Biggest EV charger your panel can handle (NEC 220.87)

It depends on your service size and your measured demand, not on a rule of thumb. By NEC 220.87 you take 125% of your 12-month peak demand, subtract it from the service rating, and the headroom (divided by 1.25 for the continuous charger) is the biggest charger that fits. For an example 100A service carrying a 60A peak that is a 16A charger on #12 Cu; the same math on a 200A service with a 90A peak allows a 48A charger on #6 Cu. Enter your own service and metered demand in the calculator for your number.

An automatic load-management device (an EVEMS) relaxes the SERVICE load calculation, which NEC 220.70 and 625.42(A) permit for a listed EMS in accordance with 750.30. It does this three different ways by device type: a shed device (DCC) disconnects the charger when the panel nears its limit, so the service stops being the binding constraint; a throttle device (Emporia PowerSmart, Wallbox Power Boost) modulates the delivered current to hold total demand under the service; a circuit-share switch (Splitvolt) time-shares one existing branch so the EV and the appliance never run together. In every case the device changes the SERVICE calculation, never the branch circuit.

No. The branch conductor and breaker are always sized to 125% of the FULL EVSE hardware output (NEC 210.20(A) / 625.41), never the shed or throttle setpoint. An external monitor or throttle does not change the charger's rating: if it fails or is bypassed the EVSE can pull its full current, so the conductor must always carry it. The only legitimate branch reduction is the EVSE's OWN restricted-access adjustable rating, a separate field-secured setting under NEC 625.42(B). NEC 625.42(A) relaxes the service and feeder only, never the branch.

A DCC shed device allows a charger up to 80% of its EV pass-through breaker, regardless of the service headroom, because it disconnects the charger before the panel is overloaded (NEC 750.30). Across the DCC breaker sizes that is 20A breaker to a 16A charger, 30A breaker to a 24A charger, 40A breaker to a 32A charger, 50A breaker to a 40A charger, 60A breaker to a 48A charger. The DCC-9, DCC-10, and DCC-12 all compute the same charger; they differ in the internal breaker, the maximum monitored service, and the application. The manufacturer also publishes a minimum-main matrix, keyed by the EV pass-through breaker: a 30A EV breaker needs at least a 60A main, a 40A breaker needs 80A, a 50A breaker needs 100A, a 60A breaker needs 125A.

Yes. NEC 625.42 defines EV charging as a continuous load, so the branch circuit and overcurrent device are sized at 125% of the charger output, and the 220.87 service check adds the charger at 125% as well. Adding the 125% once to the existing demand and once to the new charger is the conservative, fire-safe reading; it understates the maximum charger rather than overstating it.