The question behind most extinguisher orders is blunt: which unit do I actually need, how many, and where do they go on the wall? The label answers the first part in a code like 2-A:10-B:C, and NFPA 10 together with OSHA answers the rest. Once you can read both, specifying extinguishers for a shop, a kitchen or a fuel yard stops being guesswork.
The five fire classes
Fires are classified by what is burning, because what is burning determines what will put it out.
- Class A — ordinary combustibles such as cloth, wood and paper.
- Class B — flammable and combustible liquids: grease, gasoline, oil and oil-based paints.
- Class C — fires involving energized electrical equipment. The C is about the agent being non-conductive, not about the fuel itself.
- Class D — combustible metals, including magnesium, sodium, potassium, sodium-potassium alloy, titanium, zirconium and powdered aluminum.
- Class K — combustible cooking media, meaning the vegetable and animal oils and fats used in commercial cooking appliances.
What the numbers in front of A and B mean
The numbers are not marketing. They come from listing tests and they are comparative measures of firefighting capacity.
Each unit of the A rating represents the equivalent of 1.25 gallons of water. A 4-A extinguisher therefore carries the knockdown power of five gallons of water against ordinary combustibles, and a 10-A unit the power of 12.5 gallons. The rating is established by testing against wood panel and wood crib fires of increasing size.
Each unit of the B rating represents one square foot of flammable liquid fire that a non-expert operator should be able to extinguish. A 10-B unit is credited with ten square feet, an 80-B unit with eighty. The listing laboratory tests against a much larger pan fire and then applies a substantial reduction factor, because a trained technician performs better than the person who will actually grab the extinguisher.
Classes C, D and K carry no number. A C on the label means the agent has been tested as non-conductive, which is a pass or fail result, not a quantity. Class D and Class K hazards vary too widely for a single numerical scale, so those extinguishers are rated by listed agent capacity and by the specific metals or appliances the manufacturer lists.
Matching the agent to the hazard
| Agent | Classes it is listed for | Best fit | Watch out for |
|---|---|---|---|
| ABC dry chemical (monoammonium phosphate) | A, B and C | General-purpose coverage for mixed occupancies: shops, warehouses, offices, vehicles | Leaves a powder residue that must be cleaned up; not accepted for commercial cooking media |
| Purple K (potassium bicarbonate) | B and C | Flammable liquid and gas risks: fuel transfer, tank farms, offshore and process areas | No Class A rating, so it will not cover ordinary combustibles in the same space |
| Carbon dioxide | B and C | Electrical rooms, labs and equipment where residue is unacceptable | Listed B:C only, short discharge range, and it displaces oxygen in confined spaces |
| Clean agent (halocarbon such as Halotron I) | A, B and C on listed models | Electronics, control rooms, telecom, aviation and other sensitive equipment | Highest cost per unit of coverage |
| Wet chemical | K | Commercial kitchens: fryers, griddles, ranges | Purpose-built for cooking media, not a general-purpose extinguisher |
| Sodium chloride dry powder | D | Machining, casting and processing of combustible metals | Agent must match the specific metal; check the manufacturer's listed metals |
The distinction between dry chemical and wet chemical for cooking fires is not a preference. Wet chemical works by saponification, forming a thick soap-like blanket over the oil surface while cooling it, and it produces a stronger, thicker blanket than dry chemical. When the UL 300 test standard for cooking fires was introduced in 1994, no existing dry chemical system could pass it, which is why a kitchen needs a genuine Class K unit rather than the ABC unit hanging by the back door.
Similarly, combustible metal fires need a listed Class D agent. Amerex's 30 lb sodium chloride unit uses a blended sodium chloride dry powder that cakes and crusts over the burning metal to exclude air and dissipate heat, which is a mechanism no ABC extinguisher reproduces. Ours is listed in the fire extinguishers section as the Amerex 30 lb sodium chloride extinguisher.
How many, and how far apart
NFPA 10 and OSHA both work in travel distance: the distance a person actually walks to reach an extinguisher, around benches, racks and walls, not a straight line across the room.
- Class A hazards: travel distance to any extinguisher must be 75 feet or less.
- Class B hazards: OSHA caps travel distance at 50 feet. Code tables tie the required rating to the distance, so for moderate flammable liquid hazards a 10-B unit within 30 feet or a 20-B unit within 50 feet is the usual pairing, and for high flammable liquid hazards a 40-B unit within 30 feet or an 80-B unit within 50 feet.
- Class C hazards: distribution follows the pattern for the Class A or Class B hazards that exist in the same space, since energized equipment is a condition rather than a fuel.
- Class D hazards: travel distance from the combustible metal working area to any extinguisher must be 75 feet or less.
- Class K hazards: maximum travel distance from the hazard to the extinguisher is 30 feet.
That is why a plant that has standardized on one large unit per bay often fails an inspection: a single high-rated extinguisher does not cure a travel distance violation. Coverage is a function of spacing first and rating second. Mounting, signage and cabinets fall under the same reasoning, and the extinguisher accessories range covers the brackets and covers that keep units visible and reachable.
Inspection, maintenance and testing
OSHA requires portable extinguishers to be visually inspected monthly, and to receive an annual maintenance check with the record retained. NFPA 10 layers on periodic internal maintenance, including the six-year procedure that requires a stored-pressure extinguisher to be opened and examined. Hydrostatic test intervals depend on the shell and agent: five years for carbon dioxide, for water and antifreeze units, and for dry chemical with stainless steel shells; twelve years for stored-pressure dry chemical with mild steel shells, for halogenated agents, and for cartridge-operated dry powder units.
Two practical consequences. First, the purchase price is not the cost of ownership — a cylinder that must be hydrostatically tested at five years costs more over a decade than one tested at twelve. Second, buying a mixed fleet of agents multiplies the service calendar, so most sites standardize on ABC dry chemical extinguishers for general coverage and add specialty units only where the hazard demands them.
Specifying by area
For mixed general use, an ABC dry chemical unit sized to the hazard is the default; a heavy-duty 30 lb unit such as the Buckeye Offshore ABC portable extinguisher suits fuel-handling and process areas where a 5 lb unit would run dry. Where residue would destroy what you are trying to save, move to CO2 extinguishers for electrical rooms or to clean agent extinguishers for electronics and control rooms; the Buckeye 15.5 lb Halotron I extinguisher is listed 2-A:10-B:C, so unlike CO2 it carries a Class A rating alongside B and C. Confirm every rating against the label on the unit you receive, and have an authorized service company sign off on the placement plan before it goes on the wall.
This guide is educational. Selection and compliance remain the employer's responsibility under the applicable standard.