Load Banking

Because real-world facility load is often dispersed, unpredictable and random, load bank testing is essential. Load banking is a technique used to determine maximum back-up power system performance and should be performed regularly. A load bank is a self-contained unit that applies controlled load on a power system. It consists of load elements with controls that allow an operator to incrementally step and vary the load. For total peace of mind, load banking is the method of choice to verify system operation, commissioning and maintenance as well as certify system capacity.

Photo 1: Load banking can verify system operation, commissioning and maintenance as well as certify system capacity. Here a load testing mobile unit is used to test a portable generator.

Why Load Bank Test?

Load banking is a critical requirement to ensure that generator owners enjoy the full potential of their emergency backup power equipment and that it will perform as expected when pressed into service. Since emergency generators must be sized to accommodate the full startup load of a facility, they are generally sized above the entity’s normal operating load if a load management system is not in place.

Generators are commonly sized as much as 30 percent over the kW rating of a facility to accommodate the surge of the building and equipment startup load. This situation creates a challenge with diesel powered generators. Conditions such as wet stacking and carbon buildup in combustion chambers, injector nozzles, piston rings, turbo chargers, exhaust piping and silencers can develop.

Wet stacking is best described as unburned fuel that accumulates in diesel exhaust. It can be detected by black seepage around exhaust connections or continuous black exhaust from the stack after warm up. Exhaust gas temperature of 275 degrees Fahrenheit must be maintained to avoid wet stacking (see Photo 2). With the increasing awareness of exhaust pollution, regular load banking is an important contribution on the generator owner’s part to minimize exhaust pollution in their communities.

Wet stacking is common when diesel engines operate for extended periods of time with little or no load applied. To operate at peak efficiency, the engine must be able to provide the proper ratio of fuel and air and maintain the right temperature to burn that fuel completely. EPA requirements have resulted in dramatic improvements in engine design while the use of electronic controls has resulted in increased performance.

Photo 2: Record temperature at turbo and after turbo during load test.

Photo 2: Record temperature at turbo and after turbo during load test.

Every client should require load bank testing when commissioning an emergency power system to ensure proper installation, adequate cooling at ambient temperature, sufficient fuel delivery, and proper load transfer.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)1 guideline 1-1996 defines commissioning as “the process of ensuring that the systems are designed, installed, functionally tested and capable of being operated and maintained to perform in conformity with the design intent…”

To test performance, load bank testing should be performed annually (for a minimum of two hours) for standby applications. During weekly exercises, the engine will not reach manufacturer’s recommended operating temperatures. The National Fire Protection Association (NFPA 110)2 has established the standards for monthly maintenance and operation for standby generators that states, “Generators should be exercised monthly at 30 percent of the nameplate rating or loaded to the minimum engine exhaust temperature recommended by the engine manufacturer.”

The Load Banking Process

1. Logistics are important. Be conscious of the load bank’s proximity to anything that could be affected by heat. The heat leaving a load bank can destroy plants, trees and discolor the paint on objects that are too close to or in the direct path of exhaust air. It is also important that the load bank is situated in free air space for adequate cooling.

2. The emergency switchgear should be disabled to avoid accidental transfer to facility load.

3. Ideally, the load bank should be connected to the load side of the generator breaker. When connecting the transfer switch, confirm that the cable size is rated to carry the full amp load encountered between the transfer switch and the generator breaker.

4. Cable rated to carry 100 percent of the rated load should be used to connect the load bank to the generator. For every additional 100 feet of cable required, cable size should be increased to the next largest cable size.

5. Notify the facility manager of start time and estimated completion time.

6. Verify that all cables point-to-point have secure termination, proper insulation, and correct phasing. All cables should be color coded prior to installation to simplify verification.

Photo 3: Use the master switch to “block” load as you increase load.

7. Start generator and allow a 5 to 10 minute warm-up period.

8. Apply load in steps allowing 15 to 30 minutes between steps.

9. Closely monitor the engine for leaks, oil pressure and temperature. Be prepared to quickly remove load if problems arise.

10. Block load (switch the existing load including last step by switching master switch) each step to record the generators ability to pickup the load in one step (see Photo 3). Measure the exhaust temperature and compare to manufacturer recommendation for engine and exhaust filter if installed. Recent emissions requirements call for the installation of particulate filters creating the need to raise exhaust temperatures to 400-800 degrees Fahrenheit to meet the manufacturer’s requirement for effective regeneration.

11. Allow a 30-minute cool down period at the end of the test with no load.


Load banking can create unexpected results. In many cases, the engine has not been tested to its capacity since the manufacturer’s testing at the factory. If regular load banking was not instituted as part of a preventive maintenance plan, then the older the engine, the greater the risk of operational breakdown during testing. As a surgeon would review the potential consequences with his patient prior to an operation, it is important that you review the worst case events with your client prior to subjecting their facility to the potential risks of generator failure during a test. Test results will reflect the level of neglect or underutilization experienced by the power system.

The facility should be disconnected from the generator while testing to prevent overloading the generator. For this reason, life support and mission critical clients need to be advised of the load bank test, and staff must be notifi ed in the event of a blackout. Backup generators should be recommended where a blackout is not an option for the facility. Testing should be scheduled during the most favorable time of the day, week and month when interruption would be most manageable.

Be aware of any local Environmental Protection regulations. For example, the New Jersey Department of Environmental Protection (NJDEP) requires you to log onto their website (www.state.nj.us/dep/aqpp/aqforecast) to check the air quality conditions and forecasts BEFORE testing a generator to avoid heavy fines.


There are two methods for load banking generators: resistive testing and reactive testing, with resistive load banking being the most common. The resistive method, which measures kW, but not kVA at the rated power factor, is ideal for testing the engine cooling system, exhaust system and the fuel delivery system. Reactive testing is a generator specific kVA test typically performed at the factory.


1. If the exhaust has not cleared (smoke-free) during the test period, engine repair will be required. Extensive smoke is most often caused by rings that have never seated properly, a fuel pump that needs to be rebuilt or bad fuel.

2. If the temperature is not within the manufacturer’s recommended range, the cooling system may need to be flushed to remove any restriction, or there could be a faulty thermostat or defective water pump.

3. If the oil pressure drops below the manufacturer’s recommended range, the oil pump may need to be rebuilt or low pressure may be an indication of excessive engine wear that could signal the need for an engine overhaul.


Ideally a digital recorder should be connected to chart frequency, voltage and amperage, and document in one-second sampling. This information should be downloaded to graph a detailed performance report. Engine oil pressure and temperature should be documented at 25, 50, 75 and 100 percent of the nameplate rating of the generator.


Photo 4: A sample load test report.

Load bank testing should not be considered an option, but rather a critical element of your emergency preparedness plan. If you are the maintenance provider, consider it your responsibility to recommend load banking to your client. As a generator owner, make sure this is included as part of your annual maintenance plan. There is no better way to have the peace of mind that you are prepared when the lights go out!


1The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) www.ashrae.org 2The National Fire Protection Association (NFPA 110) Standard for emergency and standby power systems. www.nfpa.org

About the author

Lyndon Risser is the CEO of DynaTech Power located in Lebanon, PA. Their primary focus is “Delivering Power and Confidence” through the sale, service and rental of emergency power equipment. For information, contact us.

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