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 wraps noncombustible contents in a combustible wood crate or cardboard; Class III is predominantly cellulosic — wood, paper, natural-fibre textile, leather — with up to 5% Group A plastic; Class IV is a Class III commodity carrying Group B plastic or some Group A plastic. Plastics split into Group A (PE, PP, PS, ABS, nylon, urethane foam — highest fire load), Group B (silicones, some fluoropolymers) and Group C (PVC, PVDF, melamine and other fluorine-containing plastics). 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 contents in combustible wood crates or very combustible cardboard Glass bottles in a wood crate, metal parts in a corrugated carton
Class III Predominantly cellulosic goods — wood, paper, natural-fibre textile, leather — that may contain up to 5% Group A plastic Book / magazine warehouse, paper reels, wood furniture, cotton textile bobbins
Class IV Class III commodity containing Group B plastic or some Group A plastic — 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

Combustible wood crates or very combustible cardboard with noncombustible contents. Examples: glass bottles in wood crate, metal parts in corrugated carton.

Class III: Combustible Goods

Wood, paper, natural-fiber textile, leather — predominantly cellulosic. May contain up to 5% Group A plastic:

Class IV: High Combustibility

Class III commodity containing Group B plastic or some Group A plastic. Heavily packaged electronics and cosmetic bottles fit here.

Plastic Groups (A, B, C)

NFPA 13 splits plastics and elastomers into three groups:

PVC is interesting: halogen content makes it self-extinguishing — hence Group C. But it releases toxic HCl when burning — a separate design concern.

Common Field Mistakes

  1. Assuming mixed storage: Design must address the worst class; averaging leaves the system undersized.
  2. Ignoring plastic packaging: PE shrink wrap around Class III can push it to Class IV.
  3. Wrong rack height: Above 3.6 m high-piled storage rules engage.

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 Chapter 5 & 20. Original NFPA post: NFPA Today - Commodity Classifications.

FS

Fatih Selvi

Mechanical engineer and software developer. MEP and fire protection experience.