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…But a few years ago after a string of really hot week days we hit 400A/phase. … so there is no real way to say “we’re close to the limit, don’t charge your car”. If they were restricted to charging overnight (10pm – 6am) that would probably help since most poeople would be in bed and the sun has gone down, reducing the heat load…
I don’t know how many units you have but if your historic peak load was 400A/phase from a 600A/phase supply, then you could supply 200A/phase to vehicle charging. Three phases x 200A would allow 10A single phase charging for 60 vehicles. If you have less than 60 units, you could safely let each unit have an ordinary 10A power point and they could all be charging in the worst possible circumstances and you would still be OK.
As it happens there are systems to deal with exactly your example, even if you have more units. A staged adoption might include 32A single phase cabling for parking spaces but terminated in ordinary 15A power points. Until there was some particular number of vehicles, the building supply would be fine. I routinely charge at 10A and it is ample (pardon the pun). Fast charging is not needed where one routinely parks.
You plan to upgrade the system once you get to more EVs. Then the ordinary cheap power points are replaced with a set of networked 32A EVSE charging outlets. These can be controlled from a central device. That device might be set to allow a maximum supply to the complete set. If every car were plugged in and charging, each car might only be allowed 6A. As cars that did not drive very far cease charging or leave the building, the system allows the remaining cars to take 10A each. Some time later, more cars have finished charging and the few remaining are allowed 15A each without exceeding the total allow supply for charging. Eventually, there are few cars remaining and all are allowed to draw 32A.
A further refinement of such systems is that the maximum supply to the set of charging outlets can be varied dynamically and automatically to reflect the difference between the maximum permitted building load and the non-charging load of the building. So, nearly all of the time, there will be lots of excess capacity that can be dedicated to charging. On the unusual occasions when the building has a high demand such as in the example, car charging can still proceed albeit more slowly and automatically shared among the vehicles requiring a change. As the evening wears on and the load from units is reduced as the evening peak passes, charging rates can ramp up and all cars will get a decent top up, even in the worst case scenarios.
These really are set and forget systems. They are described in more detail and with diagrams in the WattBlock publications I mentioned.