If your building or plant has a standby generator, this guide explains what has to be inspected and tested, how often, and what to record. It also explains why a generator that rarely carries much load may need an extra annual test, and when a problem during a test calls for an electrician or the generator dealer. A monthly test log is at the end.
For generators covered by NFPA 110:
NFPA 110, the Standard for Emergency and Standby Power Systems, applies when a code requires an emergency power supply system and points to the standard. The standard sorts systems into two levels:
Many commercial generators are optional standby systems, installed to keep a business running rather than because a code requires them. For those systems, the manufacturer's maintenance schedule usually governs, though many owners follow the NFPA 110 routine anyway because it is a proven program.
Not sure which you have? Your generator's installation records, your fire code official, or your electrical contractor can tell you.
The current edition of NFPA 110 is 2025, but your jurisdiction may enforce an older edition referenced by its adopted fire or building code. The schedule below follows the 2025 edition. Where an earlier edition differs in a way that matters, it is noted.
NFPA 110 calls for Level 1 systems to be inspected weekly. The 2025 edition allows monthly inspections for Level 2 systems. Because older editions required weekly checks for both levels, plan on weekly inspections unless your fire code official tells you otherwise.
A typical weekly walk-around checks:
Each generator set should run at least monthly, for at least 30 minutes, under load. The 30 minutes is counted from when the generator takes the load, not from when it starts.
For diesel units, the load must do one of two things. It must keep the exhaust at the manufacturer's recommended minimum temperature, or it must reach at least 30 percent of the generator's standby nameplate kW rating. Natural gas and other spark-ignited units run with whatever emergency system load is available.
Also operate transfer switches monthly by electrically moving them from normal power to the generator and back. If you have several transfer switches, rotate which one starts the monthly test.
Generators are often sized for future growth, so a building's real emergency load may be well under 30 percent of the generator's rating. A diesel engine that repeatedly runs lightly loaded doesn't burn its fuel completely. Unburned fuel and carbon build up in the exhaust, a condition called wet stacking. Over time, it reduces performance and can lead to failures.
That is why NFPA 110 adds an annual supplemental load test when monthly runs don't reach the required load.
A diesel unit that cannot meet the monthly loading rule must still be exercised every month on the available load. It then needs one supplemental load test each year.
Under the 2025 edition, the generator runs at least 50 percent of nameplate kW for 30 continuous minutes, then at least 75 percent for 60 continuous minutes, for a total of 1.5 hours. Earlier editions started with an extra step at 25 percent, for a two-hour total.
A portable load bank provides the extra load. If your generator carries at least 30 percent of its nameplate rating, or reaches the manufacturer's exhaust temperature, during every monthly run, this annual test is not required. Your monthly log is the evidence, which is one more reason to record kW during every run.
At least once every 36 months, a Level 1 system must run continuously for the duration of its class rating. The test may stop after four continuous hours. Plan this one ahead, because it is the longest run in the schedule.
NFPA 110 requires a record of every inspection, test, exercise, repair, and modification, available to the authority having jurisdiction. For the monthly run, record at least:
The log below follows that sequence.
Stop and call for service when:
Who to call depends on the problem. Generator work is usually split three ways:
If you aren't sure where a problem starts, call the company that installed the system. They can tell you which trade it belongs to.
Download the Monthly Generator Test Log Spreadsheet
Trained facility staff can do the weekly inspection, start and watch the monthly run following the manufacturer's procedure, record readings, and keep the log. Load bank testing and transfer switch maintenance need the generator dealer. Fuel piping needs a plumbing contractor. Breaker work, wiring repairs, and changes to the system's loads need a licensed electrician.
CDI Electric installs and services Kohler, Generac, CAT, and Cummins Onan generators for commercial and industrial facilities across Northern Kentucky and Greater Cincinnati.
We handle the electrical side of the system: wiring, breakers, load connections, and changes to which loads the generator serves.
The generator dealer handles engine service, transfer switch service, and load bank testing, and a plumbing subcontractor installs fuel piping.
CDI coordinates with both so the system is installed and tested as one job. If your generator is failing tests, isn't carrying enough load, or needs to support new equipment, see our commercial electrical services or industrial electrical services.
You can also call 859-291-9189.