Load bank testing
A load bank is a device that draws a controlled, measurable electrical load from a power source and turns that energy into heat, so a generator, UPS or other source can be proven at known loads without connecting real equipment to it.
A load bank works by passing current through load elements and getting rid of the energy as heat. In the simplest form, as ASCO describes it, precise resistive elements are cooled by an integral fan; the electrical energy in kilowatts leaves the unit as heat. Because the elements are switched in known steps, the load is consistent and repeatable, and it can be controlled, measured and recorded, which is the point: the source under test sees a load you chose, not whatever the building happens to draw that day.
Control ranges from pilot relays, toggles and decade switches to embedded controllers and PC software that capture data, record transients and produce reports with graphs. Larger systems network several load banks under one controller, so a site can apply more load than a single unit could carry.
There are four types, defined by the kind of load they present: resistive, inductive, capacitive and combined.
| Type | Load it presents | Rated in | What it is used for |
|---|---|---|---|
| Resistive | Real power at unity power factor | kW | The most common type; loads an engine, a UPS or a power system to its kW rating |
| Inductive | Lagging power factor (current lags voltage) | kVAR | Used with a resistive unit to reach a source's full kVA rating at a lagging power factor |
| Capacitive | Leading power factor (current leads voltage) | kVAR | Used with a resistive unit to simulate leading loads, typical of computer and UPS equipment |
| Combined | Resistive plus reactive elements, switched independently | kVA | Testing at a chosen power factor from one unit; can include resistive, inductive and capacitive elements (RLC) |
The difference matters most for generators and is covered in resistive vs reactive load banks.
They are the three sides of the power triangle: kW is real power, kVAR is reactive power and kVA is the apparent power that combines them. A resistive load bank only draws kW, at unity power factor. That fully loads an engine, but ASCO notes that only 80% of a generator set's nameplate kVA can be reached with a resistive unit. Generator sets are usually rated at 0.8 power factor, so proving the alternator at its full kVA takes an inductive element alongside the resistive one.
For a UPS the question is different: what load do you need to prove the power train at its rating? That depends on the UPS and on what the test is meant to show, and it is settled when the test method is written.
Load banks come in six common forms, chosen for where they will be used:
Load banks are used to proof-test power sources: generators first, then UPS systems, switchgear and other parts of a power system. ASCO lists the main uses:
A load bank presents a steady, chosen load, which is its strength and its limit. It tells you how the source behaves at the steps you applied; it does not reproduce every behaviour of a real IT load, and a resistive-only test says nothing about reactive performance. For a UPS it does not prove battery runtime, which needs a capacity test; see UPS and battery load bank testing.
Four things decide whether a test day goes to plan, and all of them are settled before anyone arrives:
No. The load bank is the equipment that creates the artificial load; the load bank test is the procedure that uses it to prove a generator, UPS or other source at known load steps.
It fully loads the engine, cooling, fuel and exhaust systems to the kW rating, but ASCO notes that only 80% of the nameplate kVA can be reached. A reactive element is needed to test the alternator at its full kVA.
Large enough to reach the rating you want to prove. Load banks can be networked: ASCO's example uses a 3000 kW load bank to test one 1500 kW generator set at full kW, or two in parallel.
Out of the load bank through its fan, so the unit must sit where the heat can be rejected safely, away from sensitive equipment, with cable run from the source under test.
ASCO recommends a third-party listing such as UL 508A, CSA or the CE marking, and design to the applicable IEC, NEMA, NEC and ANSI standards.
Sources, checked 2026-10-04:
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