How Do You Classify a Mixed Commodity Under NFPA 13?

A mixed commodity is classified under National Fire Protection Association (NFPA) 13 by working out how much Group A plastic a unit load contains, by weight and by volume, and then applying three breakpoints: 5 percent, 15 percent by weight and 25 percent by volume. Plastic at or below 5 percent is ignored and the load keeps its Class I to IV rating. Plastic above 5 percent and up to 15 percent by weight (nonexpanded) or 25 percent by volume (expanded) moves the load to Class IV. Anything above those limits makes the load a Group A plastic commodity, and the whole storage array is then protected for the highest class it holds unless the higher-hazard loads are kept to the small quantities or the segregated areas that NFPA 13 allows.

The calculation is the same in every state DGA works in; only the NFPA 13 edition changes. NFPA moved commodity classification from Chapter 5 to Chapter 20 in the 2019 edition and renumbered parts of it again in 2022, so the section numbers below are given for 2022 with the 2019 and 2025 numbers noted where they differ. This article covers the inputs, the formula, two worked examples, the errors that most often produce the wrong class, and the cases where a licensed Professional Engineer (PE) has to make the call.

What Inputs the Mixed Commodity Calculation Needs

The calculation is only as good as the product data behind it. NFPA 13 classifies the unit load as stored, which means the product, its packaging, any inserts or trays, the stretch wrap and the pallet all count. The table lists each input, its units and where a design team normally gets it.

Two points about the inputs matter more than the rest. Nonexpanded plastics are judged by weight and expanded plastics by volume, so both sets of numbers have to be collected for a load that contains both. And the share of the array is a separate input from the content of any one load, because NFPA 13 applies a second test to how the loads are mixed on the floor.

How the Calculation Works, Step by Step

NFPA 13 (2022) states the plastic content limits in the Class IV and Group A definitions (Sections 20.4.4 and 20.4.5), and Figure 20.4.3.3(a) covers loads that hold both expanded and nonexpanded plastic. The result is one of three outcomes: the base class (Class I, II, III or IV), Class IV, or Group A. The method below reproduces that decision in four steps without reproducing the figure.

Step 1: Define the unit load. Take the load as it sits in the rack or pile: product, carton, inserts, film and pallet. Record its total weight in lb and its total volume in cu ft from the outside dimensions of the wrapped load, pallet included.

Step 2: Total the plastic. Add up the nonexpanded plastic by weight and the expanded plastic by volume. Group B plastics (chloroprene rubber, fluoroplastics and silicone rubber) and Group C plastics (for example, polyvinyl chloride, PVC) do not count toward the Group A total. Nylon (nylon 6 and nylon 6/6) is also Group B, while PVC with more than 20 percent plasticizer is Group A; the commodity classification guide lists which plastics fall in each group.

Step 3: Apply the breakpoints. Compute both percentages. If both are at or below 5 percent, the plastic is ignored and the load keeps the class of its base product. If either exceeds 5 percent but the weight stays at or below 15 percent and the volume at or below 25 percent, the load is Class IV. If either limit is exceeded, the load is a Group A plastic commodity, cartoned or exposed and expanded or nonexpanded according to how it is packaged.

Then adjust for the pallet under NFPA 13 (2022) Section 20.3: an unreinforced plastic pallet raises a Class I to IV load by one class, and a reinforced plastic pallet by two, unless the pallet is listed for the purpose.

Step 4: Apply the array rule. NFPA 13 (2022) Section 20.4.13 requires a storage area holding more than one commodity class to be protected for the highest class and arrangement present, and it does not allow averaging. The 2019 edition carries the same rule in Section 20.4.14, the 2025 edition in Section 20.4.15, and the 2016 edition in Chapter 5. Two allowances relax this: up to 10 pallet loads of a higher-hazard commodity per 40,000 sq ft may be dispersed in a lower-hazard area (Section 20.4.13.3), and a higher-hazard commodity may be segregated in a designated area that is protected for its class (Section 20.4.13.4). The higher design extends 15 ft beyond that area unless a draft curtain, barrier or partition separates it (Section 19.1.2).

Worked Example 1: A Pallet of Cartoned Canned Goods

A food distributor stores canned vegetables in corrugated cartons on wood pallets. Each pallet load measures 48 in by 40 in by 48 in high including the pallet, which is 92,160 cu in or 53.3 cu ft, and weighs 1,800 lb. The load carries 12 lb of nonexpanded plastic: stretch film, polyester strapping and thin polyethylene terephthalate (PET) trays inside each carton.

Plastic by weight is 12 lb divided by 1,800 lb, or 0.67 percent. Plastic by volume uses the density of the film and PET, about 57 lb per cu ft for polyethylene film and 85 lb per cu ft for PET, so 12 lb of mixed plastic occupies roughly 0.2 cu ft. Divided by the 53.3 cu ft load, that is about 0.4 percent by volume. Both numbers are under 5 percent, so the plastic is ignored and the load keeps the class of cartoned metal cans with noncombustible contents, which is Class I.

The result changes if the trays change. Suppose the same distributor switches to expanded polystyrene (EPS) trays, 40 in by 48 in by 1 in thick, with 10 trays per load. The EPS volume is 19,200 cu in or 11.1 cu ft, which is 20.8 percent of the 53.3 cu ft load. At about 1 lb per cu ft the EPS weighs only 11 lb, 0.6 percent by weight, so a weight-only check would have missed it.

The load is now Class IV, and if the trays were thicker or the cartons shorter it would cross 25 percent and become a Group A expanded plastic commodity.

Worked Example 2: A Mixed Array with 20 Percent Group A Positions

A 40,000 sq ft rack storage area holds 1,000 pallet positions. The owner’s inventory shows that 80 percent of positions hold Class II cartoned goods and 20 percent hold cartoned nonexpanded Group A plastics (small appliances with plastic housings, cartoned without foam inserts), and the plastics are slotted wherever space is free.

The array rule in NFPA 13 (2022) Section 20.4.13 asks two questions. First, how many higher-hazard loads are present per 40,000 sq ft: here 200, far above the 10 dispersed loads that the standard allows in a lower-hazard area (Section 20.4.13.3). Second, are the higher-hazard loads confined to a designated area: no, they are scattered. The sprinkler design for the entire 40,000 sq ft area must therefore be based on cartoned nonexpanded Group A plastics at the stored height, which is a different and heavier design than Class II.

The same warehouse has two ways to lower the design. The owner can dedicate a block of racks to the plastics and protect that block, plus 15 ft beyond it (Section 19.1.2), for Group A, leaving the rest of the area on a Class II design. Or the owner can select a ceiling-only scheme that covers both commodities anyway; many Early Suppression Fast Response (ESFR) and Control Mode Specific Application (CMSA) designs protect cartoned Group A plastics and Class I to IV with the same design, and the ESFR or CMSA comparison for warehouses explains when that is the simpler route. The high-piled storage plan has to show the chosen arrangement; the high-piled storage plan article describes what the fire code official expects on that sheet.

DGA classifies commodities and prepares high-piled storage plans and sprinkler design criteria across New York, New Jersey, Pennsylvania, Maryland and Delaware. Project briefs get confirmation and preliminary code feedback within one business day. See where DGA works →

Which NFPA 13 Edition Applies in Each State

The breakpoints and the array rule have not changed in substance since the 2016 edition, but the section numbers have. A commodity letter or a high-piled storage plan should cite the edition the authority having jurisdiction (AHJ) has adopted, which is set by the building code in force.

Editions as of October 2026. Local amendments are not shown. The International Fire Code (IFC) Chapter 32 high-piled storage guide explains when the fire code requires the commodity classification to be submitted at all.

Common Errors in Mixed Commodity Calculations

Ignoring the pallet. The pallet is part of the unit load. A wood pallet adds nothing, but an unlisted plastic pallet raises the class under NFPA 13 (2022) Section 20.3, and a load that calculated out as Class III on wood can become Class IV or Group A once the warehouse switches to plastic pallets.

Treating a carton as paper when it holds foam. Electronics, appliances and glassware ship in corrugated cartons with expanded polystyrene inserts. The carton is paper, but the inserts are expanded Group A plastic, and by volume they often fill a quarter or more of the carton.

Measuring by weight only. Weight is the easy number because it comes from the shipping manifest. Expanded plastic weighs 1 to 2 lb per cu ft, so a load that is 1 percent plastic by weight can be 20 percent by volume. Volume governs for expanded plastics, and the calculation has to be done both ways.

Averaging across the array. A warehouse that is 90 percent Class II by count is not a Class II warehouse if the other 10 percent is Group A and scattered. NFPA 13 bases the design on the highest class present, and the only relief is the small dispersed quantity or a designated, separately protected area.

Using the product’s class without the packaging. A commodity letter that classifies “steel fasteners” as Class I is wrong if the fasteners are sold in plastic blister packs on plastic display trays. The class belongs to the load as stored, not the product as sold.

Limits of the Method and When a PE Calculation Is Required

The percentage method answers one question: how the plastic content of a conventional cartoned or palletized load affects its class. Several conditions fall outside it, and a licensed Professional Engineer should classify the commodity and set the design basis when any of them apply.

Exposed expanded plastics. Once a load is Group A, the next questions are whether the plastic is exposed or cartoned and expanded or nonexpanded, and those answers change the sprinkler design more than the percentage did. Loads with exposed foam, open-top plastic bins or plastic crates need an engineering review of the arrangement, because the container can matter more than the contents.

Unusual packaging and products. Mixed loads that combine expanded and nonexpanded plastic are checked against NFPA 13 (2022) Figure 20.4.3.3(a), where neither straight limit applies on its own. Products that are themselves plastic but contain liquids, lithium-ion batteries, aerosols or flammable liquids are classified by other sections of NFPA 13 or by other standards entirely, and the percentage method does not apply.

Idle pallets. Stacks of empty wood or plastic pallets are a separate storage hazard under NFPA 13 (2022) Chapter 20 with their own height and area limits, and they are never folded into the commodity calculation of the goods stored on them.

FM Global differences. Insurers that follow FM Global Data Sheet 8-1 use their own commodity definitions, which group products differently and in some cases are stricter than NFPA 13. A warehouse that is code compliant on an NFPA 13 Class IV design may still receive an FM recommendation for a plastics design, so the owner should confirm which basis the insurer expects before the sprinkler criteria are fixed. The industrial and high-hazard sector page describes how DGA handles projects that need both.

Frequently Asked Questions

This article is general information, not a substitute for a project-specific commodity classification. Code editions, local amendments and reviewer practice change; confirm the adopted code and the authority having jurisdiction for the project.