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ISO adds Class L fire rating for lithium-ion as New York enforces 1 kWh storage rule

ISO adds Class L fire rating for lithium-ion as New York enforces 1 kWh storage rule

Rule Changes

New international fire classification puts lithium-ion in its own category; New York City requires FDNY approval above 1 kWh and reports a 41% rise in e-bike battery fires this year.

October 21st, 2026: Large residential battery system tested in New York

Overview

Updated Oct 21

Lithium-ion battery fires now have their own fire class, after ISO published the third edition of ISO 3941 in January 2026 adding Class L for cells and batteries that contain no metallic lithium. New York City had already set a lower regulatory bar: its amended NFPA 855 requires FDNY approval for lithium-ion storage above 1 kilowatt-hour, against 70 kilowatt-hours for lead-acid.

The fire problem is getting worse. As of mid-October, e-bike battery fires in the city were up 41% year-over-year, and 61% of battery fires happened outside homes or businesses, according to Streetsblog. Five people had died in battery fires this year.

Why it matters

Lithium-ion batteries now have their own fire class, and New York City's 1 kWh rule puts fire department review on small storage systems.

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Key Indicators

1 kWh
NYC lithium-ion ESS approval threshold
Energy storage systems above this capacity require FDNY design approval and testing.
70 kWh
NYC lead-acid ESS approval threshold
Lead-acid systems get 70 times more capacity before the same regulatory requirements kick in.
41%
Rise in NYC e-bike battery fires, 2026
E-bike battery fires rose 41% year-over-year as of mid-October 2026; 61% occurred outside homes or businesses.
5
Deaths in NYC battery fires, 2026
Five people had died in battery fires this year as of mid-October 2026, according to Streetsblog.
277
FDNY lithium-ion battery fires in 2024
Fires rose from 268 in 2023 to 277 in 2024; deaths fell from 18 to 6. By mid-June 2025, FDNY had logged 110 fires and 25 injuries.
60%
2023 lithium-ion fires not charging
FDNY analysis found nearly 60% of reviewed 2023 fires occurred while batteries were not plugged in.

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Organizations Involved

Allstate Sprinkler
Allstate Sprinkler
Fire protection contractor
Published September 2026 analysis linking ISO Class L to NYC's storage threshold

Bronx-based fire suppression inspection, testing, and installation contractor.
International Organization for Standardization (ISO)
International Organization for Standardization (ISO)
International standards body
Published ISO 3941 third edition adding Class L in January 2026; no later revision reported

Global standard-setting organization that publishes fire classification norms.
Fire Department of New York
Fire Department of New York
Municipal fire department
Reports a 41% rise in e-bike battery fires in 2026; most occurred outside homes or businesses as of mid-October

Enforces New York City fire codes and issues Certificates of Approval for energy storage.
National Fire Protection Association (NFPA)
National Fire Protection Association (NFPA)
Standards organization
Does not recognize ISO Class L in US-adopted codes

Publisher of fire and life safety codes, including NFPA 855 for energy storage.
Exponent
Exponent
Engineering and scientific consulting firm
Published guidance interpreting ISO Class L for US stakeholders

Engineering firm that analyzed the implications of ISO Class L for facility operators and insurers.

Timeline

2023 October 2026

9 events Latest: October 21st, 2026 · 3 weeks ago
Tap a bar to jump to that date

  1. Large residential battery system tested in New York

    Latest Development

    A 3.75 MWh lithium-ion system at the Peace Bridge Place affordable housing complex is among the largest in a U.S. residential building, manufacturer Viridi and developer Good Carbon said.

  2. E-bike battery fires up 41%; most occur outside homes

    Incident Data

    Streetsblog, citing FDNY data, reported e-bike battery fires up 41% year-over-year, with 61% of battery fires non-structural and five deaths.

  3. No later ISO 3941 revision reported after third edition

    Standards

    Searches found no later revision of ISO 3941 beyond the January 2026 third edition, which added Class L for lithium-ion fires.

  4. Allstate Sprinkler links ISO Class L to NYC threshold

    Analysis

    The Bronx contractor publishes a study connecting the new fire class to New York's 1 kWh storage rule and two years of FDNY incident data.

  5. ISO publishes Class L fire classification

    Standards

    ISO 3941 third edition adds Class L for lithium-ion cell and battery fires without metallic lithium, citing thermal runaway and re-ignition hazards.

  6. New York state cord labeling rule takes effect

    Regulation

    Charging cords for micromobility devices, limited-use motorcycles, and e-bikes must carry warning tags in English and Spanish.

  7. FDNY tallies 110 battery fires by mid-2025

    Incident Data

    The department counted 110 lithium-ion fires and 25 injured residents year to date, with analysis showing most 2023 fires occurred while not charging.

  8. FDNY records 277 lithium-ion fires, 6 deaths

    Incident Data

    Fires rose to 277 while deaths fell 66% from 18 to 6 across the year.

  9. FDNY records 268 lithium-ion battery fires

    Incident Data

    The department logged 268 fires and 18 deaths across the year, the peak of recorded mortality.

Scenarios

1

NFPA brings Class L into US fire classification

Possible Resolves by End of 2028

Discussed by: Exponent, whose guidance notes NFPA does not recognize the ISO Class L designation

As lithium-ion storage spreads, NFPA faces pressure to adopt Class L into NFPA 10 or related suppression standards. Fire departments and insurers would then specify Class L-rated strategies. Adoption would require a formal code revision cycle, which NFPA typically runs on multi-year schedules.

2

NYC storage compliance wave follows 1 kWh rule

Possible Resolves by Q2 2027

Discussed by: Allstate Sprinkler, which recommends documented hazard assessments and FDNY approval for ESS above the threshold

Property owners with indoor lithium-ion storage above 1 kWh seek FDNY Certificates of Approval, UL 9540 listings, and large-scale fire testing. The rule is already in city law, so the open question is how aggressively FDNY enforces it and how many approvals it issues. Owners who do not comply face FDNY action.

3

Other US cities copy New York's low storage threshold

Possible Resolves by End of 2028

Discussed by: Trade press including Building Design, which notes the shift toward lithium-specific fire rules

Cities and states amend fire and building codes to adopt lithium-ion storage thresholds well below lead-acid, following New York's 70-fold gap. The ISO Class L designation gives jurisdictions a classification hook on which to base such rules, and FDNY's incident data provides the justification.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

Established classification

NFPA Class D for combustible metals

Fire classification systems long ago added a dedicated class for combustible metals like magnesium, titanium, and sodium, which burn at extreme temperatures and react violently with water. These fires require dry powder extinguishing agents, not conventional water or foam.

Then

Suppression equipment was rated and labeled specifically for metal fires, and responders were trained to avoid water on these hazards.

Now

Class D became a standing precedent that certain chemical hazards need their own fire class and dedicated suppression methods.

Why this matters now

Class L follows the same logic: lithium-ion fires resist conventional suppression, so regulators now treat them as a distinct hazard rather than an ordinary combustible.

February-March 2022

Felicity Ace cargo ship fire (2022)

The car carrier Felicity Ace caught fire in the Atlantic carrying roughly 4,000 vehicles, including electric cars, and burned for days before sinking on March 1. Salvage crews could not board for weeks due to re-ignition and heat.

Then

The ship sank with its cargo, and insurers faced billions in losses from the vehicles and vessel.

Now

The incident pushed shipping and fire authorities to treat lithium-ion battery fires as a re-ignition risk requiring specialized suppression and extended monitoring.

Why this matters now

It showed the same hazards ISO cites in Class L: rapid growth, vented gases, cascading thermal runaway, and re-ignition after apparent control.

2019

NFPA 855 first edition (2019)

NFPA published the first edition of NFPA 855, the Standard for the Installation of Stationary Energy Storage Systems, setting spacing, ventilation, and suppression requirements for battery storage. It became the framework most US jurisdictions adopted for ESS.

Then

Jurisdictions adopted NFPA 855 as the baseline for battery storage approvals, giving fire departments a consistent review standard.

Now

New York City amended its adoption of NFPA 855 to set the 1 kWh lithium-ion threshold, well below the standard's typical capacity triggers.

Why this matters now

NFPA 855 is the exact framework New York City modified to impose its strict lithium-ion threshold.

Sources

(8)
+2
PR Newswire Business Insider Markets Lexology Exponent via PMRA Building Design ANPI Streetsblog New York City Los Angeles Times