If you're protecting a warehouse with sprinklers, the system must be sized to what's stored inside. NFPA 13 uses the commodity classification system for this. Wrong classification means undersized density — in a real fire, the system can't control the load. Here's Class I-IV and Group A/B/C plastics, with real warehouse examples.

Short answer: NFPA 13 sorts stored goods into 4 commodity classes plus 3 plastic groups, and each carries its own design density. Class I is noncombustible material, or a small amount of combustible in noncombustible packaging; Class II is noncombustible product in slatted wooden crates, solid wood boxes or multiple-layered corrugated cartons (a single-layer corrugated carton keeps it Class I); Class III is product made of wood, paper, natural fibres or Group C plastics, with up to 5% by weight nonexpanded (or 5% by volume expanded) Group A plastic; Class IV is product made partly or wholly of Group B plastic, free-flowing Group A plastic, or containing 5–15% by weight nonexpanded (5–25% by volume expanded) Group A plastic. Plastics split into Group A (PE, PP, PS, ABS, PET, nylon, polyurethane foam — highest fire load), Group B (chloroprene rubber, silicone rubber, ECTFE/ETFE/FEP fluoroplastics) and Group C (PVC up to 20% plasticiser, PVDC, PVDF, PTFE, PCTFE, melamine, phenolic, urea). In mixed storage the design must address the worst class present, never an average.
Class Definition Warehouse examples
Class I Noncombustible material, or a small amount of combustible in noncombustible packaging — the lowest fire load Metal parts (automotive spare parts warehouse), glass bottles (even in cardboard boxes), cement bags and brick, palletized canned food
Class II Noncombustible product in slatted wooden crates, solid wood boxes or multiple-layered corrugated cartons (NFPA 13-2025 20.4.2) Glass bottles in a wood crate, metal parts in a multiple-layered corrugated carton
Class III Products of wood, paper, natural fibres or Group C plastics, permitted to contain up to 5% by weight nonexpanded or 5% by volume expanded Group A plastic (20.4.3) Book / magazine warehouse, paper reels, wood furniture, cotton textile bobbins
Class IV Group B plastic products, free-flowing Group A plastics, or products with 5–15% by weight nonexpanded / 5–25% by volume expanded Group A plastic (20.4.4.1) — the highest combustibility of the four classes Heavily packaged electronics, cosmetic bottles

Why Classify?

Toilet paper and rubber tires on a 5-meter rack aren't the same fire load. Toilet paper burns fast but light; rubber starts slower but releases enormous heat and toxic smoke. NFPA 13 groups these into 4 classes plus 3 plastic groups, each with its own design density.

Class I: Lowest Fire Load

Noncombustible materials or small amounts of combustible in noncombustible packaging:

Class II: Lightly Combustible

Noncombustible contents in slatted wooden crates, solid wood boxes or multiple-layered corrugated cartons (20.4.2). Examples: glass bottles in a wood crate, metal parts in a multiple-layered corrugated carton. A noncombustible product in a single-layer corrugated carton stays Class I (20.4.1).

Class III: Combustible Goods

Products of wood, paper, natural fibres or Group C plastics (20.4.3.1). May contain up to 5% by weight nonexpanded or 5% by volume expanded Group A plastic (20.4.3.2):

Class IV: High Combustibility

Products made partly or wholly of Group B plastic, free-flowing Group A plastic, or containing 5–15% by weight nonexpanded / 5–25% by volume expanded Group A plastic (20.4.4.1). Heavily packaged electronics and cosmetic bottles usually fit here.

Plastic Groups (A, B, C)

NFPA 13-2025 (20.4.5–20.4.7) splits plastics, elastomers and rubber into three groups:

PVC is interesting: halogen content makes rigid and lightly plasticised PVC self-extinguishing — hence Group C. Flexible PVC with more than 20% plasticiser (cable sheathing, hoses, some films) is Group A, so always ask for the plasticiser content. But it releases toxic HCl when burning — a separate design concern.

Common Field Mistakes

  1. Averaging mixed storage: Design must address the highest class present (20.4.15.2); averaging leaves the system undersized.
  2. Ignoring plastic packaging: If Group A plastic, packaging included, exceeds 5% by weight of a Class III commodity, it becomes Class IV.
  3. Wrong storage height: Above 12 ft (3.7 m) the storage chapters apply; storage up to 3.7 m may qualify as miscellaneous storage under conditions (4.3.1.4.1).

Conclusion

Commodity classification is the bedrock of sprinkler design. Which density? Which area? Which sprinkler type? Answers depend on knowing what sits on the rack. When tenants change and contents shift, the sprinkler system needs re-evaluation.

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Sources & Further Reading

Core reference: NFPA 13-2025 Chapter 20 (20.4 Commodity Classes). Original NFPA post: NFPA Today - Commodity Classifications.