12 gauge should be just fine for 50ft. Even 14 guage is technically OK. If you where doing 100 feet I'd say 10 gauge. The guy who had a 12 gauge that got hot probably had a defective cord. The wiring in your wall is only 12 gauge at best and is good for 20 amps. The wiring from the EVSE itself to the J1772 plug is only 16 gauge believe it or not! And the short length from the EVSE to the plug is 14 I believe.
Now the long version:
The basic rule of thumb for house wiring and extension cords is 14 gauge for 15 amps, 12 gauge for 20 amps. Additional code requirements is for an 80% derate for continuous multi-hour steady use, which is the reason that the EVSE is 12 amps. It's 80% of 15 amps and is safe for "standard" 15 amp/14 gauge household circuits. This level is safe from a standpoint of overheating the wires. There is the additional caveat that very long runs can cause unacceptable voltage loss. Generally a 3% voltage drop from the panel to the appliance is the acceptable limit. There's some math involved here which I'll summarize:
Stranded 12 gauge wire is .00198 Ohms/foot, 14 gauge is .00314 Ohms/foot, and 10 gauge is .00124 Ohms/foot. Since V=I*R and the distance is in both directions we can calculate the length of wire that causes a 3% voltage drop at 240V (7.2V)
Length of wire = Voltage Drop / (Current x 2 x Ohms per foot)
So,
14 gauge, Length = 7.2 / (12 x 2 x .00314) = 95 feet
12 gauge, Length = 7.2 / (12 x 2 x .00198) = 151 feet
10 gauge, Length = 7.2 / (12 x 2 x .00124) = 242 feet
These distances are for the *total length of wiring* so you need to take into consideration the length and gauge not only of the extension cord, but of wire that is feeding your outlet. If you have, say, 50 feet of wire between the panel and your outlet, and an additional 50 feet of extension cord, 12 gauge is absolutely fine for this application and 10 gauge is overkill. If you have 50 feet from the panel to the outlet, and a 100 foot extension cord, then 12 gauge is right on the limit and it might be wise to upsize to 10 gauge.
But the way you are doing it with plug > adapter > extension cord > EVSE is IMHO taking a somewhat dodgy setup (the adapter) and making it even dodgy-er. The "more right" way to do it is to cut the ends off and make the cord itself 240V plugs. Reduces the chances that some unsuspecting person sees the extension cord lying around and decides to plug a power tool or something into it, thereby frying the tool and maybe starting a fire.
I do use an adapter myself but I leave it zip-tied to the EVSE plug so that nobody can easily unwittingly separate them and leave an unsafe 120V style plug exposed.
