Because the efficiency of the Tesla Chargers is about 90% for the AC to DC conversion, and the max DC output of a Supercharger Cabinet is 135 kW, the max AC input of a Supercharger Cabinet is 150 kW. 4 Supercharger Cabinets at maximum (8 cars drinking heartily!), need a maximum of 600 kW of AC in. Because the power factor of a Tesla Charger is very close to 1, that means a max draw of 600 kVA.
The way most utilities rate and install transformers, 500 kVA could probably work for a 4 Supercharger Cabinet site, but every 4-cabinet site that I have seen, has had a 750 kVA or larger transformer.
The Tesla spec sheet for Gen II superchargers that showed up in a UK planning application (permit) shows for various input voltages:
380V 126kVA in, 115kW out (305A max)
400V 133kVA in, 120kW out (310A max)
480V 160kVa in, 145kW out (335A max)
AC input current in all cases 192A per phase, 200A breaker specified.
99% power factor.
This was in connection with a European installation, but the fact that they quote ratings at the common EU voltages and at 480V (not seen here other than via a private transformer) suggests that the Superchargers are now universal. If so, 4 cabinets is a load of 640kVA under USA conditions.
The efficiency is specified as 91.5% across all voltages, although calculation from the other figures gives 91.3%/90.2%/90.6% respectively - near enough the same.
The max output currents are slightly odd, as the rated power could never be achieved at those currents when charging a Model S. I'm wondering if those specifications refer to the current at the connector, implying a 335A connector limit, and hence the full power rating can be achieved when two cars are connected (at lower voltages but a higher aggregate current).