Generator Load Tests

Q: Is it allowed to combine the 3-year 4-hour generator load test along with the annual 2-hour load test? Our generator test company plans on running the generator at 50% load for the first two hours and then elevate to 75% load for the last two hours. In your opinion would this satisfy both the 2 hour and the 4 hour load test?

A: You are allowed to combine the 2-hour load test and the 4-hour load test. The 2-hour load test is required to be conducted once per year when the generator cannot meet the load test of 30% of nameplate rating every month. When this occurs, you still conduct the monthly load tests but once per year you need to conduct a 2-hour load test (usually by connecting the generator to a resistive load bank) that consists of the following sequences:

  • 25% load for 30 minutes, then
  • 50% load for 30 minutes, then
  • 75% load for 60 minutes for 2 continuous hours.

The scenario that you described allowed 50% load for the first two hours and then a 75% load for the last two hours. This would be acceptable in meeting both test requirements since the percentages listed in the standards are minimum settings, and you are permitted to exceed them. But you need to be careful, because if you combine these two tests and start out at 25% load (as required for the 2-hour load test) for the first 30 minutes, then you are out of compliance with the 4-hour test, unless you run an extra 30 minutes after you reach or exceed 30%.