Classification of Lab units

How to Classify Laboratory Fire Hazard Levels Under NFPA 45 (2024)

Correctly classifying a laboratory under NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals (2024) drives allowable quantities of ignitible liquids, storage layout, ventilation, and protection features. The classification is based on ignitible liquids only (flammable and combustible). Gases and reactive solids fall under other standards.

What the standard does—at a glance

NFPA 45 Chapter 4 defines four Laboratory Unit Fire Hazard Classes determined by the quantity of ignitible liquids permitted by Tables 9.1.1(a) (SI) and 9.1.1(b) (US):

• Class A (high): highest quantities permitted.
• Class B (moderate).
• Class C (low).
• Class D (minimal).

Key US limits (Table 9.1.1(b))

• Class A: Class I liquids—20 gal per 100 ft²; total I/II/IIIA—40 gal per 100 ft²; lab maxima 480/1,600 gal.
• Class B: 10 / 20 gal per 100 ft²; maxima 480/800 gal.
• Class C: 4 / 8 gal per 100 ft²; maxima 300/400 gal.
• Class D: 2 / 2 gal per 100 ft²; maxima 150/150 gal.
  (Approximate SI: per 9.3 m² and liter maxima appear in Table 9.1.1(a).)

Floor level reductions: Reduce Class B limits by 50% above the 3rd floor; reduce Class C/D by 25% on 4th–6th floors and 50% above 6th.

Special occupancies

• Educational/Instructional labs: must be Class C or D. If designated educational, limit to 50% of Class C quantities.
• Health-care labs: Class C or D only; Class C also limited to 50% of normal Class C quantities.

What counts as “in use” vs “in storage”

• In storage: in a safety can or approved storage cabinet and not physically connected to equipment.
• In use: physically connected to equipment or handled in a manner that supports an active process.
• Open systems: the maximum in use is limited to 10% of the table quantities.
• PLDCs (pressurized liquid dispensing containers): always considered “in use.”
• Waste ignitible liquids count toward totals.

How to classify—five practical steps

Try the calculator

1. Map the laboratory unit. Confirm gross area and whether any inside liquid storage room (NFPA 30) is excluded from unit totals.
2. Inventory ignitible liquids. Split by class (I, II, IIIA), and by in use vs storage (include waste; identify open systems).
3. Apply Table 9.1.1(b) (US) or (a) (SI). Calculate per 100 ft² (9.3 m²) and per laboratory unit amounts for in use and in use + storage.
4. Apply modifiers. Floor-level reductions; educational/health-care 50% reductions where applicable.
5. Assign Class A–D to the most restrictive result and document the basis (date, inventory, assumptions).

example:
A 1,200 ft² teaching lab stores 120 gal Class I and uses 6 gal (open system). “In use” cap is 10% of table values, so 6 gal must be below 10% of Class C limits. Apply 50% educational reduction and check per-100 ft² and per-lab maxima. Result: Class D after reductions. 

Common pitfalls to avoid

• Ignoring waste cans: solvent waste can push you over the class limit.
• Overlooking open-system 10% cap: especially with bench-scale distillations or reactors.
• Counting refrigerator contents incorrectly: Class I liquids must be in closed containers; verify storage design.
• Not recalculating after floor moves: moving above the 3rd floor can change your class.
• Container size caps: see Table 9.1.2 for maximum capacities (e.g., safety cans, glass allowances under specific conditions).

Compliance note: Final design and operations must follow the Authority Having Jurisdiction (AHJ) and the latest NFPA/OSHA/CCPS guidance. Site policies may be stricter.

Q&A

How do educational labs differ from research labs under NFPA 45?
Educational/Instructional labs must be Class C or D; educational labs are capped at 50% of Class C quantities.

Do gases affect my lab’s NFPA 45 class?
No. Classification is based on ignitible liquids only. Manage gases via other sections of the standard and other standards (e.g., NFPA 55). Exception: liquefied flammable gases (LFGs) (e.g., propane, some olefins) are counted as Class I flammable liquids for the purpose of determining the lab class/

What happens if a lab spans multiple floors?
Class is set per laboratory unit. If the unit is above certain floors, apply the reduction factors before determining the class.

Are PLDCs treated differently?
Yes. Pressurized liquid dispensing containers are always “in use” and must meet listed/labeled and capacity requirements. 

How often should I re-classify?
At least annually, and whenever inventory, process, or location changes affect ignitible-liquid totals.