When you said "Rivian will only accept 50A at 240" I had taken that as a 50 amp circuit where only 40 amp is usable, hence the different calculation. At any rate, yes, 48 amp (60 amp breaker) at 240v would be the most you would need with a Rivian to approach that 11 kW on-board charger. But why limit your EVSE to the specs of a Rivian? I would think anyone investing in an EVSE would not just size it to a Rivian, or at least at its current spec, but instead get one powerful enough to be effective for that future replacement or second EV that probably will have a more powerful on-board charger. (This assumes the person has already calculated the need for the most powerful home EVSE charger, say, due to driving many miles in a day.) I was under the impression the EV software and/or on-board charger will only draw what it needs / can handle. I could be way off base here as a yet-to-be-EV-owner and trying to soak up all I can to help determine which kind / size of EVSE to get while also future proofing (to a degree) my set up to get the most out of that investment.