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UL 9540 Energy Storage System (ESS) Requirements - Evolving to Meet Industry and Regulatory Needs

In recent years, installation codes and standards have been updated to address modern energy storage applications which often use new energy storage technologies.

Energy Storage Systems

May 20, 2020

Authored by Laurie B. Florence and Howard D. Hopper, FPE

Energy storage systems (ESS) are gaining traction as the answer to a number of challenges facing availability and reliability in today’s energy market. ESS, particularly those using battery technologies, help mitigate the variable availability of renewable sources such as PV or wind power. ESS are a source of reliable power during peak usage times and can assist with load management, power fluctuations and other grid related functions. ESS are used for utility, commercial/industrial and residential applications.

In recent years, installation codes and standards have been updated to address modern energy storage applications which often use new ESS technologies. The 2018 editions of the International Fire Code, International Residential Code and the NFPA 1 Fire Code first introduced requirements aimed specifically at modern ESS applications, with a focus on lithium-ion battery installations. Requirements were further refined in the 2021 editions of those model codes, and in the 2020 edition of NFPA 855, the Standard for the Installation of Stationary Energy Storage Systems.

These codes and standards have one thing in common: they all require electrochemical ESSs to be listed in accordance with UL 9540, the Standard for Safety of Energy Storage Systems and Equipment, which was first introduced in November 2016.

As installation code requirements are updated to reflect new industry developments, research, and testing, UL 9540 has also evolved to better meet the safety needs of industry and the regulatory community. ESS size and separation requirements in particular have been addressed in the second edition of UL 9540.

ESS installation codes contain size and separation requirements designed to prevent a fire originating in one ESS unit from propagating to adjacent ESS units or adjacent battery room walls and exposures. The size requirements limit the maximum electrical storage capacity of nonresidential individual ESS units to 50 KWh while the spacing requirements define the minimum separation between adjacent ESS units and adjacent walls as at least three feet. Exceptions in the codes allow the code authority to approve installations with larger energy capacities and smaller separation distances based on large-scale fire testing conducted in accordance with UL 9540A, the Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems Standard.       

To approve increased installation sizes or decreased spacings, a code authority will need to evaluate a data intensive UL 9540A fire test report that describes the fire and explosion characteristics of the battery ESS. Changes recently made to UL 9540 will help simplify this process for designers and jurisdictions, but code authority review of the UL 9540A report remains an important part of installation acceptance.

The second edition of UL 9540 has new requirements that limit the maximum energy capacity of individual nonresidential electrochemical ESS to 50 kWh unless they comply with UL 9540A fire test performance criteria. Similarly, there are new requirements for nonresidential electrochemical ESS intended for indoor installations with separations less than three feet to comply with UL 9540A fire test criteria. The minimum separation distances to adjacent units and walls established by UL 9540A fire testing will be reflected in the installation instructions.

The significance of the new UL 9540 requirements is as follows: Prior to the changes, there were no requirements in the first edition of UL 9540 that limited the maximum energy capacity of ESS units. For example, it was possible to certify (list) a 200 KWh unit with no UL 9540A fire testing. To approve an indoor installation of this larger ESS unit or an installation with separations less than three feet, the code official would have to ask for the UL 9540A test report, review the detailed findings, then determine if the proposed ESS size and spacing should be approved. This can be a very complicated process for many code authorities. 

With the new UL 9540 requirements in place, the process is simplified. ESS larger than 50 KWh or with separations less than three feet cannot be listed to the second edition of UL 9540 without complying with appropriate UL 9540A fire test performance requirements. To determine compliance with a specific installation’s size and separation requirements, code authorities simply have to confirm that the ESS is certified (listed) to the second edition UL 9540, and is installed in accordance with the listing and the manufacturer’s installation instructions, which includes minimum separation distances. The system designer and code authority still need to review the UL 9540A report to evaluate flammable gas release data. The data may be needed to design code-mandated explosion control systems. As can be seen, better aligning the UL 9540 second edition requirements with UL 9540A large-scale fire testing and code requirements will help the system designer and code authority determine code compliance.  

These are just some of the many changes that have been incorporated into the second edition of UL 9540 to address modern ESS installation code requirements. For additional information on the second edition UL 9540 requirements, please feel free to visit

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