Why can a three phase UPS support larger loads than a single phase UPS?

A three-phase (3-phase) supply delivers about three times the power of a single-phase (1-phase) supply for the same current per conductor. Three-phase power is calculated as √3 × 400V × current, while single-phase power is 230V × current. At 100A, a single-phase supply delivers about 23kVA and a three-phase supply about 69kVA. This means smaller cables and breakers can carry large loads. Single-phase UPS systems typically top out at around 20kVA because of the cable size needed to carry the current; above that, three-phase is the standard, practical choice. Three-phase UPS systems start at around 10kVA and scale to several megawatts when units or modules are run in parallel.