I don't mean to hijack the conversation, but if I was going to run a new circuit for a 120V 20A wire (5-20), what wiring should I use? Romex 10/3? 10/2? Should I use 12 gauge which supports 20A, or should I push for 10 gauge which supports 30A, for better cooling/resistance properties? It will have a 20A breaker, regardless, and an industrial 5-20 outlet on the other end in a weatherproof outdoor enclosure.
12AWG is acceptable for a 20A circuit, but will generate more waste heat that will translate into a larger voltage drop on the wire; if the voltage drop is large enough (typically > 10V), the car will reduce its draw from 16A to 12A, eliminating the benefit of installing a 5-20 in the first place. If, your breaker box is in/near your garage and the wire run is, say, 50' or less, 12 *should* be fine, but personally I'd err on the side of caution and use 10AWG if you can.
If you go to this
calculator, you can see the voltage drop between the two gauges at your circuit length (remember this is a round trip circuit, so enter double the length of your wire run). At a 100' circuit length (50' wire run), you'll see that you'll lose 5.2V at 16A with 12AWG wire, but only 3.3V with 10AWG. At 120V, that's a 30.4W difference in wasted power - not a huge amount, but probably adds up over time.
I have a particularly long run - about 150' - but I'm 10AWG all the way, including a 10/3 extension cord; I'm measuring ~112V at the car, and charging at around 6-7mph at current PNW outdoor temperatures, more than enough for my daily needs. I do expect the efficiency to go down in the winter due to battery conditioning, but per other accounts, it shouldn't go down by much.
