This article is educational, not engineering or code advice. Your authority having jurisdiction (AHJ) and engineer of record govern.

Battery fires are rare per gigawatt-hour deployed — and industry incident databases such as EPRI’s show failure rates falling sharply as designs have matured — but when a fire happens, it defines public perception for years. If you work in this industry, you owe it to yourself to understand the failure physics, the standards stack, and what changed in the 2026 edition of NFPA 855. It changed a lot.

To ground the design discussion, it helps to picture the hardware. This BESS Container Structure visual shows how cells, modules, racks, and the enclosure fit together — the very geometry that decides whether vent gas can accumulate and where explosion control has to live.

This article is the anchor of the BESS fire-safety hub — the standards, the failure physics, site design, and the permitting side, each linked out in one place.

Thermal runaway: the failure that matters

Thermal runaway is a self-accelerating exothermic failure inside a lithium-ion cell. The simplified chain: something drives cell temperature up (an internal defect, overcharge, external heating, mechanical damage) → protective interfaces begin breaking down at roughly 90–120 °C → the separator fails → cathode decomposition adds heat and, in nickel chemistries, releases oxygen → the cell vents hot flammable gas: hydrogen, carbon monoxide, hydrocarbons, electrolyte vapor.

Two consequences follow, and they are different problems:

  1. Fire — if the vent gas ignites immediately.
  2. Explosion — if the gas accumulates unburned inside an enclosure and then finds an ignition source.

The second one is what injures people. The 2019 McMicken incident in Arizona is the canonical case: a cell failure filled a container with flammable gas that had not ignited, and when firefighters opened the door, the resulting deflagration seriously injured responding firefighters. Nearly every gas-detection, ventilation, and explosion-control requirement in today’s codes traces back to that event.

A hard-won nuance: the industry’s shift to LFP improved fire behavior but did not solve the explosion problem. LFP vent gas is hydrogen-rich. In many ways the modern design challenge is less “how do we stop fire spreading” and more “how do we prevent a flammable gas volume from ever assembling.”

Moss Landing: the incident that reset public perception

On January 16, 2025, the Moss Landing 300 battery building at Vistra’s site in Monterey County, California caught fire. Facts worth knowing, drawn from the US EPA’s response documentation and subsequent reporting:

  • The facility was a 300 MW system holding about 100,000 lithium-ion batteries (EPA’s figure) of NMC chemistry, installed indoors in a former turbine-hall building — an architecture the industry has moved decisively away from.
  • EPA estimates roughly 55% of the batteries were damaged; independent studies put the range at 55–80%. The site flared up again on February 18, 2025.
  • A peer-reviewed study later estimated on the order of 25 metric tons of nickel, manganese, and cobalt deposited across surrounding wetlands.
  • Cleanup is still running as I write this: EPA-supervised battery removal is expected to continue into late 2026, with Phase 2 demolition of the burned building starting mid-2026.
  • The cause remains under investigation at the time of writing. Anyone telling you definitively why it happened is ahead of the evidence.

The industry lesson is not “batteries are dangerous.” It’s that this specific architecture — hundreds of MW of NMC sharing one indoor air volume — concentrates consequences. A CPUC survey found the majority of California’s grid-scale fleet is the opposite design: outdoor, dispersed containers, LFP chemistry. Same industry, very different risk profile. The regulatory aftermath was real either way: California’s CPUC adopted General Order 167-C, taking oversight of BESS operations and maintenance and requiring emergency response plans to be filed with local fire departments.

The standards stack, in one map

  • UL 1973 — safety listing for the battery system (racks and modules).
  • UL 9540 — safety listing for the complete ESS (battery + PCS + controls).
  • UL 9540A — not a pass/fail listing but a test method that characterizes thermal runaway propagation at cell, module, unit, and installation level. Its data feeds everything else. A new edition was released in the same cycle as NFPA 855 (2026), with large-scale fire testing requirements and a greater focus on system-level safety.
  • NFPA 855 — the installation standard: where systems can go, how far apart, and what protection they need. It reaches projects through the fire codes (IFC, NFPA 1) as jurisdictions adopt them.
  • NFPA 68 / NFPA 69 — explosion protection: deflagration venting (68) and explosion prevention through gas control (69).
  • Internationally, the IEC 62933-5 series and Canada’s CSA C800 play parallel roles.

What changed in NFPA 855 (2026 edition)

The 2026 edition was released in September 2025, and it’s the most consequential revision yet:

  1. Hazard Mitigation Analysis becomes the default. Earlier editions required an HMA only when a project exceeded certain stored-energy thresholds; the 2026 edition removes that threshold table entirely. An HMA — a formal engineering risk analysis covering thermal runaway initiation, propagation, and consequences — is now the expected baseline for essentially all in-scope ESS installations, with exemptions reserved for well-characterized chemistries such as lead-acid and aqueous nickel-based systems.
  2. Large-scale fire testing (LSFT) alongside UL 9540A. The 2023 edition allowed testing to stop early if propagation was contained at module level, leaving a data gap for large installations. The 2026 edition explicitly expects large-scale fire testing to demonstrate that systems can withstand and contain severe thermal runaway events.
  3. Explosion control shifts from venting to prevention. Installations must now incorporate explosion control designed per NFPA 69 — active prevention, including combustible concentration reduction (CCR) systems that keep gas concentrations below ignitable levels — or a performance-based alternative backed by installation-level testing. Deflagration venting per NFPA 68 is no longer accepted as the primary strategy on its own. It's a philosophical shift: don't manage the explosion, prevent the gas cloud.
  4. Thermal runaway propagation prevention (TRPP) is defined as an active method with new requirements, including documenting piping compliance (ASME B31.1/B31.3) as part of the UL 9540 listing.
  5. Detection options broadened. Early fire detection may now use smoke detection, thermal-imaging detection, or radiant-energy detection installed per NFPA 72 — a more flexible menu than the previous edition allowed.
  6. Emergency response gets teeth. A compliant Emergency Response Plan must address mitigation, preparedness, response, and recovery, be reviewed annually, and include annual refresher training with the AHJ notified.
  7. The scope table was rebuilt. Chemistries are now listed explicitly (rather than lumped into an "other technologies" bucket), which reduces ambiguity for newer entrants like sodium-ion and gives AHJs clearer thresholds.

Worth watching: NFPA is developing NFPA 800, a Battery Safety Code intended to cover battery hazards across the whole lifecycle; it may eventually absorb some of what NFPA 855 covers today.

Adoption reality check: most jurisdictions still enforce codes based on the 2020 or 2023 editions via IFC 2021/2024. California adopted fire-code updates based on NFPA 855 (2023) effective January 1, 2026, and the State Fire Marshal has signaled early adoption of the 2026 edition targeted for July 2027. Always confirm which edition your AHJ actually enforces before you design to one.

What good design looks like in practice

Across projects, the pattern that satisfies both the codes and common sense:

  • Outdoor, dispersed, non-occupied enclosures with propagation resistance demonstrated by unit-level UL 9540A data and large-scale fire testing.
  • Off-gas detection tied to system shutdown, plus combustible-gas concentration control so a flammable volume never assembles.
  • Spacing and site layout consistent with the tested listing, with clear fire-apparatus access and water supply planned for exposure protection.
  • A real emergency response plan, walked through with the local fire department before energization — not a PDF that lives in a drawer.
  • Response tactics that accept a burning enclosure may be allowed to burn out while exposures are protected — a decision that always belongs to the fire service on scene.

FAQ

Are LFP batteries safe from thermal runaway? Safer, not safe. Harder to ignite and less energetic in failure — but hydrogen-rich vent gas keeps explosion prevention the central design problem.

Does NFPA 855 (2026) apply to my project? That depends on what your jurisdiction has adopted. Many AHJs still enforce earlier editions through their fire codes. Ask first — designing to the newest edition is often wise anyway, since it’s where regulation is heading.

Should communities fear BESS projects? The honest answer: modern outdoor LFP container plants designed to current codes carry a very different risk profile than the legacy indoor NMC design that burned at Moss Landing. Scrutiny is fair; conflation is not.

Fire safety, UL 9540A test data, and how safety flows into permitting and insurance get full modules in my Grid-Scale BESS: Complete Guide.