NFPA 13 Commodity Classification: Calculating Warehouse Fire Load

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.

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:

• Metal parts (automotive spare parts warehouse)
• Glass bottles (even in cardboard boxes)
• Cement bags, brick
• Canned food (palletized)

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:

• Book / magazine warehouse
• Paper reels
• Wood furniture
• Cotton textile bobbins

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:

• Group A: Highest fire load. PE, PP, PS, ABS, nylon, urethane foam. Excludes PVC.
• Group B: Medium. Silicones, some fluoropolymers.
• Group C: Lowest. Fluorine-containing plastics, PVDF, melamine.

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.

Commodity-based design in SprinkCalc

Density selection for Class I-IV and Group A/B/C plastics, hydraulic calculation, and PDF report.

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