Stationary Generator Annual Load Test Procedure

Why this matters

A standby generator that exercises weekly under no load proves nothing about its ability to carry the actual building load when the power fails. Cylinder glaze, wet stacking, and fuel system varnish only show up under load. NFPA 110 requires annual load testing for emergency systems (Level 1 and Level 2), and even on residential standby installs the same procedure catches problems weekly exercise hides. This is the procedure for a single resistive load bank test on a residential or light commercial standby up to 60 kW. Larger units follow the same steps with bigger gear.

What NFPA 110 actually requires

NFPA 110 Chapter 8.4 requires the emergency power supply system (EPSS) to be exercised at least monthly under load of at least 30 percent of the nameplate kW rating, OR at the actual building load if it exceeds 30 percent, for a minimum of 30 minutes. If the monthly exercise does not reach 30 percent of nameplate, an annual load bank test of 2 continuous hours is required, ramped through:

• 30 minutes at 25 percent of nameplate kW.
• 30 minutes at 50 percent.
• 60 minutes at 75 percent, or at 100 percent for the final 30 minutes if the manufacturer permits.

Residential automatic standby units (under 22 kW typical) are not under NFPA 110 by code, but the test catches the same failure modes and is good practice annually.

Pre test verification

Before any load bank is connected:

1. Confirm the generator has had its annual service: oil and filter changed, air filter inspected, coolant level and freeze protection verified, battery load tested.
2. Test the battery under cranking load. Voltage should not drop below 9.6 V during a 15 second crank simulation. Replace the battery if it does.
3. Inspect for any visible fluid leaks, exhaust system damage, loose belt, or hose deterioration.
4. Verify the day tank or fuel supply is at minimum 50 percent. A 2 hour load test on a 30 kW diesel burns approximately 3 to 4 gallons per hour under load.
5. Verify the ATS is in normal (utility) position. The load bank will be connected on the generator side independently of the ATS.
6. Confirm exhaust discharge is clear and oriented away from any building openings or fresh air intakes.
7. Notify the building owner / occupants that a 2 hour test is starting.
8. Have shutdown access ready (E stop, breaker, fuel shutoff valve location all known and reachable).

Load bank connection

A resistive load bank (typical units: 25, 50, 100, 200 kW capacities) connects to the generator at the load side terminals or via a dedicated load bank connection point if installed.

1. Verify load bank is rated for the generator voltage and phase configuration. A single phase 240 V residential generator needs a 240 V single phase load bank; a 3 phase 208 or 480 V commercial generator needs the matching 3 phase bank.
2. Inspect load bank cables for damage, melted insulation, loose lugs.
3. With the generator OFF, connect load bank cables to the generator output terminals or load bank tap, following the load bank manufacturer's connection sequence (typically ground first, then neutral, then phase conductors).
4. Confirm the load bank's step controls are all OFF before generator start.
5. Position the load bank with at least 6 ft of clearance on the air intake side and unrestricted exhaust airflow above. Load bank discharge air is hot (often 200 to 400 F); do not direct it at people, vehicles, or combustible structures.

Test sequence

1. Start the generator manually from the controller (do not initiate via ATS transfer).
2. Allow 5 minutes of no load warmup. Voltage and frequency should stabilize within 30 seconds.
3. Record baseline data with engine warm and no load:
   • Coolant temperature (typically 160 to 200 F after warmup).
   • Oil pressure (typically 40 to 70 psi at rated speed depending on engine).
   • Battery / charge voltage (13.5 to 14.5 V).
   • AC voltage (within 1 percent of nameplate, typically 240 V single phase or 208 / 480 V 3 phase).
   • AC frequency (60.0 Hz +/- 0.3 Hz).
4. Apply 25 percent load using the load bank step switches. Engine speed should not droop below 59 Hz. Voltage should hold within 5 percent of nominal. Run for 30 minutes. Record all parameters every 10 minutes.
5. Step to 50 percent load. Run 30 minutes. Record every 10 minutes.
6. Step to 75 percent load. Run 60 minutes. Record every 15 minutes.
7. If the manufacturer permits 100 percent and the engine has shown no abnormality, the final 30 minutes can be at 100 percent. Watch coolant temperature; should not exceed 220 F on most liquid cooled diesels.
8. After the load run, step back to 0 percent and run a 5 minute cool down before shutting down.

What you are watching for

Voltage regulation: voltage must hold within 5 percent of nominal across the load steps. A voltage that droops more than 10 percent or hunts visibly between load steps indicates an AVR (automatic voltage regulator) problem.

Frequency stability: frequency must hold 60.0 Hz +/- 0.3 Hz under load. A frequency that droops below 59.4 Hz indicates a governor problem or undersized engine.

Coolant temperature: should stabilize between 180 and 210 F under full load. Climbing past 220 F indicates radiator or coolant flow problem.

Exhaust smoke: clear or very light haze under load. Black smoke under load indicates fuel system over fueling or air starvation; white smoke indicates incomplete combustion (cold engine, water in fuel) or coolant leak into combustion chamber.

Oil pressure: should not drop more than 10 percent below baseline as the engine warms and loads. A continuous drop indicates oil pump wear or bearing problems.

Wet stacking diagnosis: if the generator has been exercising at no load weekly, watch for black liquid weeping from the exhaust manifold during the first 30 minutes of load. This is unburned fuel in the exhaust system being burned off; the test itself is the cure for the wet stack condition.

Post test documentation

Record the following in the customer's maintenance log:

• Date of test, technician name, ambient temperature.
• Generator make, model, serial, hour meter reading.
• Each load step duration and percent.
• Voltage, frequency, coolant temp, oil pressure, battery voltage at each load step.
• Fuel consumed.
• Any anomaly observed.
• Recommendations.

Per NFPA 110 Chapter 8.3, this record must be retained for the life of the EPSS. For residential, retain for the life of the generator and provide a copy to the homeowner.

Common findings

References

• NFPA 110 Standard for Emergency and Standby Power Systems, Chapter 8 (Routine Maintenance and Operational Testing).
• NFPA 110 Chapter 4 (Classification) and Chapter 5 (Energy Source Considerations).
• NFPA 70 (NEC) Articles 700 (Emergency Systems), 701 (Legally Required Standby Systems), 702 (Optional Standby Systems).
• IEEE 446 Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications.
• Generator manufacturer service manuals: Kohler, Generac, Cummins, Caterpillar, Onan, Briggs and Stratton.
• EGSA Performance Standards (Electrical Generating Systems Association).