Generator Load Bank Test
Purpose
Prove that a standby generator set can actually carry load, not merely start. The monthly exercise most sites run is a no-load or light-load run: it proves the batteries, the starter, the fuel solenoid and the transfer signal. It proves nothing about the cooling system at rated output, the alternator's ability to hold voltage into a real load step, or whether the engine has been wet stacking for a year because it never got hot enough to burn its own fuel.
Sites find out which of those is true during the outage, when there is no second chance. A loaded test converts a set nobody has stressed into a set with a known limit, and produces a number a facility can defend to an inspector or an insurer.
Scope
Covers a supplemental-load test of an installed diesel or gaseous standby generator set at 600 V or less serving emergency, legally required standby, or optional standby loads, using a portable resistive load bank connected downstream of the generator's output breaker.
Does not cover the monthly operational exercise with building load, which is the facility's own routine; the transfer switch installation and its timing verification, which the transfer-switch install SOP owns; fuel polishing, tank cleaning or engine overhaul; or medium-voltage sets, where the connection and PPE requirements change.
Roles and handoffs
| Role | Owns | Hands off |
|---|---|---|
| Office | Confirms the test window, the building's outage tolerance, fuel level, and who on site can authorize an abort | Names the on-site authority in writing to the lead before mobilization |
| Lead electrician | The connection, the load steps, every reading, and the abort decision | Hands the completed test log and any deficiency to the office within one business day |
| Facility contact | Notifies occupants, confirms nothing critical rides on the set during the window | Signs the log acknowledging the result, including a failed or aborted result |
Procedure
Walk the exhaust path, the room and the fuel supply before anything is started. A load bank test runs the set at load for hours, not the few minutes the monthly exercise runs, so an exhaust defect that has never mattered will matter today. Check the exhaust for holes, loose clamps and a termination that discharges clear of intakes and occupied space, and wear a personal carbon monoxide monitor for the whole test if you will be in the generator room or downwind of the termination. Acceptance: exhaust intact and clear, room ventilation openings unobstructed, CO monitor on and zeroed. Wrong looks like soot streaks at a manifold joint. Stop rule: a leaking exhaust inside an occupied building means the set does not run today; evacuate the room, get outside, and quote the repair.
Confirm the set will not be needed and the abort authority is on site. Establish that no critical load depends on this generator during the window, that the transfer switch will be left in its normal position, and that a named person can authorize an abort. Acceptance: written confirmation of the window and the named authority, plus the transfer switch verified in normal. Wrong looks like a test starting because the tech assumed a facilities email was approval. Stop rule: no named authority means no test, because the whole value of an abort rule is that someone can invoke it.
Verify fuel volume against the run's consumption plus the site's required reserve. A two-hour loaded run consumes real fuel and a set that ends the test below its required reserve is now less ready than before you arrived. Size it from the consumption table in that set's own manual; where you have no table, a planning figure near 0.07 gallons per kilowatt-hour for a mid-size diesel gets you close enough to decide, and it must be replaced by the manual's number before you commit. Acceptance: fuel on hand covers the full sequence plus the reserve the facility is required to hold. Wrong looks like starting a test on a tank at a quarter. Stop rule: short on fuel, reschedule after a delivery. Hazard clause: no fuel sampling or filter work with the engine hot, because diesel on a hot manifold ignites.
Connect the load bank as electrical work, not as a hookup. Connect downstream of the generator output breaker at a point rated for the current. De-energize that connection point, lock and tag it under 29 CFR 1910.333(b)(2), and prove it dead live-dead-live per NFPA 70E-2021, 120.5, before landing a lug; the prime mover itself is separately isolated under 29 CFR 1910.147 so nobody can crank it while hands are in the enclosure. Acceptance: every lug torqued to the marked value, cables supported off the ground, load bank grounded, doors closed. Wrong looks like cables draped across a doorway or a lug landed by hand-feel. Stop rule: an unrated tap point is not made rated by a short test.
Establish the baseline before load: start, stabilize, and record the no-load set. Start on the panel, let the set stabilize, and record no-load frequency, line-to-line voltage on all phases, oil pressure, coolant temperature and battery charging voltage. Acceptance: frequency and voltage within the manufacturer's stated no-load band, oil pressure in range, no alarms. Wrong looks like a voltage that will not settle or a hunting governor. Stop rule: a set that cannot hold a stable no-load condition does not get load applied; that is an engine or governor deficiency to report before you make it worse.
Apply load in steps and hold each one, watching the trend rather than the instant reading. Where the site qualifies under NFPA 110 for supplemental-load testing because the monthly exercise cannot reach 30 percent of nameplate kilowatts or the manufacturer's minimum exhaust gas temperature, the standard sequence in Chapter 8, in the edition your authority having jurisdiction has adopted, is 25 percent of nameplate for 30 minutes, then 50 percent for 30 minutes, then 75 percent for 60 minutes, for two continuous hours. Acceptance at each step: frequency and voltage hold within band, coolant temperature reaches a plateau below the alarm point, oil pressure steady. Wrong looks like a coolant temperature still climbing at a steady rate late in a step. Stop rule: project the climb to the shutdown set point and abort the step before the set protects itself, because a protective shutdown under test tells you less than a controlled abort and risks the equipment.
Record readings on a fixed interval, not when something looks interesting. Log every 10 minutes through the whole sequence: kilowatts, amps per phase, line-to-line and line-to-neutral voltage, frequency, coolant temperature, oil pressure, and exhaust appearance. Acceptance: a complete interval log with no gaps, and each abnormal entry annotated at the time it was taken. Wrong looks like a log filled in from memory in the truck. Hazard clause: read gauges from outside the machine's guarded envelope, never with a hand on a hot exhaust shield or near a running belt, and use hearing protection for the whole run rather than pulling it off to talk.
Unload in reverse, cool down under no load, then shut down and reconnect. Step the load off in the reverse order it went on, run the set at no load for the cool-down period the manufacturer specifies, then shut down. Acceptance: cool-down completed, set returned to auto, transfer switch confirmed in normal, battery charger confirmed operating, load bank cables removed and the tap point re-torqued and re-covered. Wrong looks like a hot shutdown straight off 75 percent, which cooks the turbocharger bearing. Stop rule: the job is not finished until the set is verified back in automatic, because a generator left in off or manual is a building with no standby power and no indication of it.
When the site does not match the assumption
The monthly exercise already reaches 30 percent of nameplate kilowatts or the manufacturer's minimum exhaust temperature: the supplemental annual test is not required by that route. Say so in the record with the monthly load figures that prove it, rather than selling a test the site does not need.
The set cannot be taken out of service because it backs a load with no tolerance for the risk: the window is the constraint, not the test. Route to the lead for a rental set or a scheduled outage, and record the deferral and its reason.
Wet stacking is visible on arrival, meaning unburned fuel at the exhaust and heavy smoke at light load: the loaded run is treatment as well as test, but say plainly that a set under-loaded for years may not clear in one run. Do not promise the smoke stops.
The record this produces
One test log per set per test. Header: date, technician, generator make, model, serial, nameplate kilowatts and voltage, engine hours in and out, fuel level in and out, ambient temperature, load bank make and size, and the facility contact who signed.
Body: the interval log from step 7 at 10-minute spacing across the whole sequence, with each load step marked, plus the no-load baseline from step 5. Footer: the result stated as passed, failed or aborted with the reason, every deficiency with an owner and a date, and the date the test is to be re-run if it did not complete.
The facility keeps it as the annual test record an inspector or insurer will ask for. The office reads the deficiency list to quote. The next technician reads last year's interval log to see whether a coolant temperature that plateaued at 92 C last year is plateauing at 99 C now, which is a trend nobody catches from a pass or fail.
Worked pass: 150 kW diesel standby, three-story office building
Nameplate is 150 kW. The facility's monthly exercise carries only the life-safety panel, and the metered load during the last three exercises averaged 34 kW, which is 22.7 percent of nameplate and below the 30 percent threshold, so the site qualifies for the supplemental-load sequence.
Step 3, fuel: the sequence is 37.5 kW for 30 minutes, 75 kW for 30 minutes and 112.5 kW for 60 minutes, which is 18.75 plus 37.5 plus 112.5, or 168.75 kilowatt-hours. At the planning figure of 0.07 gallons per kilowatt-hour that is about 11.8 gallons; the manual's table gives 12.4 gallons for the same sequence, and the manual's number governs. Tank shows enough for the run and the site's reserve, so the test proceeds.
Step 5 baseline: 60.1 Hz, 481 V line to line, oil pressure in range, coolant 71 C, no alarms.
Steps 6 and 7 run clean through 25 percent and 50 percent. Coolant plateaued at 88 C on the first step and 94 C on the second.
Step 6 fails on the 75 percent step. At 41 minutes into the 60-minute step, coolant reads 99 C and the last four log intervals show it rising 1 C every 3 minutes with no sign of plateau. The high coolant temperature shutdown is set at 104 C. Five degrees at one degree per three minutes projects to a shutdown in about 15 minutes, and 19 minutes of the step remain, so the set would protect itself roughly four minutes before the step ended. Stop rule taken: the step is aborted at 41 minutes, load is stepped off in reverse, the set cools down at no load and shuts down normally.
The test is logged as aborted, not failed and not passed, with the reason and the projection printed. Inspection with the set cold finds the radiator core packed with cottonwood and the discharge louver actuator opening to roughly half travel. Both go on the deficiency list with the lead as owner. The re-run is booked for two weeks out, and the facility contact signs the aborted result, which is the record that matters if the set is asked to carry the building before the re-run happens.
References
- NFPA 110, Chapter 8, in the edition your authority having jurisdiction has adopted, for monthly operational testing thresholds and the supplemental annual load-test sequence used in step 6
- 29 CFR 1910.333(b)(2) for the electrical lockout and verification practice at the load-bank connection, and 29 CFR 1910.147 for isolating the prime mover against cranking
- NFPA 70E-2021, 120.5, for the live-dead-live proving sequence, in the edition your employer's electrical safety program has adopted
- Manufacturer's operation manual for the specific set: fuel consumption table, no-load voltage and frequency bands, alarm and shutdown set points, and cool-down period
- See related: Generator Transfer Switch Install SOP, Utility Outage Coordination and Customer Notification SOP