When you look at a fire extinguisher label, you’ll see codes like “2A:10B:C”. These tell you what classes of fires the extinguisher can fight (A, B, C, etc.) and how big a fire it can handle. For example, “2A” means Class A fires (wood, paper, cloth) and “10B” means Class B fires (flammable liquids). These ratings aren’t arbitrary – Underwriters Laboratories (UL) tests every extinguisher under standard fire scenarios (UL 711) to “earn” its numbers and letters. In this post, we’ll explain the UL rating system, the actual fire tests behind each class, and why only a UL-listed extinguisher is acceptable for code compliance. We’ll also show how Holmes Fire & Safety uses this knowledge to help Ohio businesses stay safe and compliant.
Portable extinguishers are labeled with letters (the fire classes) and numbers (the size of fire they can fight). Here’s what they mean:
Each extinguisher’s nameplate shows all its qualified classes (often with pictograms). For example, a “3A:40B:C” extinguisher can fight Class A fires up to 3A, Class B fires up to 40B, and is safe on energized equipment (C). In short, the letters tell you what kinds of fires you can use it on, and the numbers tell you how big a fire it can handle for A and B classes.
UL maintains Standard 711 (ANSI/UL 711), which specifies exactly how extinguishers are tested for each rating. In a UL lab, extinguishers are subjected to full-scale fires to prove their ratings. In simple terms, to earn a given rating an extinguisher must completely put out a defined test fire under controlled conditions. Below are key UL test scenarios for each class:
UL’s Class A test uses a standardized wood “crib” fire. In these tests, a stack of 2″×2″ wooden sticks (“wood crib”) and an adjacent wood panel are ignited. For example, UL 711 specifies that a 3A extinguisher must quench a fire from a 12′×12′ wood panel and a crib of 144 wooden pieces (18 layers of 8 sticks). A 2A uses 112 sticks and a 10′×10′ panel. After igniting the fuel (often with a heptane pan burn), the operator sprays the extinguisher from a set distance until the fire is out. To pass, the extinguisher must completely extinguish the crib and panel fire with no reignition. The numeric rating is essentially the size of that test fire. In fact, each “A” is calibrated to ~1.25 gallons of water, so a 3A unit roughly equals 3.75 gal of water in extinguishing power. (By comparison, 2A ≈ 2.5 gal and 4A ≈ 5 gal.)
Class B ratings come from liquid fuel fires. UL 711 uses a large steel pan (at least 8″ deep) filled with a 2″ layer of heptane or gasoline. The fuel surface is kept 6″ below the pan rim (often by placing water under the fuel). After a one-minute “pre-burn,” the operator attacks the fire from one side with the extinguisher. The extinguisher must extinguish the flames completely in the allowed discharge time. Each B-rating corresponds to a larger pan area. For example, UL’s 10B test uses a 10′×10′ pan (~25 ft²) with about 31 gal of heptane, and a 20B test uses a 10′×20′ pan (~50 ft²). If the extinguisher put out the fire (no reflash) under those conditions, it earns the 10B or 20B rating. In practical terms, a higher B-number means the unit can tackle a proportionally larger liquid fire (10B covers roughly twice the fuel area of 5B).
All told, UL’s testing regimen is rigorous. These standardized fires give each extinguisher a “stamp of approval” – if it’s rated 4A:80B:C, you know it actually put out the fires corresponding to 4A and 80B under lab conditions. The UL mark and labels (e.g. “2A:10B:C, UL” on the nameplate) are your guarantee of performance.
Understanding these ratings isn’t just trivia – it’s a legal and safety necessity. Fire codes, OSHA and NFPA standards require that any portable extinguisher in a business be listed and labeled by a recognized testing lab (UL is the most common). In practice, that means the extinguisher must have official classification symbols on its nameplate. A random canister of “fire suppressant” without a UL rating is not acceptable. During inspections, an unlabeled or unloved extinguisher can be cited as non-compliant. Moreover, in an actual fire, only a properly rated extinguisher can be trusted to perform. For example, a restaurant is required by code to have UL-classified Class K wet-chemical units for its fryers, as well as appropriate A/B/C units elsewhere. If an extinguisher lacked a UL rating for the hazard it’s placed at, it could fail miserably in an emergency.
In short, a UL-rated extinguisher gives you assurance: it has been “trial by fire” tested to handle the hazard it’s marked for. No rating label means no proof of effectiveness. Always check that the nameplate clearly shows the UL logo and the class/number ratings (like “4A:80B:C”). This is your evidence that the unit is listed for your hazards. An improperly rated or fake extinguisher not only jeopardizes safety but can invite fines and liability under OSHA/NFPA rules.
At Holmes Fire & Safety, we take fire extinguisher expertise seriously. Our technicians undergo continual training on UL standards, fire classes, and code requirements so we can advise customers with confidence. When we visit your facility, we’ll assess each hazard (electrical panels, flammable liquids, kitchen equipment, etc.) and select the correct UL-rated extinguisher(s) for that area. We’ll ensure you have enough extinguishers of the right class in the right places to meet NFPA 10 and OSHA rules. For example, a warehouse might need 4A:80B:C units strategically placed, whereas a restaurant’s kitchen will get dedicated Class K wet-chemical stations.
We also offer regular inspections and maintenance – both code-mandated. During service calls, we verify that all extinguishers still carry their UL ratings and are pressurized and ready. If any unit is damaged or out-of-date, we’ll replace it with a proper UL-listed replacement. Our goal is to make compliance effortless for you.
Stay prepared and stay safe. The next time you see a fire extinguisher, remember that “2A:10B:C” isn’t just a label – it’s a precise measure of firefighting capability established by UL testing. With the right UL-rated extinguishers and the Holmes Fire & Safety team on your side, you can be confident your facility is protected. If you have any questions about extinguisher ratings or need service – whether it’s a new installation, inspection or training – contact Holmes Fire & Safety today. Our expert advice and professional service will help keep your people and property safe. Your safety is our passion!
