Every fire extinguisher sold in the United States carries a UL rating on its label — something like 4A:80B:C. It looks like a part number. It isn't. It's the record of a series of physical fire tests the extinguisher had to pass, and once you know what those tests are, you can compare any two units on the market in seconds.
Most explanations of extinguisher ratings online repeat a rule of thumb that isn't in the standard. What follows comes from Amerex's own technical documentation and product catalogs — the actual test conditions behind each number.
The format
4A : 80B : C
The letters say which classes of fire the extinguisher is approved for. The numbers record how large a test fire it put out. C, D and K never carry a number, and there's a specific reason for each.
The letters
- Class A — ordinary combustibles: wood, paper, textiles
- Class B — flammable liquids and gases: oils, greases, paints, solvents, propane
- Class C — fires involving energized electrical equipment
- Class D — combustible metals: magnesium, titanium, zirconium, sodium, lithium and potassium
- Class K — cooking oils and animal fats in commercial cooking appliances
Class K is the newest classification. Before the 1998 edition of NFPA 10, cooking oil and fat fires were treated as Class B; extensive testing established that they behave differently enough to warrant their own class. Class D hazards are dangerous precisely because the metals involved can react violently with water, foam and halogenated agents — which is why you never treat a Class D fire with a general-purpose extinguisher.
The Class A number: wood cribs and panels
The number before the A is not a volume of water. It's an index tied to a specific, physically defined test fire. Amerex publishes the dimensions:
| Rating | Wood crib | Wood panel fire |
|---|---|---|
| 1A | 72 members, 2"×2"×20" | 8' × 8' |
| 2A | 112 members, 2"×2"×25" | 10' × 10' |
| 3A | 144 members, 2"×2"×29" | 12' × 12' |
| 4A | 180 members, 2"×2"×32" | 14' × 14' |
| 6A | 230 members, 2"×2"×36" | 17' × 17' |
| 10A | 324 members, 2"×2"×43" | — |
| 20A | 256 members, 2"×4"×55" | — |
| 30A | 324 members, 2"×4"×64" | — |
| 40A | 400 members, 2"×4"×69" | — |
Units rated 1A through 6A must pass both the wood panel and the wood crib fire. A 10A unit must extinguish the 6A panel fire plus the 10A crib. At 20A and above, only the crib test applies.
The test is deliberately brutal. A pan of heptane beneath the crib ignites it and is allowed to burn out completely. The crib then keeps burning until it has lost 45% of its original weight — which for ratings up to 10A can take another eight to ten minutes. Only then does the operator attack, from no closer than six feet, with the extinguisher held fully open under continuous discharge. For any unit rated 2A or higher, minimum discharge time is 13 seconds.
One historical note worth knowing if you're comparing an old label to a new one: in August 2002 the 6th Edition of ANSI/UL 711 eliminated the excelsior fires and increased crib fire sizes by roughly 40%. A 4A rating earned after 2002 represents a materially larger fire than a 4A earned before it.
The Class B number: what the test pan actually holds
This is where most published explanations go wrong. You'll frequently read that "the number before the B is the square footage you can put out." The number is an index, and the test fire behind it is considerably larger than the numeral suggests.
Class B tests use a square steel pan at least 8 inches deep, holding at least a two-inch layer of heptane with six inches of freeboard. Ratings up to 20B are tested indoors in a draft-free room; above 20B the test moves outdoors into still air of 3–8 mph. After a one-minute pre-burn, the operator attacks — from one side only, forbidden from reaching any part of themselves past the edge of the pan.
| Rating | Minimum discharge time | Test pan size | Heptane |
|---|---|---|---|
| 1B | 8 sec | 2½ sq ft | 3¼ gal |
| 2B | 8 sec | 5 sq ft | 6¼ gal |
| 5B | 8 sec | 12½ sq ft | 15½ gal |
| 10B | 8 sec | 25 sq ft | 31 gal |
| 20B | 8 sec | 50 sq ft | 65 gal |
| 30B | 11 sec | 75 sq ft | 95 gal |
| 40B | 13 sec | 100 sq ft | 125 gal |
| 60B | 17 sec | 150 sq ft | 190 gal |
| 80B | 20 sec | 200 sq ft | 250 gal |
| 120B | 26 sec | 300 sq ft | 375 gal |
| 160B | 31 sec | 400 sq ft | 500 gal |
| 240B | 40 sec | 600 sq ft | 750 gal |
| 320B | 48 sec | 800 sq ft | 1000 gal |
The test pan is consistently 2.5 times the numeral in square feet. An 80B extinguisher was tested on a 200 square foot pan holding 250 gallons of heptane. A 320B wheeled unit was tested on 800 square feet and 1,000 gallons.
That margin exists because a laboratory operator running a defined attack pattern performs better than someone meeting a real fire for the first time. Treat the numeral as a comparative scale, not a promise about the square footage you personally can handle.
Class C: no fire test at all
There is no Class C fire test, which is why there's never a Class C number. What gets tested is the agent's electrical conductivity.
The extinguisher is discharged onto a copper target charged with 100,000 volts AC across a ten-inch air gap between the target and the nozzle. To pass, no measurable current may be detected at the nozzle. Water-based agents must show a conductivity of 1 microsiemens or less.
Pass, and a C is added to the label. That's the whole thing. The C tells you the agent won't carry current back to your hands; the A and B numbers still tell you the capacity. This is why Amerex points carbon dioxide at high-voltage electrical distribution, welding shops and auto repair shops — and, for server rooms and data centers, points at a clean agent or the HT Series instead — and why a plain water extinguisher can never carry a C.
Class K: the deep fat fryer test
Class K carries no number because the hazard varies too much between appliances. The test is specific and unusually demanding.
A commercial gas deep fat fryer holding 80 lbs of shortening is heated until the oil auto-ignites — which cannot occur below 685°F. The fire burns freely for one minute. The extinguisher is then discharged with the nozzle fully open while the burner stays on, and no burning shortening may be ejected from the fryer. The heat is then shut off and the fire must stay out for twenty minutes. The unit has to do this three times to earn the rating.
Class K agents work by saponification — the agent reacts with the oil to form a foam blanket that cuts off oxygen while cooling the fuel below its ignition point. Our wet chemical units are rated 2A:K.
What this means when you're choosing
Weight tells you very little
Two Amerex units, both 10 lb, both ABC dry chemical:
- B456 — 4A:80B:C
- 720 (High Flow series) — 1A:20B:C
Same agent, same weight, one quarter the Class A rating. The 720 isn't a worse extinguisher — it's a High Flow unit built to exceed 1 lb per second discharge rate, emptying its 10 lbs in about 10 seconds instead of 22. That buys you knockdown speed on a fast-developing fuel fire and costs you sustained capacity. Same trade in the 20 lb units: the standard A411 is 10A:120B:C, while the High Flow 760 is 4A:60B:C.
The same trap waits a size down. The B402 and B500 sit on the same aluminum valve platform and weigh the same on the shelf, yet the nameplates read 3A:40B:C versus 2A:10B:C from the same 5 lb cylinder — the rating, not the size, is the specification.
Stepping up a size is often cheap
The B441 is a 10 lb ABC unit rated 4A:80B:C. The A411 is 20 lb, rated 10A:120B:C — two and a half times the Class A rating and half again the Class B — and it currently costs slightly less. Where the mounting location and the operator can handle 20 lbs, check the larger unit before defaulting to the smaller one.
Match the letters before the numbers
A 320B wheeled Purple K unit is formidable on fuel and carries no Class A rating at all — Purple K is a Class B:C agent. Right tool for a flight line or fuel depot, wrong tool for a paper storage room. Our water mist units are the inverse: 2A:C, no B rating, residue-free and safe around energized equipment, which is why they turn up in operating theatres and clean rooms.
The one thing the rating doesn't cover
UL ratings measure extinguishing capability. They say nothing about what the agent leaves behind.
ABC dry chemical gives you the most capability per dollar, and its residue is mildly corrosive and travels everywhere. In a warehouse that's a cleanup. In an avionics bay, a server room or a shop full of precision equipment, the cleanup can cost more than the fire did. That's why clean agents like Halotron BrX and CO2 exist, and why they carry lower ratings for their size — you are trading capacity for leaving nothing behind.
Specifying an extinguisher means matching three things: the class of hazard, the size of the hazard, and what you can afford to have happen to the room afterwards. The rating covers the first two.
Sources
Test procedures, crib and pan dimensions, and the Class C and Class K test conditions above are taken from Amerex Corporation's technical publication Fire and Fire Extinguishment: A Brief Guide to Fire Chemistry and Extinguishment Theory, which cites ANSI/UL 711 Standard for Fire Extinguishers, Rating and Fire Testing of, NFPA 10 Standard for Portable Fire Extinguishers, and the NFPA Fire Protection Handbook. Product ratings are taken from Amerex's published hand-portable and wheeled extinguisher catalogs.
Where a rating is critical to your application, confirm it against the extinguisher's own label and Amerex's current documentation. Placement, coverage and inspection requirements are set by NFPA 10 and your local authority having jurisdiction.
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