Generator Load Tests
by Brad Keyes, CHSP, on Apr 12, 2021 1:00:00 AM
Q: My question is related to emergency generator testing. On my last building (and my current one) the emergency generator was really small for the building, therefore only supplying power to a life safety branch and some critical branches including just one elevator. This arrangement is keeping up just fine but we are unable to add anymore load. During load tests, there is no way for me to use a load bank, since the generator is always at 100% load when doing the monthly load test. Now I read about the 2-hour and 4-hour tests. How would I even perform those tests if I have no control of the loads? Is my monthly load test enough to satisfy NFPA requirements? As you can imagine it is really disruptive to perform a load test using the main disconnect and having the generator only supplying limited stuff.
A: You are required to do monthly load tests on the generator, no fewer than 20 days and no later than 40 days from the previous test for a minimum of 30 minutes, not including the cool-down period. If any one of the monthly load tests does not meet or exceed a load of 30% of the nameplate rating, then you are required to do an annual load-bank test. The annual load test operates the generator at 50% of nameplate capacity for 30 minutes, then 75% of the nameplate capacity for 60 minutes for a 90-minute continuous test. But based on your comments, your monthly load tests exceed the 30% load threshold so you do not have to perform an annual load test.
All generators have to have a 4-hour load test once every 3 years. This 4-hour load test must operate the generator at least 30% of the nameplate rating for 4 continuous hours, which again it appears you can meet without any difficulty.
The monthly load test and the 3-year load test should only energize the emergency power branches (i.e., life safety, critical, and equipment) and not affect the normal power circuits. I do not see this as a big problem for your facility. You do not cut power to the entire facility and only operate on EM power. You flip the ‘Test’ switch at the Automatic Transfer Switches (ATS) for the different branches of emergency power, and only those branches are operating under EM power. The rest of the facility is operating under normal power and for the most part, are not even aware there is a load test under way.