I didn't have access to 240 in the garage and am renting, so didn't want to go to the expense and surgery on someone else's house to get 240. But I realized that I had two 240 circuits in the garage (one for the garage door opener and one for everything else). Since 120 is just half of a 240 circuit, I realized if I made a Y-cord (a homebrew extension cord with one female end but two male ends, one to go to each 120 volt circuit) I could bring my own 240 volts together. This only works if your two 120 volt circuits are on opposite phases (halves of the mains power from the utility); otherwise this setup gives 0 volts. I had that issue at first, but was able to swap breaker positions around in the panel to get those two 120 volt circuits on opposite phase. The female end of the Y-cord has the 5-15 connector so the stock EVSE plugs in (and it likes it just fine). Also note that this setup only works if there are no GFCIs involved; it trips them every time. And of course note that the female end of such a cord needs a big red warning tag on it to indicate that it is not carrying the expected 120 volts. This allows 12 amp charging at 240, so it doubles charging speed with very little expense and no permanent modification to building w
For the wires and male ends of the cords, I bought 14 gauge extension cords (12 is best, 14 is ok for maybe 25 feet max, and I'd say 16 is marginally dangerous at any meaningful length for this much load). I cut the female ends off the extension cords, stripped them to bare wires, and put one hot to each of the hot and (usually) neutral pins in the female connector, and a single ground to ground the connector. This meant I could even cut the cords to desired length, since the 120 outlets were not equal distances away. Make sure your 5-15 female connector is super duty so the strain relief will be big enough to accept and clamp on to your two cords where one is usually expected.