Load bank testing
A resistive load bank draws real power (kW) at unity power factor; a reactive load bank adds inductive or capacitive load (kVAR) so a source is tested at its full kVA rating and at a realistic power factor. Which one you need depends on what the test must prove.
Resistive load turns current into heat with voltage and current in step, so all of it is real power; reactive load stores and releases energy in a magnetic or electric field, so current runs out of step with voltage. ASCO's paper describes inductive reactance as resisting a change in current, making current lag voltage (motors, transformers and chokes behave this way), and capacitive reactance as resisting a change in voltage, making current lead it. A source that only ever meets resistive load in a test has not been shown working at the power factor it will see in service.
| Resistive | Inductive | Capacitive | Combined | |
|---|---|---|---|---|
| Power factor | Unity | Lagging | Leading | Chosen |
| Rated in | kW | kVAR | kVAR | kVA |
| Used alone? | Yes | With a resistive unit | With a resistive unit | Yes |
| Typical use | Engines, UPS, power systems at kW rating | Generators and transformers at full kVA | Leading loads such as IT and UPS equipment | Generators at rated power factor, parallel systems, switchgear |
Because a generator is rated in kVA at a lagging power factor, and resistive load cannot draw the reactive part. ASCO notes that resistive load fully exercises the engine, its cooling, fuel and exhaust at 100% of nameplate kW, but brings the set to only 80% of nameplate kVA; testing the alternator to its rating needs a combined load bank at the set's rated power factor, which in most cases is 0.8. Generac draws the same line between a 1.0 resistive test and a 0.80 reactive test.
Reactive load also matters when generators run in parallel: ASCO notes that loading the reactive component is usually required to calibrate load sharing and voltage regulation.
It depends on the UPS and on the question being asked, which is why the load type belongs in the written method. ASCO lists UPS systems among the uses of resistive and reactive load banks, and notes that capacitive load banks simulate the electronic loads typical of computer and UPS equipment. Some large UPS test themselves at a set real and reactive power: Vertiv's safe capacity test lets the engineer set active power up to 120% of the UPS's maximum active power and reactive power within plus or minus 120% of its apparent power, lagging or leading, without an external load bank.
For the UPS load bank method as published, see procedure and requirements.
They are the acceptance classes for engine-driven generating sets in ISO 8528 part 6, which defines functional and acceptance load bank tests. ASCO's paper summarises the classes: G1 for simple sets with undemanding loads, G2 broadly equivalent to commercial power, and G3 for sets powering critical loads that need stable, accurate power. Combined resistive and reactive load banks are used in factories to class a set and on site to confirm that it still meets its class.
Power factor and kVA, in addition to volts, amps, frequency and kW. ASCO notes that combined units need to monitor power factor and kVA, and that software control lets an operator store load profiles, capture transient responses and log data, so the same test can be repeated and compared over time.
For proving an engine or a UPS at its kW rating, often yes. For proving a generator's alternator at full kVA, or tuning paralleled generators, no: that needs reactive load.
Kilovolt-amperes reactive: the reactive power an inductive or capacitive load bank draws. Combined with kW it gives the kVA the source must supply.
Because the loads they supply, motors and transformers among them, draw reactive power. ASCO notes that combined load banks test a set at its rated power factor, in most cases 0.8.
Some can. Vertiv's safe capacity test lets the engineer set reactive power, lagging or leading, on the Liebert Trinergy Cube and EXL S1.
Sources, checked 2026-10-04:
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