Three Phase Uninterruptible Power Supplies (UPS) Selection
Here are many aspects which affect the selection of module. Totally single phase units are rarely available beyond 20 kVA, above this size it's usually necessary to use a 3 phase input for the rectifier, even where the inverter output is a single phase. Because 3 phase supply can deliver more electric power than one phase supply.
When feeding this sort of system via a generator it's important to recollect that the bypass line is going to be single-phase, demanding a better current on one phase only. Larger units with three-phase input/output are more easily distributed across generators and may even be wont to feed single-phase loads, and with good load balancing, needn't be oversized.