For facility owners, one of the biggest questions in generator exercising is determining when supplemental loading is needed and when their facility’s load is enough.
NFPA 110 addresses this issue through its testing requirements for emergency and standby power systems, including provisions for diesel generators that cannot achieve the required loading during monthly exercises.
This guide from Curtis Power Solutions explains when supplemental generator load bank testing may be necessary, what to consider when planning a test, and how to verify that your emergency power system is being tested appropriately.
Generator load bank testing applies a controlled electrical load to an emergency generator to evaluate its performance under load. Under this test, a controlled electrical load is applied to an emergency generator using a load bank.
This helps verify generator performance under the required test conditions and prevents buildup of carbon in the exhaust system known as "wet stacking”. The NFPA 110 standard also covers the ATS and Electrical System Testing, control panels and auxiliary components that are necessary for the system to function safely and automatically.
Supplemental generator loading can help reduce carbon buildup and wet stacking while supporting reliable generator operation. Supplemental loading may be appropriate in the following situations
A standby generator may run normally during monthly exercising but carry too little electrical load to meet the applicable testing criteria. In such cases where the facility cannot provide sufficient demand during Testing, a load bank can supply the additional load required to achieve the prescribed operating conditions.
Repeated operation of a diesel generator at a light load can contribute to incomplete combustion and wet stacking. A load bank allows controlled loading to help the engine reach the appropriate operating conditions.
A facility's normal electrical demand may be insufficient for a generator's required performance or acceptance test. This can happen in healthcare facilities, data centers and commercial buildings where standby capacity exceeds the load used during testing. A load bank supplements the connected building load to achieve the prescribed conditions.
A newly installed generator or one that has undergone major repairs may require additional performance testing to verify its operation under load. A load bank can help with Acceptance Testing to assess the electrical output, engine response, and cooling performance.
Under NFPA 110 generator testing requirements, the rules for supplemental loading are quite specific and apply when the generator cannot meet its applicable load requirements during routine testing.
NFPA 110 requires diesel generators to be exercised at least once a month for a minimum of 30 continuous minutes. In case the generator does not achieve the required load of at least 30% of its nameplate kW rating or maintain the minimum exhaust gas temperatures recommended by the manufacturer, an annual supplemental load bank test is required.
The exact requirements for this annual test vary according to the applicable edition. The 2025 edition requires 30 continuous minutes at 50% and one continuous hour at 75% of the generator's nameplate kW rating.
See our guide to The Key NFPA 110 Requirements for more details.
To determine the supplemental load bank capacity required for your facility, you should consider the monthly loading thresholds required under NFPA 110. This means subtracting your lowest recorded building load from the 30% compliance threshold.
For instance, if your facility has a standby generator of 2,000 kW, the minimum required threshold for compliance would be 600 kW. However, during monthly building transfers, if your facility components only pull a baseline of 350 kW then you must install or rent a supplemental load bank with a capacity of at least 250 kW. It is also wise to add an additional safety margin of 10% to 20% to this calculation.
If you plan to use the supplemental load bank to handle the annual load bank test without relying on building power at all, you must size the total load bank capacity to at least 75% to 100% of the generator's nameplate rating (including the safety margin).
You can also refer to our facility manager’s checklist for NFPA 110 Generator Testing Requirements to be better prepared for compliance reviews.
A load bank test shouldn’t just establish that the generator can reach a particular output. The testing team should also monitor the equipment's response as the load increases and throughout the required operating period.
They should monitor the following key equipment parameters:
Confirm that the generator maintains acceptable electrical output as the load changes.
Record the applied load in kW and kVA, and power factor where relevant to confirm that the generator has achieved the specified test conditions.
Monitor applicable coolant and oil temperatures, oil pressure, exhaust conditions and other manufacturer-specified parameters.
Watch for unexpected shutdowns, warning indicators, leaks, excessive vibration or unusual noise. Also verify relevant protection and control functions according to the approved test procedure.
The technician should document load levels, test duration, operating readings, alarms, and any recommended corrective actions. This gives facility managers a useful record for Maintenance Planning and compliance reviews.
Poorly planned load bank testing can create avoidable risks or produce results that do not demonstrate what the facility needs to verify. Common mistakes that can compromise testing accuracy, safety, and compliance include:
Using the wrong NFPA 110 edition - Applying a testing sequence from a newer edition without checking local adoption can lead to an incorrect compliance plan. You should confirm the applicable requirements for your facility before scheduling the test.
Assuming a successful start proves generator readiness - Starting and running are important but they do not demonstrate the ability to sustain the required electrical output. Generator testing should address the applicable load and operating conditions.
Selecting a load bank based only on generator size - The generator's rating matters, but other factors such as the connected building load, required load steps, electrical characteristics, and testing objectives are equally important.
Ignoring site safety and essential loads - Load bank placement, hot exhaust air, temporary electrical connections, and cable routing require careful planning. The testing team should also ensure that essential services remain protected and the approved procedure addresses the risk of losing normal utility power during testing.
Failing to document the results - Without clear records, it becomes difficult to demonstrate what was tested, whether the required conditions were achieved, or what needs follow-up.
Treating load bank testing as a substitute for maintenance - A successful test does not eliminate the need for scheduled servicing, inspections, fuel-system checks or investigation of faults identified during operation.
You can also download our NFPA 110 Forms to get the schedules for maintenance and testing, and logs for recordkeeping.
A generator testing specialist can help facility owners to make informed decisions about generator performance and maintenance. You should consult a generator testing specialist when your facility's electrical load is insufficient for the required testing, or the compliance requirements are unclear.
Professional support becomes particularly useful when:
Routine tests fall short - The generator cannot achieve the required load during monthly exercising.
Low-load operation persists - You notice recurring wet stacking, unusual exhaust smoke or other performance concerns.
Installation or major repairs are completed - You need to verify the generator’s performance under load.
Compliance requirements are unclear - You need to confirm the applicable NFPA 110 edition and testing procedure.
Your facility has critical power needs - You need a carefully planned test that accounts for essential services and site safety.
At Curtis Power Solutions, we supply generators for reliable backup power and offer generator rental services for temporary power needs. We also help facility teams evaluate generator testing requirements and determine the appropriate load bank testing approach.
Supplemental generator load bank testing is required when a facility's normal electrical demand is too low to meet the applicable generator exercise or testing requirements. It helps demonstrate reliable generator performance under load and help identify problems that can develop during prolonged light-load operation.
For facility owners, the starting point is to review the generator's actual operating data, confirm the NFPA 110 edition adopted by the local jurisdiction, and establish which testing requirements apply.
From there, a qualified generator specialist can help determine the necessary load bank capacity, testing procedure, and documentation. Contact Curtis Power Solutions to determine the right approach for your facility.
Generator load bank testing is a compliance test under NFPA 110 where an artificial electrical load is connected to a standby generator to test its performance under specified test conditions.
Supplemental loading is needed for a standby generator if it fails to meet the minimum load threshold during its mandatory monthly operational tests.
The NFPA 110 generator testing requirements for minimum loading are -
The system should run for at least 30 minutes at a minimum of 30% of its nameplate kW rating.
Or it must be operated at the manufacturer’s recommended minimum exhaust gas temperature to ensure adequate combustion.
Prolonged low-load operation can contribute to incomplete combustion and carbon buildup in the exhaust system, commonly referred to as wet stacking.
The capacity of the load bank required for generator testing depends on whether you need it for monthly operational testing (at least 30% of the generator's nameplate kW rating) or an annual supplemental load bank test (at least 80% to 100% of the generator's nameplate kW rating).
Supplemental load bank testing is generally performed once annually if the monthly test requirements are not met. For Level 1 systems, an extensive load test is required every 36 months.
No. Testing requirements depend on the applicable NFPA 110 edition, system classification, operating conditions, and whether required load or temperature criteria are met during routine testing.