How do I select the breaker rating and cable size for a three phase UPS?

A qualified electrician or electrical contractor should carry out all sizing and electrical work. They will need the maximum input current, usually provided in the UPS manufacturer’s installation manual. The input current is higher than the UPS output current because of energy losses within the UPS AC-DC-AC conversion and also when the UPS battery is recharging. As a rough guide, a 30kVA UPS draws about 43A per phase at full output, rising to roughly 50A while charging the batteries, so a 63A triple-pole and neutral breaker is typically specified.

UPS Manufacturers usually call for Type C or D curve breakers to handle inrush currents. The rectifier input, bypass input, UPS output and battery DC circuit each need their own protection.

Cables for 3-phase UPS system installations can be H07RN-F (Heavy-Duty Flexible Rubber) or XLPE-insulated, steel wire armoured (SWA), in 5-core (L1, L2, L3, N, E) or 4-core plus a separate earth. Low smoke zero halogen (LSZH) sheathing is the usual choice inside occupied buildings and data halls. Size the cable for the breaker rating, installation method, grouping, ambient temperature, voltage drop and fault current. Larger systems often use single-core cables on cable trays.