Fire extinguishers are classified, or rated, according to the type of fire they are intended to extinguish; this classification, or rating, is represented by a letter (A, B, C or D). A fire extinguisher may be rated for one or more classes of fire, depending on the type of extinguishing agent used (Water, Dry Chemical, CO2, Dry Powder). In addition to the letter rating, colors, symbols and pictures may be used to show what class of fire the extinguisher is rated for.
Class A Ratings
A fire extinguisher rated for class A fires is intended for use on fires involving Ordinary Combustibles such as wood, paper, cloth or plastics. Class A extinguishers are rated from 1 - A through 40 - A. The number in front of the A represents an amount of water or, if another extinguishing agent is used, how much fire that extinguishing agent will put out as compared to the rated amount of water. A Class 1 - A water extinguisher contains 1 1/4 gallons of water; a Class 2 - A extinguisher contains 2 1/2 gallons of water, or twice the 1 - A capacity.
Class B Ratings
A fire extinguisher rated for class B fires is intended for use on fires involving Flammable and Combustible Liquids and Gases such as gasoline, oil, lacquer, paint, mineral spirits and alcohol. Class B extinguishers are rated from 1 - B through 640 - B. The rating is based on the approximate square foot area of a flammable liquid fire that a non-expert operator can extinguish. The non-expert operator is expected to be able to extinguish 1 square foot of fire for each numerical rating or value.
Class C Ratings
A Class C fire is essentially a Class A or B fire where the fuel involved is energized with electricity. A Class C rating confirms that the extinguishing agent will not conduct electricity. The Class C rating is assigned in addition to the rating for Class A and/or Class B fire extinguishers.
Class D Ratings
A fire extinguisher rated for class D fires is intended for use on fires involving Combustible Metals such as aluminum, magnesium, titanium, zirconium, sodium and potassium. A Class D fire extinguisher is rarely needed outside of the industrial or professional firefighting setting.
To put all of this information together, a fire extinguisher with a rating of 2 - A, 10 - BC should extinguish the same amount of fire as a 2 1/2 gallon water extinguisher or extinguish a liquid fire that is 10 square feet in size and the extinguishing agent will not conduct electricity.