TLDR: Best value is 90A wire called #2. It's OK that it's aluminum, really. Scrimping on number of spaces is a costly blunder: think BIG and over-buy spaces. Don't need a main breaker until you have at least 8-10 circuits in the panel.
I have an outbuilding which is going to have a car charger in it (48A 240V),
We do a lot of EVs here. I know those numbers!
48A is the actual charge rate. Unless you have industrial gear rated for continuous use, you must apply a 125% derate to a long list of loads including EVs. (this is more commonly known as an 80% derate, but Code says 125%).
Thus 48A charge rate requires a 60A circuit and wire. Your EVSE instructions say this. You must follow them (NEC 110.3).
and one or two 120v 20A circuits. The car charging circuit won't be in use when any of the other circuits are in use.
Promises don't work. You would need a physical interlock. But as things work out, this won't be a problem at all.
FYI you must use a proper NEC Article 220 Load Calculation, you can't freestyle this calculation.
I would like to run a 60A circuit to the building to minimize wire size and cost.
You'd think, right? that there's a linear relationship between ampacity and cost. Good news: there's not.
- The cheapest wire in town is 50A.
- Next is a tie between 90A wire and 30A wire.
- Then 100A wire. Then a tie between 120A and 40/50A.
The X-factor here is that long before people experimented with misapplying aluminum wire in small branch circuits, it was building a proven track record as heavy feeder. You might not have been planning heavy feeder, but it sure makes sense. The pricing "sweet spot" is at #2 aluminum, which is 90A for feeders. Plenty for run-of-the-mill EV charging + a few other things.
By the way, there is no such thing as 60A wire. 55A wire and 65A wire can be had, but 60A has never existed. Why do people imagine it for subpanels? Because that once was a size the power company offered for services. A dwelling service gets an 83% favorable derate, allowing 60A service to use 49.8A = 50A wire. (see above).
I seem to be unable to locate an outdoor panel which has a small enough main breaker.
Because it's unnecessary. Let's get clear: Your subpanel does not need a local main breaker. It does need a local disconnect switch.
For the disconnect, you can rely on the Rule of Six - the disconnect can be up to 6 hand throws. So if you have <= 6 branch circuit breakers you're done. That was easy.
If we have too many breakers, we need a literal main disconnect e.g. knife switch. But when you price that, simply buying a panel with a main breaker works out cheaper. No one cares about the number on the breaker, as long as it's not in our way.
There is no requirement for the panel to be small. Panel size and bus rating are redline limits, not best practices. For instance if you drive in Wyoming and roads are posted for 80 mph, would you want 85 mph or 130 mph tires?
So what actually are the restrictions?
- The feeder wires must be sufficient for the load to be served. And you don't get to use promises and procedures here, it must undergo a NEC Article 220 Load Calculation.
- The supply breaker (at the main panel) must be <= feeder wire ampacity (with a "round-up" allowed to the next available breaker size).
- The rating of the subpanel's bus bars must be >= the smallest breaker in the chain. In fact we consulted with someone who had a 200A breaker feeding 4/0 wire to a 125A-bussed subpanel with a 100A breaker.
Does it need to be an outdoor rated panel?
Yes.
Is an oversized breaker on the downstream end legal?
As discussed, totally fine.
Is it legal to use a lug panel as the panel in an outbuilding?
As discussed yes, as long as there are <=6 hand throws to shut everything off, you don't need ONE disconnect.
However, you do yourself NO FAVOR by using a small panel. Nobody ever said 5 years later "man, I so regret buying a huge panel because if I bought a smaller panel I could have afforded a pizza also back then." However lots of people say "HELP, I am out of breaker spaces!" Lavishly over-buying a panel is the second best advice we can give. It is funded and then some by the best advice we can give, which is that there's nothing wrong with aluminum heavy feeder.
Is it legal to feed a lug panel through a 2-pole 240v breaker serving as a main breaker, if (somehow) properly labelled?
Yes, however the breaker requires a tie-down kit. Most panels are designed with this in mind.
Now some people get hung up on the idea of trying to get one breaker or the other to trip preferentially. That's not offered as a product in the consumer space, but you can "wing it" by choosing a subpanel so the supply breaker and backfeed breaker will be the same model. (note that breakers are NOT interchangeable across brands; they "seem to fit" but will arc and burn up the bus stab). Then if the "wrong one" trips, you can just swap them. However, you are doing something gravely wrong if you see a trip ever. Breakers have very generous trip curves.
