How Often Should a Commercial Generator Be Load Bank Tested?

We get this question a lot, and the honest answer is: it depends on your monthly testing. NFPA 110, the Standard for Emergency and Standby Power Systems, does not simply say "load bank test once a year." It ties the annual load bank test to whether your generator is carrying enough load during its regular exercise. Plenty of buildings either over test or, much more often, under test because nobody explained the rule.

This post walks through the NFPA 110 testing schedule as it applies to typical commercial diesel generators, and what we recommend for buildings in South Florida. Always confirm which edition of NFPA 110 your authority having jurisdiction enforces, and whether your facility has other rules on top of it, such as healthcare requirements under NFPA 99 or state rules for nursing homes and assisted living.

Monthly exercise

NFPA 110 requires generator sets in service to be exercised at least once a month for at least 30 minutes. The run has to meet one of two conditions: either the load keeps the engine at the minimum exhaust gas temperature the manufacturer recommends, or the generator carries at least 30 percent of its standby nameplate kW rating.

That second condition is where most buildings fall short. A 500 kW generator needs to carry at least 150 kW during the monthly test to meet it. Many buildings only put the emergency or life safety loads on the generator, and those might total 60 or 80 kW. The generator runs, it looks fine, and it never gets close to 30 percent.

The monthly test also includes the transfer switch. The automatic transfer switch should be operated monthly so you know it actually transfers.

Why light loading is a problem

Diesel engines are designed to work. When a diesel runs at light load for long periods, it does not get hot enough to burn all the fuel completely. Unburned fuel and carbon build up in the exhaust system and cylinders. You will sometimes see black oily residue dripping from the exhaust. That condition is called wet stacking.

Wet stacking reduces performance and can damage the engine over time. More important, it means you do not actually know whether the generator can carry its full rated load when you need it. A generator that has only ever run at 15 percent load is an unproven generator.

The annual load bank test

If your diesel generator does not meet the monthly loading requirement, NFPA 110 requires an annual test with supplemental load. Under the current language, that means at least 50 percent of nameplate kW for 30 continuous minutes, followed by at least 75 percent of nameplate kW for 1 continuous hour, for a total of at least 1.5 continuous hours. Some older editions and some jurisdictions describe the steps a bit differently, so check what yours enforces.

The supplemental load usually comes from a portable resistive load bank connected at the generator. The load bank lets the engine work hard without putting the building at risk, and it burns off the carbon buildup from light loading.

If your generator does carry at least 30 percent of nameplate during its monthly exercises, or reaches the manufacturer's recommended exhaust temperature, the annual load bank test is not required by NFPA 110. Some owners do it anyway as a proof test. We think that is a reasonable choice for critical buildings.

The 36 month test for Level 1 systems

NFPA 110 classifies systems as Level 1 or Level 2. Level 1 systems are those where failure could result in loss of life or serious injury, which typically includes many emergency systems in hospitals and high rises. For Level 1 systems, NFPA 110 also requires a test at least once every 36 months. The generator runs continuously for its assigned class duration or 4 hours, whichever is less, at not less than 30 percent of nameplate kW. For diesel units, a load bank can supply that load.

Healthcare facilities often follow this schedule closely because accreditation and CMS surveys look at it.

What we recommend for South Florida buildings

We look at it practically. Hurricane season runs June 1 to November 30. That is exactly when you need the generator, and exactly when every load bank and every technician in the region is booked.

So our general advice:

• Do the monthly exercise every month and log it. Record the load in kW or percent, not just "ran 30 minutes."

• If the monthly load is below 30 percent of nameplate, schedule the annual load bank test in the spring, well before June.

• Check fuel quality at the same time. Diesel stored in a humid climate picks up water and grows microbial contamination. Fuel polishing or testing is worth doing annually.

• Replace starting batteries on a schedule. Dead batteries are the most common reason a generator fails to start.

• Exercise the transfer switch and inspect it. A transfer switch that sticks is as bad as a generator that will not start.

Records matter

NFPA 110 requires records of inspections, tests, exercising, operation and repairs. Fire marshals, insurers, healthcare surveyors and sometimes building owners' lenders ask to see them. A clean log that shows monthly runs with load, annual load bank results and maintenance is your proof that the system is ready.

When we do a load bank test, we record the load steps, voltage, frequency, coolant temperature, oil pressure and exhaust temperature over the test, and we give the owner a written report.

Gas and bi fuel generators

The annual supplemental load test in NFPA 110 is written for diesel powered systems because wet stacking is a diesel problem. Natural gas generators have different concerns, and the 36 month rules are written differently for them. If you have a natural gas generator, follow the manufacturer's recommendations and the NFPA 110 requirements for your system.

Common mistakes we see

The most common is a building that runs the generator every week with no load at all. That is worse than nothing for a diesel engine. The second is a building that does not record the load during monthly runs, so nobody can tell whether the load bank test is required. The third is scheduling the load bank test in August.

Need a load bank test?

We perform generator load bank testing, transfer switch service and generator maintenance across Broward, Miami Dade and Palm Beach. See our generator services or call 954.577.7923 to schedule a test.