MEMBERS-ONLY FACT SHEET
UL 9540, UL 9540A, and Reduced Spacing for ESS Units
Last Updated April 2, 2025
The purpose of this fact sheet is to clarify the complexities surrounding reduced spacing between energy storage systems (ESS) units, and the current permitting challenges in certain jurisdictions. It examines the 2022 California Fire Code (CFC) and the 2022 California Residential Code (CRC), along with the requirements for UL 9540 certification and UL 9540A testing.
UL 9540 Certification Requirements:
All ESS, including residential, commercial, and industrial applications, must be certified to UL 9540 in accordance with 2022 CRC Section R328.2 and 2022 CFC Section 1207.3.1. UL 9540 is a system-level evaluation of ESS. It ensures that all components of the ESS are compatible with each other, as well as the environment in which they are installed. Individual components must also comply with their respective safety standards. For example, lithium battery cells must be certified to UL 1642; inverters and controllers must be certified to UL 1741. ESS can be configured as AC ESS where the battery, battery management system (BMS), and inverter are housed within a single manufactured package. It can also be configured as DC ESS where the battery and BMS are in a UL 9540 listed enclosure, but the inverter is a separate component. UL 9540 also requires large-scale fire testing for specific installation locations and applications.
UL 9540A Testing and Criteria:
UL 9540A testing becomes mandatory under the UL 9540 product listing for the following applications and scenarios:
Systems with increased energy capacities as required in codes and standards
Systems with decreased separation distances to adjacent ESS units, doors and windows or exposures.
Indoor systems
Systems for installation in dwelling units
The 2022 CRC Section 328.3.1 for one- and two-family homes and 2022 CFC Section 1207.5.1 for non Group R-3 and R-4 applications, establish prescriptive requirements for unit-to-unit spacing of 3ft. However, they also provide a pathway for installing more ESS units within a contained area using separation distances less than 3ft, contingent upon documented evidence of adequate safety as demonstrated by “large-scale fire testing” in accordance with 2022 CFC Section 1207.1.5, specifically UL 9540A.
UL 9540A assesses the thermal runaway propagation of a given battery technology and evaluates the fire and explosion hazard characteristics. The test exposes ESS to a thermal event and/or mechanical damage to determine its ability to contain and prevent the propagation of thermal runaway and fire. In order to pass the test, there must not be flaming outside the ESS enclosure. Testing does not rely on integral safety features and/or the battery management system. This performance data is then used to apply for exceptions in the fire code to reduce unit-to-unit setback restrictions.
Code Interpretation and Compliance:
The code is clear. UL 9540A is the “large-scale fire test” as defined by 2022 CFC Section 1207.1.5. UL 9540A also designates itself as the definitive large-scale fire test for battery ESS specifically intended to be used for validating safety and the basis for exceptions outlined in the fire code. Moreover, UL 9540A test reports are required as part of an ESS’s certification to UL 9540 edition 2 and later. (UL 9540 edition 2 is required by the 2022 CRC and CFC.) The minimum separation distances to adjacent units and allowable indoor/outdoor installation locations established by UL 9540A fire testing are then reflected in the manufacturer’s installation instructions (MII).[1] The MII is a controlled document under the supervision of the Nationally Recognized Testing Laboratory (NRTL). The NRTL is responsible for evaluating the UL 9540A test results and only under their oversight, the MII may reflect less than 3ft separation between ESS units as part of their UL 9540 listing.
For non-one-and two-family dwellings, 2022 CFC Section 1207.5.1 also allows ESS larger than 50 kWh provided they adhere to 2022 CFC Section 1207.1.5 large-scale fire testing and UL 9540A. In correlation with the code, the UL 9540A test is mandatory for UL 9540 edition 2 certification when a nonresidential ESS exceeds 50 kWh. Meaning, a nonresidential ESS surpassing 50 kWh cannot receive UL 9540 certification without NRTL evaluation of its performance under the UL 9540A fire test.
In summary, if an ESS is listed under UL 9540 edition 2 or later, any reduced unit-to-unit spacing and indoor allowance stipulated in its MII has undergone scrutiny by the NRTL and is sanctioned by its certification.
Permitting Challenges and Jurisdictional Variations:
Notwithstanding the aforementioned clarity, the 2022 CRC and 2022 CFC grant fire code officials the authority to review and approve UL 9540A test reports. This can make it difficult to obtain permits for installations with less than 3ft separation between ESS units.
Jurisdictions listed below have been identified as those posing significant challenges to reduced unit-to-unit spacing and non-residential ESS exceeding 50 kWh. Some fire code officials may require full copies of all UL test reports and/or a certified letter from a Fire Protection Engineer. Contractors should work with the product manufacturer to obtain approval for reduced unit-to-unit spacing.
Contra Costa County Fire Protection District
Los Angeles City Fire Department
Los Angeles County Fire Department
Palo Alto (City)
Riverside County Fire Department
San Jose Fire Department
Santa Clara County Fire Department
Sunnyvale (City)
Woodside Fire Protection District
If you have been rejected by an AHJ, please report it to permitting@calssa.org. We may be able to help, and either way, CALSSA will continue to serve as a clearinghouse of information for our membership.
Future Code Revisions and Advocacy Efforts:
Thanks to CALSSA’s advocacy, the 2025 California Residential Code, effective January 1, 2026, will reference “the ESS listing and the manufacturer’s installation instructions” for less than 3ft spacing between units. We anticipate this will simplify permitting for projects on most one- and two-family dwellings. Unfortunately, this is unlikely to help in jurisdictions where the fire code official references the California Fire Code for Group R-3 and R-4 occupancies.
CALSSA is working hard to make it easier for contractors to safely install ESS through our work in the I-Code development cycle for the 2027 International Fire Code and 2027 International Residential Code, as well as NFPA 855 and UL 9540. The process is long and arduous, and it may be years before we see more concrete results. In the meantime, contractors should exercise caution in proceeding with projects requiring reduced spacing in unfamiliar jurisdictions, and work with manufacturers for approvals.
Additional Resources:
SEAC Informational Bulletin on the UL 9540 Safety Standard and the UL 9540A Test Method
UL 9540 Second Edition: Understanding the Impacts of Requirement Changes
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[1] UL 9540 Energy Storage System (ESS) Requirements - Evolving to Meet Industry and Regulatory Needs, https://www.ul.com/news/ul-9540-energy-storage-system-ess-requirements-evolving-meet-industry-and-regulatory-needs