11.2 NFPA 110 Emergency System Load Tests

NFPA 110 Classification

NFPA 110 classifies an Emergency Power Supply System (EPSS) with three labels, and you must read all three before testing because the Class drives the triennial test duration.

• Type — the maximum time, in seconds, that the load terminals can be without acceptable power. Type 10 = 10 seconds, Type 60 = 60 seconds, Type U = uninterrupted, Type X = AHJ-specified.
• Level — criticality. Level 1: failure could result in loss of life or serious injury (hospitals, life safety loads). Level 2: less critical to human life.
• Class — the minimum time, in hours, the EPSS can supply the load at its continuous rating without refueling. Class 0.083 = 5 minutes, Class 2 = 2 hours, Class 48 = 48 hours, Class X = AHJ-specified.

A common spec is "Type 10, Level 1, Class 48" — 10 seconds to restore power, life-safety grade, 48 hours of fuel on board. Read the Class carefully, because it sets the triennial test duration: per NFPA 110 §8.4.9 a Level 1 EPSS runs continuously for the duration of its assigned Class or 4 continuous hours, whichever is less, at not less than 30 percent of nameplate kW. Do not confuse §8.4.9 (the 36-month test) with §8.4.2 (the monthly 30-minute exercise) — they are different intervals, durations, and purposes, and swapping them is a reliable exam distractor.

The Monthly Generator Load Test

Per NFPA 110 §8.4.2, every Level 1 and Level 2 EPS must be exercised under load at least monthly for a minimum of 30 minutes. For diesel generators the load must either maintain the manufacturer-specified minimum exhaust gas temperatures or be at least 30 percent of the EPS standby nameplate kW rating. The test is initiated by simulating a power outage — using the ATS test switch, the parallel gear, or the remote control system — not by running the generator off-load at the generator. Where multiple ATSs share one generator, rotate which ATS initiates the test so each starting circuit is verified. Spark-ignited units may use the available EPSS load. Level 1 EPS are inspected weekly; Level 2 EPS are visually inspected at least monthly.

Annual and Triennial Full-Load Tests

If a diesel cannot meet the monthly 30 percent loading, NFPA 110 §8.4.2.4 requires an annual supplemental load-bank test: at least 50 percent of nameplate kW for 30 continuous minutes, followed by at least 75 percent of nameplate kW for 1 continuous hour — total of at least 1.5 continuous hours.

The triennial (every 36 months) test for Level 1 EPSS, per NFPA 110 §8.4.9, runs continuously for the duration of its assigned Class or 4 continuous hours, whichever is less. Minimum loading is at least 30 percent of nameplate kW (diesel) or available EPSS load (spark-ignited/fuel cell). The triennial may be combined with the annual load-bank test into a single 4-hour run: the first three hours at not less than 30 percent of nameplate kW (or the minimum exhaust gas temperature), and the final hour at not less than 75 percent of nameplate kW. Acceptance testing of a new installation is a separate requirement under NFPA 110 Chapter 7 and is specified by the project documents — do not assume the triennial numbers apply to commissioning.

NFPA 110 Test Intervals

Wet-Stacking Avoidance

Wet-stacking is unburned fuel and carbon accumulating in the diesel exhaust when the engine runs too cool or too lightly loaded — a direct consequence of chronic underloading. The monthly 30 percent loading rule (or maintaining manufacturer exhaust gas temperatures) is the defense. If you find a generator that has been run off-load for years, expect wet-stacking; a full load-bank run at ≥75 percent for an hour is the typical remedy before returning the unit to a 30 percent monthly regime. Wet-stacking reduces output, increases exhaust smoke, and can eventually damage the turbo and exhaust manifold.

What Qualifies as an Alternative Load Test

Where the connected load is too small to reach 30 percent of nameplate, a portable or permanently installed load bank supplies the deficit. NFPA 110 treats a load-bank-augmented monthly run as equivalent to a connected-load run as long as the 30 percent / 30-minute threshold is met. A no-load run never qualifies. Document the load source (connected load versus load bank), the kW achieved, the percentage of nameplate, the duration, and the exhaust temperature if the temperature method was used.

Documentation

Per §8.5, records of every inspection, test, and repair must be maintained — including the date, equipment ID, test duration, load (kW and percent of nameplate), any deficiencies, and the corrective actions taken. Records are subject to AHJ and accreditation review; gaps in the monthly log are a common Joint Commission and NETA accreditation finding. A NETA test report for a load test includes the instrument IDs and calibration dates for any meters used to record voltage, current, frequency, and exhaust temperature.