Ok we are talking extreme what if's!!! Simply designate one car as primary and the other car that charges at between 3am to 7am. The primary car can plug in anytime prior to 11pm and suck down maximum power to be recharged.
In extremely rare off case where the primary car will charge outside of this time frame, you can modify the HPWC with a switch to over-ride the internal dip switch to charge at 40amps until morning!!!!
From a code perspective, modifying the HPWC disqualifies its listing and does not eliminate the load requirement. A user-selectable switch does not permit you to count it as a 40A charging load / 50A circuit requirement, if it is possible to charge at 80A (100A circuit) via a user-selectable switch. This is why it is based on DIP switches.
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If you set both HPWCs to 40A you can still charge both cars fully overnight so why worry?
There are times you will want to have the faster charging. Getting out a T20, looking up DIP switch setting, changing it, cycling power at the breaker, and then plugging in is a pain in the ass.
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Why is it ok for you to have two 80A loads on a 200A panel but not ok for the OP to have two 40A loads on a 100A panel? (I'm assuming the other loads are trivial.)
You can't assume other loads are trivial for NEC. See my post above for the ways to calculate lighting/receptacle loads other than charging.
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So the OP would be OK if he had no other loads concurrent with the two 40A chargers? Couldn't he reasonably argue that the other loads are unlikely to be on during the charging cycles? We're pushing the envelope here I guess. But given the OP's situation it seems like he doesn't have any other good options. I suppose he could limit the HPWCs to 35A each leaving room for 10A of other loads. That would still be sufficient to charge both cars overnight except when they're almost down to zero.
You'd have an easier time suggesting that 220.60 applies to 2 EV's than you would that other loads don't apply. Here's an example -- plug the car in, it starts charging, then you close the garage door. Boom - 5A motor load plus charging load.
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I'll stick with my original suggestion and that would be to get a dedicated 200A panel for two 80A (100A breaker) HPWCs.
That's what I recommend to anyone who is adding charging to an existing structure.
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My house has 200A service. I have an HPWC set at 80A. And the usual plethora of little stuff for a big house (the sum of those breakers is far more than 200). But I'm about to remodel my kitchen and replace the gas cooking appliances with electric. That will add another 90A to my total. Can I reasonably argue that the cooking will never be concurrent with the charging?
No inspector would ever allow you to say that.
The intent of 220.60 is to address things like large, high-HP tools. For example, it would be unlikely for you to use a 5 HP lathe and 5 HP planer at the very same time in a personal workshop. I don't necessarily agree with wk057 that EV loads should not apply to 220.60. There is also a provision in 220 (.53, I think?) that allows 4 or more appliances to have a 75% load factor applied to them. So 4 HPWC's installed @80A charging could be sized with 300A feeder.
There are specific calculations for a home, and you may end up needing a service upgrade.