Class A ratings: 1A - 40A. For a 1A rating, 1-1/4 gallons of water is required. A 2-1/2 gallon water extinguisher is rated at 2A. Therefore, a dry chemical extinguisher rated at 10A is equivalent to five 2-1/2 gallon water extinguishers.
Class B ratings: 1B - 640B. This rating is based on burning flammable liquid n-heptane in square steel pans. The rating is determined from the approximate square foot area of a flammable liquid fire that a non-expert can extinguish.
Class C ratings: This is only to assure electrical non-conductivity.
Class D ratings: No specific test given. Factors considered; reactions, toxicity of agent, fumes produced, etc.
Presently extinguishers are labelled with pictographs to show what class of fire they can be used on. Previously, colored geometric shapes were used.
FPT 102 - Fire Prevention and Inspection // Instructor: Kevin M. Kolb Slide #2