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g/electrical • Posted by u/gen_set_gina •

Generator will not transfer under load but tests fine on exercise — Kohler 20RESCL Troubleshooting

Residential standby, RXT transfer switch. Weekly exercise runs perfectly, no-load, every time. Actual outage last week and it cranked, ran for about 40 seconds, then shut down and threw an underfrequency fault before the ATS transferred.

Checked the obvious: fuel pressure good, air filter clean, oil level fine, battery at 12.9V resting and holds 11.1V under crank.

The thing that is bothering me is that the exercise cycle is the only test the customer ever sees, and it does not load the machine at all. It has been "working" for two years.

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u/admin_jake ADMIN •
✓ Accepted Solution

Almost certainly gas supply. The exercise cycle draws maybe 10% of rated fuel flow, so a regulator that cannot deliver full flow passes the weekly test forever and fails the one time it matters.

Get a manometer on the fuel inlet at the generator and watch the pressure while it is loaded. Kohler wants 5 to 14 inches WC at the inlet under full load for that unit. If it is fine at idle and sags below 5 when the load hits, your problem is upstream — undersized pipe, an undersized meter, or a second-stage regulator that was sized for the house without anyone adding a 20kW generator to the load.

Nine times out of ten on a resi standby it is the gas meter. The house had a 250 CFH meter and somebody hung a generator on it that needs 300 CFH by itself.

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✓ Solution
u/gen_set_gina OP •

Manometer on it under load this morning. 11 inches WC at idle, dropped to 3.2 the moment the ATS picked up the house. Meter is a 250. Generator wants 299 CFH at full load.

Two years of clean exercise reports on a system that was never going to work. Utility is scheduling a meter upgrade.

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u/boiler_bill •

This is the single most common standby generator failure and it is a gas fitting problem that gets sold as an electrical job. Whoever priced that install either did not do the CFH math or did it and hoped.

Worth noting for anyone quoting these: add up every gas appliance in the house plus the generator at full load, because the generator runs during an outage when the furnace is also running.

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u/gen_set_gina OP •

That is exactly the number the original installer missed. Furnace, water heater, range and dryer plus 299 for the generator puts it at about 460 CFH. Nothing about a 250 meter was ever going to work.

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u/vfd_victor •

Underfrequency at 40 seconds is the engine not making power, which is fuel or air. Since it runs fine unloaded, it is a delivery problem that only shows when the governor opens up. Check the fuel line sizing and the regulator, not the engine.

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u/osha_omar •

Tangent worth saying: the customer believed this thing worked because it made noise every Tuesday. Exercise cycles are a maintenance function, not a test. If you sell standby systems, sell an annual load bank test with them, or you are selling a very expensive noise.

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