I'd really like to advise you to please watch Technology Connections' superb video on home EV charging, and the section at 28:15 seems written for you. I hear this kind of "huge charging" plan from EV novices a lot, and obviously, so does Alec.
It seems like either you believe EVs can only charge at one specific speed (if the above didn't put that right, see Alec's other video on how EV charging works), or you're a "Fastest Charge Possible" type personality. It's nice for travel, where you need 0-100% overnight, but it doesn't make any practical sense for real world EV charging at home.
You can really do one circuit and use Power Sharing with equipped EVSEs (not the ChargePoint; honestly we're not big fans of it since it provides so few options for provisioning power to the EV). A single 48A circuit is going to be plennnnty for 3 cars with Power Sharing, given that it's unlikely all 3 are going to need 150 miles of recharge in the same night.
I am skeptical about your panel having 120A of service capacity free in it. A Load Calculation must be done on the panel and service. We see a lot of people "freestyling" their Load Calcs based on hopes, wishes, and formulas their AHJ or insurer would not approve. Sacramento's worksheet accurately captures NEC 220.82, the most appropriate for EV charging. EV charging is a crushing load on a panel, and must not be underestimated. That's why less is better! Anyway, if there are Load Calculation issues, we have a variety of options to get you plenty of charging (despite the Load Calc) if you need it. Come on back and ask questions about that if applicable.
New car charges at 11Kw onboard charge (with 240v, that’s 45.8A)
That's not how that works. They're giving you the sales number, or a presumption that the voltage is 230V. Actually, the EV spec does not specify volts or watts, but rather amps. In the US or Canada, you'll have honest 240V, so 11.56 kW nominal.
The charge rate, in amps, is decided by the EVSE not the EV. I guarantee you they did not program the car to artificially limit the charge rate to 45.8 amps. It supports up to a given ampacity, but that will be a "round by EV standards" number such as 24, 32, 40 or 48 amps.
2 hots- 6 AWG THHN/THWN-2 copper 19 strand cerrowire 1 ground - 8 AWG THHN/THWN-2 copper 19 strand cerrowire
That's fine in conduit, but the ground can be 10 AWG.
Both runs, even in the unfinished basement ceiling, in 1” conduit. These will be two runs of 1” conduit (schedule 40, 90C rated) so there are only 2 load carrying wires and the one ground per conduit run.
There's no Code necessity to run 2 conduits. If you put both circuits in the same conduit, you would require an 80% derate from the highest temp permitted for the wire, which is 90°C. For 6 AWG wire that's 75 amps. An 80% derate off that puts you at 60 amps, which means it's fine to run both circuits in the same conduit.
That sucker is going to get HOT (whether it's in 1 conduit or 2). After all, you're running it almost at 75°C limit, so it's hardly a surprise if it runs at 75°C. The PVC conduit may sag or deform from all this heat; I would use EMT. It will also cool better. Some people find this heat coming off the conduit objectionable, but like Alec says in the video, the Fastest Charge Possible (tm) comes with headaches you hadn't bargained for. That's the least of them.
In fact, I think I could go up to a 75A breaker and set the charger to 60A if I wanted to in the future. Am I missing anything?
Does not work that way, in 2 dimensions.
Yes, you're missing the labeling on your load center / panel, which limits you to 75C both inside the enclosure and at terminals. UL approved that labeling as part of giving the panel a UL listing. If you want to run the 90C thermal numbers, you need either an industrial 90C panelboard, or to pigtail with 75C wire into an external enclosure rated 90C, then use 90C splices. At both ends. That costs more than the next size of wire. I would not play games like that unless I was boxed in by an underground conduit constraint.
Even so, "60 amps" is not a commonly supported charging rate/speed, so finding EVSEs with that speed would be an adventure. The next size up is generally 80A / 100A circuit. In that case you're not going to worry about the 6 AWG THHN wire (which is cheap anyway, under a dollar a foot) and you'll just go straight for #1 aluminum to the mandatory disconnect that is required above 48A, and then #3 copper the last 2 feet to the EVSE.
I have seen a lot of posts about 6 AWG being rated only to 55A (I guess at 60C)
They're referring to NM and UF type cable, which has inferior insulation. UF is stated directly in Table 310.16, and NM is set in NEC 334.80.
it seem like I should be able to use the 90c amperage of 70A.
That works inside the conduit... that's why you can derate 4 wires in the conduit. Does not work in consumer-tier load centers rated 75C.
