In short. A rental load bank applies a known, controllable electrical load to a generator so its output, stability, cooling and fuel system can be checked independently of the facility load. Choose a resistive unit for engine and cooling checks, a resistive-reactive unit when alternator and power-factor behaviour matter, and write the test plan before booking. A load bank proves the generator; it does not prove the transfer switch, the cables beyond it or the facility sequence.
What a load bank is for
Most standby generators in India run a few minutes a week on no load and a few hours a month on whatever load the building has. Neither tells the owner whether the set will carry its rated load for eight hours in May. A load bank is a bank of resistors, and sometimes reactors, in a ventilated enclosure with switched steps, that lets an engineer put 25, 50, 75 or 100 percent of rated load on the generator in a controlled way and watch what happens. It is used at commissioning to prove the set against its nameplate, after a major overhaul to confirm the repair, at annual intervals as part of a maintenance contract, and whenever a generator has run lightly loaded for long enough that wet stacking is suspected. Sizing the bank itself is covered in load-bank sizing.
Resistive, reactive or combined
| Type | What it loads | Use it when |
|---|---|---|
| Resistive | Real power only, unity power factor; engine, governor, cooling, fuel and exhaust are fully exercised, alternator current is below rated | Engine performance checks, wet-stacking burn-off, routine AMC testing, most commissioning |
| Reactive (inductive) | Reactive power; adds lagging current without engine load | Combined with a resistive bank to reach rated kVA at 0.8 power factor |
| Combined resistive-reactive | Real and reactive power together at a chosen power factor | Full nameplate kVA tests, AVR and alternator heating checks, data-centre and paralleling acceptance |
The distinction matters because a diesel generator is rated in kVA at 0.8 power factor. A 500 kVA set delivers 400 kW. A resistive bank loaded to 400 kW puts the engine at full load but the alternator at only 80 percent of rated current. That is sufficient to prove the engine, which is where most faults appear, and it is what the majority of rental load tests use. Where the alternator, the AVR response or the behaviour at rated current under lagging power factor must be proven, the reactive element is added. See the power-factor guide for the relationship.
Writing the test plan first
A load test without a defined question produces a sheet of numbers and no conclusion. Before the bank is mobilised, agree the following in writing.
- Purpose: commissioning acceptance, post-overhaul verification, annual maintenance proof or wet-stacking clean-up.
- Load profile: steps, hold time at each step and total duration. A common commissioning profile is 25, 50, 75 and 100 percent for 30 minutes each, then 110 percent for one hour where the manufacturer permits it (illustrative; the manufacturer’s test procedure governs).
- Measurements: voltage, frequency, current per phase, kW, power factor, coolant temperature, oil pressure, exhaust temperature, ambient temperature, fuel consumption and any alarms, recorded at fixed intervals.
- Acceptance criteria: for example voltage within plus or minus 2.5 percent and frequency within 50 Hz plus or minus 1 percent at steady state, coolant temperature stabilising below the manufacturer’s limit, no alarms, no visible smoke after warm-up.
- Block loading: the size of the step to be applied and the allowed transient dip and recovery time, if load acceptance is part of the test.
The plan also fixes the connection point, which decides what the test proves.
Where the load bank is connected
Connected directly at the generator terminals, the bank proves the generator alone. Connected downstream of the generator breaker and the main cable run, it also proves the breaker and the feeder under load, which is useful on new installations. Connected on the load side of the ATS with the utility isolated, it proves the generator path through the transfer switch, but it does not prove the transfer logic, which needs its own functional test as described in automatic transfer switches. Each step outward adds evidence and adds risk to the facility, so the connection point is an engineering decision, agreed with the people responsible for the installation.
Connection, safety and site practice
A 500 kW resistive bank at 415 V draws about 700 A. The temporary cables between the generator and the bank must be sized and protected like any other temporary feeder, as set out in temporary power cables,. The bank converts all of that power to heat, so it needs clear space around its air intake and discharge, placement where the hot air does not recirculate into the generator radiator. Fuel consumption at full load is roughly 0.25 to 0.30 litres per kWh for a modern diesel set, so a four-hour test on a 500 kVA set can use 400 to 500 litres (illustrative); confirm the tank has enough, because running out of fuel under load is a poor way to end a test. Keep personnel away from the discharge and ensure the bank’s interlocks drop the load if its cooling fan fails.
What the test does not prove
A clean load-bank result shows that the generator produces its rated power with stable voltage and frequency and acceptable temperatures for the test duration. It does not show that the ATS will sense a mains failure and transfer, that the largest motor on site will start against the generator’s transient capability, that the facility’s step load on restoration is within the set’s acceptance, or that a UPS on the output will be happy with the generator’s frequency behaviour during recharge. Each of those needs its own test. A load bank is one instrument in a commissioning programme, not the whole programme. For motor-starting and UPS interaction see motor starting and UPS and non-linear loads.
Common mistakes
- Booking the bank before deciding what the test is for. The day is spent arguing over load steps and the report answers nothing.
- Loading a resistive bank to the generator’s kVA figure instead of its kW figure. The engine is overloaded by 25 percent and the test is invalid or trips.
- Connecting the bank with undersized or unprotected temporary cables. The cables overheat and the test introduces a hazard the installation did not have.
- Placing the bank so its hot discharge enters the generator radiator. The set trips on high coolant temperature and the generator is blamed.
- Reporting a terminal load test as proof of the whole emergency power system. The ATS or the step load fails on the first real outage.
What to send us
Send the generator nameplate photograph with kVA, kW, voltage and alternator details, the purpose of the test, the load profile and duration you want or the manufacturer’s procedure if you have it, the location of the generator with the distance to a suitable position for the bank, and the fuel available on site. We will propose the bank size and type, the cables and the measurement schedule. Send a rental enquiry for a load bank, or if the test is part of a maintenance programme, send a service and AMC enquiry.
Questions engineers ask
How often should a standby generator have a load-bank test?
Once a year is a sound practice for a standby set that rarely sees more than 30 percent load in service, and after any major engine or alternator work. Sets that regularly run above 50 percent load on real demand may not need a separate test, because service duty already proves them.
What size load bank do I need for a 500 kVA generator?
For a resistive test to full engine load you need 400 kW, which is the set’s kW rating at 0.8 power factor. For a full kVA test at rated current you need a combined bank able to deliver 400 kW and 300 kVAr. The bank’s voltage and frequency must match, 415 V and 50 Hz for most Indian sets.
Can the facility load and the load bank be used together?
Yes, and it is common where the facility cannot supply enough load on its own. The bank tops up the facility demand to the target. The building load must be stable and the total monitored, because the bank cannot react if the building load changes suddenly.
What is wet stacking and does a load bank fix it?
Wet stacking is the build-up of unburnt fuel and carbon in the exhaust system of a diesel that runs at light load for long periods, visible as black oily deposits and smoke. Running the set at 75 percent load or more for a few hours on a load bank raises exhaust temperature enough to burn most of it off, and the test also reveals whether the deposits have affected performance.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.