Sizing a UPS: why load characteristics matter as much as load size

Most UPS specifications start with a headline capacity figure. But a system that looks right on paper can still trigger alarms, transfer to bypass or run inefficiently if the character of the load hasn't been taken into account.

A good first step is to record the details from each piece of equipment's rating label: supply voltage and frequency, number of phases, load current, power factor and power consumption. Once you have these, Rehlko's free kVA load calculator will total them up and give you a starting figure for sizing your UPS.

Check phases and power factor

A three-phase UPS can supply both single and three-phase loads, but a single-phase UPS can only support single-phase loads. Power factor needs attention too. Modern switched-mode power supplies have pushed loads towards unity, and sometimes into a leading power factor, which can significantly derate older UPS designs. Transformerless UPSs are far less affected.

Single-phase loads should be spread as evenly as possible across the three output phases. This makes full use of the UPS rating and presents a balanced load to the mains or generator if the UPS goes to bypass. Even modern UPSs with independently regulated phases shouldn't carry more than a third of their total rating on any one phase.

Watch for problem loads

Motors, compressors, air conditioning, laser printers, and discharge or dimmable lighting can all draw high or pulsed currents. Without soft-start, inrush current at start-up can reach 15 to 20 times the normal running current, causing overload alarms or unwanted transfers to bypass. A UPS supporting these loads should be oversized accordingly.

Measure, then plan for growth

A label survey is only an approximation. A measured site survey shows the real load and how it varies over time. Looking ahead, modular UPS systems make it easy to add capacity as demand grows, simply by plugging in extra modules without taking the system offline.