Fire extinguishers are rated by the class of fire they fight: A for ordinary combustibles, B for flammable liquids, C for energised electrical equipment, D for combustible metals and K for cooking oils and fats. Most buildings use ABC dry chemical as a general-purpose unit, with CO₂ around electronics and a Class K unit in any commercial kitchen.
The letters on the label describe what the extinguisher is designed to put out. Using the wrong one is not a neutral mistake — water on burning grease spreads the fire violently, and dry chemical in a server room destroys equipment the fire had not touched.
| Class | Fuel | Where you find it |
|---|---|---|
| A | Wood, paper, cloth, most plastics | Offices, retail, storage, homes |
| B | Gasoline, oil, solvents, paint, flammable gases | Auto shops, workshops, paint stores, garages |
| C | Fires in energised electrical equipment | Electrical rooms, plant rooms, anywhere with live gear |
| D | Combustible metals — magnesium, titanium, sodium | Machine shops, foundries, laboratories |
| K | Cooking oils and animal fats | Commercial kitchens — required by code |
ABC dry chemical is the general-purpose workhorse and covers three classes at once. It is inexpensive, effective, and leaves a corrosive powder that has to be cleaned up quickly — which is exactly why it is the wrong choice in some rooms.
CO₂ leaves no residue at all. It is the right unit for server rooms, electrical closets, laboratories and anywhere with sensitive equipment. It is heavy for its capacity and has a short discharge range, so it has to be closer to the hazard.
Class K wet chemical is required in commercial kitchens. It works by saponification — converting burning fat into a soap layer that smothers it and stops reignition — and it is designed to be used after the hood system has discharged, not instead of it.
Water and water mist suit Class A hazards where powder residue would be a problem, such as hospitals and some schools. Water mist is safe around energised equipment; plain water is emphatically not.
Class D units use a specialist dry powder matched to the specific metal and cannot be substituted with anything else. If your shop machines magnesium or titanium, this is not optional and no ABC unit will do.
A rating like 4A:80B:C tells you capacity as well as class. The number before the A is a rough measure of water equivalence — a 4A unit has about the extinguishing capacity of five gallons of water. The number before the B is the approximate square footage of a flammable liquid fire a trained user should be able to put out.
There is no number before the C. The C simply means the agent is non-conductive and safe to use on live electrical equipment.
NFPA 10 sets a maximum travel distance — the distance somebody actually walks to reach a unit, along corridors and around obstacles, not the straight line on a floor plan. This is the most misunderstood rule in the standard and the most common reason a well-equipped building fails.
The standard mnemonic is PASS, and the part people get wrong is the third letter. Pull the pin. Aim at the base of the fire, not the flames. Squeeze the handle. Sweep side to side across the base.
Aiming at the flames does nothing at all — the fuel is at the base, and that is what has to be smothered or cooled. Also know your exit before you start: a portable extinguisher is for a fire you can still walk away from, and if it does not go out within a few seconds, leave.
Class K means the unit is rated for cooking oils and animal fats. It uses a wet chemical agent that turns burning fat into a soap layer, smothering the fire and preventing reignition. A Class K extinguisher is required in commercial kitchens, within 30 feet of the cooking appliances, in addition to the hood suppression system.
It is not the right unit and it is not what the code requires in a commercial kitchen. Dry chemical does not cool cooking oil enough to stop it reigniting once the powder settles, which is exactly the failure Class K wet chemical was developed to solve. Commercial kitchens need a Class K unit.
Dry chemical covers more classes and is cheaper, but leaves a corrosive powder residue over everything in the room. CO2 leaves no residue at all, which makes it the right choice for server rooms, electrical closets and laboratories where cleanup would cost more than the fire.
It is set by travel distance and hazard, not floor area. Nobody may have to walk more than 75 feet to reach a unit for a Class A hazard, less for Class B, and no more than 30 feet from cooking appliances to a Class K unit. The route is measured as walked, so partition walls and racking change the answer.
Call 312-877-2355 during business hours. We'll tell you what applies to you — there's no charge for a straight answer on the phone.
1840 N Clybourn Ave · Chicago, IL 60614
