Yes there are a lot of confusing elements in the EPA regulations 40 CFR 63 Subpart ZZZZ, however the requirements for re-testing are pretty straight forward.
Here’s the Cliffs notes version:
Engines between 300 HP and 500 HP (200kW-375kW generators) do not require any additional testing – beyond the original EPA performance / compliance test done to confirm that the emissions retrofit equipment (diesel oxidation catalyst) was providing the required CO reduction.
Engines of 501 HP / 400kW and above require re-testing at 3 or 5 years, depending on annual non-emergency operating hours.
3 or 5 years?
Limited Use affected engines = used for less than 100 hours of non-emergency* operational hours – need to be re-tested every 5 years.
All other affected engines require re-testing every 3 years.
* The simple definition of Non-Emergency operation is anytime the engine generator is running, while the electrical utility is available. This includes regular exercising or load bank testing and utility control periods – really anytime that the utility could be providing power.
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