Fire extinguisher classes (A, B, C, D, K): the US guide

In the United States, fires are grouped into five classes: A, B, C, D and K. Each extinguisher is rated for one or more of them, and using the wrong one can make a fire worse. Here is what each class covers and which extinguisher fits.

The five classes

• Class A: ordinary solid combustibles such as wood, paper, cloth, rubber and many plastics.
• Class B: flammable and combustible liquids such as gasoline, alcohol and some paints. NFPA 10 also groups flammable gases and greases here. For a gas fire, shut off the supply first if you can do so safely.
• Class C: energized electrical equipment. Once the power is off, the fire becomes whatever class the burning material belongs to.
• Class D: combustible metals such as magnesium, titanium and sodium. These need a special dry powder agent made for the metal.
• Class K: cooking oils and fats in commercial cooking appliances such as deep fryers and griddles.

Which extinguisher fits which class

• Water: Class A only. Never on liquids, grease or electrical equipment.
• Foam (such as AFFF): Class A and B.
• CO₂: Class B and C. It leaves no residue, but it doesn't cool, so a fire can restart.
• ABC dry chemical: Class A, B and C, which is why it is the most common general-purpose unit. It leaves a corrosive residue and reduces visibility in a small room. BC dry chemical units cover B and C only.
• Wet chemical: Class K, and often Class A as well.
• Dry powder (Class D agents): specialist units for combustible metals.

How to read the label

Extinguishers carry letter symbols or pictograms showing the classes they are rated for. Many are multipurpose, so one unit may show A, B and C. A number in front of the letter, such as 2-A or 20-B, shows relative capacity: the bigger the number, the bigger the fire it can handle. Always check the label on the actual unit before you use it.

Unlike the UK, where a colour band identifies the contents, most US extinguishers have a red body whatever they contain. Don't rely on colour. Read the label.

What OSHA requires

Where an employer provides portable extinguishers for employees to use, 29 CFR 1910.157 sets the rules:

• Accessible and charged: extinguishers must be readily accessible, fully charged and in their designated places.
• Travel distance: no more than 75 feet to a Class A extinguisher, and no more than 50 feet from a Class B hazard area.
• Monthly visual inspection and an annual maintenance check, with the annual date recorded.
• Hydrostatic testing at set intervals, for example every 5 years for stored-pressure water and AFFF units.
• Training: an education program on extinguisher use and the hazards of fighting early-stage fires when staff are hired, and at least annually after that.

An emergency action plan under 29 CFR 1910.38, where one is required, must cover how to report a fire, evacuation and exit assignments, and how to account for everyone afterwards. Employers with 10 or fewer employees may communicate it orally. State and local fire codes may add requirements, so check yours.

How the US system differs from the UK

• Electrical fires are Class C in the US. The UK has no electrical class and uses a spark symbol instead.
• Cooking oil is Class K in the US and Class F in the UK.
• Flammable gas is grouped with Class B in the US, and is Class C in the UK.
• Identification: a label in the US, a colour band on a red body in the UK (see the UK colour chart).
• Law and standards: OSHA and NFPA in the US, the Fire Safety Order 2005 and BS standards in the UK.
• Emergency number: 911 in the US, 999 in the UK.

Using one safely: PASS

1. Pull the pin.
2. Aim low, at the base of the fire.
3. Squeeze the handle.
4. Sweep from side to side.

Only fight a small fire, once the alarm is raised and with a clear exit behind you. If it is spreading, if smoke is building, or if you are unsure, get out, stay out and call 911.

Related guides

• The fire triangle and the classes of fire, explained
• Which fire extinguisher for which fire? (UK)
• Fire extinguisher colours and chart: what each band means (UK)
• Fire safety checks: the daily, weekly, monthly and annual schedule

Sources

• 29 CFR 1910.157, Portable fire extinguishers (OSHA)
• 29 CFR 1910.38, Emergency action plans (OSHA)
• ABCs of Fire Extinguishers (UT Austin Fire Prevention Services)

This guide is for learning and applies to US federal workplace rules. State and local codes may differ, and you should always follow the label on your own extinguisher and your local fire code.