A generator that never works hard is dying — wet stacking, NFPA 110, and what the test report tells you.
Standby generators spend their lives waiting — then must carry full load instantly. But diesel engines that idle or run light-load deteriorate: unburned fuel and carbon build up in the exhaust (wet stacking), injectors foul, and the set that “tests fine” monthly fails when the outage comes. Load-bank testing is the cure: artificial load that makes the generator actually work. Here is why it matters and what the standards require.
Load-bank reports are gold — ask for them. Buying guide →
Diesel engines are designed to run hot and loaded. Below ~30% load, combustion is incomplete: unburned fuel and carbon collect in the exhaust system — wet stacking, named for the wet, black ooze at the stack. Symptoms: black exhaust, carbon-fouled injectors, glazed cylinder walls, and power loss. Standby sets are the prime victims — monthly “tests” that run the engine unloaded for 15 minutes actively cause the condition they are meant to prevent. The fix is heat and load: regular loaded runs burn the deposits out.
NFPA 110 (emergency and standby power systems) requires monthly testing of Level 1 systems — and crucially, if the monthly test does not load the generator to at least 30% of nameplate, a load-bank test is required (annually at minimum, using the prescribed load profile). In plain terms: if your monthly test is just an unloaded exercise run, code wants a real load-bank test on the record. Facilities that skip this discover wet stacking during the outage — the most expensive possible classroom.
Big resistor grids that convert the generator’s output to heat — the standard tool. Sized to the set (a 500 kW bank for a 500 kW set), connected at the generator or transfer switch, run through stepped load profiles: 25/50/75/100% and back down.
Voltage and frequency regulation under load, transient response (the dip and recovery when load steps on), cooling system capacity at full heat, fuel system under full flow, and whether the set actually makes nameplate kW. A set that cannot hold rated output on the bank cannot hold it in the outage.
A proper report shows load steps, voltage/frequency at each step, oil/coolant/exhaust temps, and fuel consumption — with pass/fail against the manufacturer’s spec. When buying used, this report is the machine’s résumé: recent, loaded, documented. No report? Price accordingly.
Unburned fuel and carbon buildup in the exhaust from chronic light-load running (under ~30%). Black ooze at the stack, fouled injectors, power loss — cured by loaded running.
NFPA 110: monthly testing for Level 1 systems; if the monthly run doesn’t reach 30% load, an annual load-bank test using the prescribed profile.
Yes — a building-load test counts if it reaches the required load levels. Most facilities can’t reliably produce full load on demand, which is why portable banks exist.
Loaded running burns deposits out — light cases clear with a proper loaded run. Severe glazing may need mechanical attention. Prevention (regular loaded runs) beats cure.
The best ones do. A recent report showing nameplate output is the strongest condition evidence a seller can provide — ask for it before the cold start.
Nameplate (make, model, kW, serial), hour meter, and overall condition. The nameplate is the entire specification.
Call or text 312-880-8174 or use the quote form. Tell us the kW, fuel, hours, and condition.
A real offer based on your photos and today’s market — not a calculator, not a guess.
The complete hub — every guide in one place.
Size it right with the interactive calculator.
Diesel and natural gas — get today’s price.
Diesel or natural gas, running or parts — send photos for today’s price.