Class IV is the commodity with ‘some’ plastic: the Group A plastic share has passed the Class III limit but is not yet high enough to make it a plastic commodity outright.
Quick reference
| Item | Value |
|---|---|
| NFPA 13 class | Class IV |
| Group A plastic content | 5–15% by weight or 5–25% by volume |
| Other routes | Group B plastic products; free-flowing Group A plastics |
| FM Global DS 8-1 | Class 4 (definitions not identical) |
| Typical protection | CMSA or ESFR; high density with CMDA; in-rack where needed |
What it is
Under NFPA 13 a commodity is Class IV if any of three conditions applies: (1) the product is made partly or wholly of Group B plastics; (2) it consists of free-flowing Group A plastic material such as pellets, flakes or powder; (3) the product or its packaging contains 5–15% Group A plastics by weight or 5–25% by volume. Because plastics release far more heat than cellulosic materials, even these modest proportions noticeably speed up fire growth.
Typical examples
- Small appliances with plastic housings (cartoned)
- Computers and electronics with plastic parts (cartoned)
- Nylon or silicone rubber products (Group B)
- PE/PP pellets or flakes in cartons (free-flowing)
- Food and cosmetics with plastic inner packaging
How packaging and plastics change the class
Both the weight and volume proportions are checked, and whichever is worse governs. Foam plastic is light, so it forms a small share by weight but a large one by volume; with foam-packed goods the volume proportion usually decides. Once the proportion passes 15% by weight or 25% by volume, the commodity is treated as a Group A plastic. Free-flowing plastics are the exception: pellets and powders tend to flow and smother the flame, so even as Group A they count as Class IV.
Protection approach
Class IV is among the heaviest classes still protectable with CMDA, and the required density rises quickly with height. That is why CMSA or ESFR are common on tall racks. Where ESFR can be used, in-rack sprinklers are often unnecessary, but ceiling height limits and obstruction rules are strict. In-rack levels and horizontal spacing are taken from the standard according to rack type. Before design, settle whether the commodity is truly Class IV or Group A; the difference changes water demand considerably.
FM Global equivalent
FM Global DS 8-1 has a Class 4, but FM's treatment of plastic proportions and its examples do not match NFPA exactly. On FM-approved projects the class is set separately using DS 8-1 and the mixed commodity rules.
The commodity class depends on what goes into the warehouse today. The commodity and storage arrangement assumed in the design report should be recorded in writing and rechecked whenever operations change. For the overall framework see NFPA 13 commodity classification, and for a quick check on whether ESFR can be used, the ESFR suitability checker.
Common site mistakes
- Checking only the weight proportion and missing the volume share of foam packaging.
- Confusing Group B plastics (e.g. nylon) with Group A, or the reverse.
- Leaving uncartoned or open-crated plastic goods at Class IV.
- Averaging the proportion across the warehouse instead of per pallet load.
- Not accounting for the added effect of plastic pallets.
Frequently Asked Questions
Where is the line between Class IV and Group A plastics?
Once Group A plastics exceed 15% by weight or 25% by volume, the commodity is no longer Class IV and is treated as a Group A plastic.
Why are plastic pellets Class IV?
Free-flowing plastics flow during a fire, cover the burning surface and limit fire growth. NFPA 13 therefore classes free-flowing Group A plastics as Class IV.
What class is a nylon product?
Nylon is a Group B plastic; products made of Group B plastics are treated as Class IV commodities.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems (current edition) · FM Global Property Loss Prevention Data Sheets (FM Global DS 8-1). This page summarises the definitions in our own words; for design densities, areas and height limits the edition in force and the decision of the authority having jurisdiction govern.