over 12A. Unfortunately most do.
Also, converting to 240 means you would reconnect the existing wires. My guess is that it is 14-2 w/ ground, meaning that if you did have a 120 volt step down transformer you would not have a neutral that goes back to the main box. A ground wire is typically not insulated, a neutral is. And if you did get a 120v step down transformer you would have to subtract that current from what is available to the EVSE.
If it is in a conduit I would suggest re-doing the feed. Probably 8-3 w/ ground if the conduit will support that, feed with a 40A 240v breaker (2 pole), then put in a breaker box in the garage.
Sorry, I know this is not the quick and dirty answer but it is the right way to do it. It will also give you a lot more power in the garage for future use.
Added note: If you can please verify the gauge of the wire going to the garage and that the outlets are connected with the wires under screws - not just stabbed into the holes on the outlet. Also make sure the outlet is nice and tight. If you have any doubts replace it.