What is the right load bank size?

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LB-applicationsPermanently-installed generator load banks are a great way to identify potential issues before they cause serious problems.  When discussing the selection of a load bank, I am often asked what load bank rating should be specified for a given generator.  Here is my approach to the question of load bank ratings.

While NFPA-110 (Standard for Emergency and Standby Power Systems) covers the formal requirements for EPSS testing (Chapter 8, par. 8.4), it does not provide a recommendation for load bank sizing.  You may have heard of a 40%-60% “rule of thumb”.  This guideline refers to selecting a load bank with a rating equal to 40%-60% of the generator’s output rating.  I think this recommendation comes from engine manufacturers that want their engines loaded to a minimum 40%-60% of the nominal power rating, mainly to avoid wet stacking issues.  While this is a legitimate reason, I think it focuses primarily on taking care of the engine, and often misses the goal of testing the generator’s performance.

Rather than an arbitrary 40%-60% rating, why not size the load bank to match the expected real-life loads that the generator could see during an emergency?  If not tested at real load levels, how can you be sure that the generator will be able to handle actual loads during a power outage? Here are a couple of reasons for my line of thought:

  • An engine with partially-clogged fuel filters could potentially handle a 50% load, but when loaded to 80% it may be starved of sufficient fuel to carry the load.
  • The engine’s cooling system could be working marginally, and ok at 50% load for the typical 30-minute test, but will it provide sufficient cooling at 80% load during a 4-hour run?

In a nutshell, I recommend that generators be tested at or near the same conditions that can be expected during an outage.  If the generator is expected to carry loads equivalent to 80% of its output rating, any test load bank should be based on a similar load profile.

For more articles on Generator Testing Recommendations be sure to visit On-Site Power Advisor.

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