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    Extinguisher Ratings: What the Letters and Numbers Mean

    By Mark Conroy

    Today’s extinguishers have the letters A, B, C, D, and K on the label to help installers and users understand which types of fires the extinguishers are to be used on. Class A and B extinguishers also have numbers that relate to sizes. Understanding the numbers and letters is essential to installing the correct extinguishers in buildings and complying with NFPA 10, Standard for Portable Fire Extinguishers. Here are the essentials that will help you in your everyday tasks and will enable you to explain capabilities of extinguishers to your customers.

    UL 711, Standard for Rating and Fire Testing of Fire Extinguishers is used to evaluate extinguishers as follows:

    • • Class A Rating: - Wood crib and wood panel test fires
    • • Class B Rating: - Square pans with flammable liquid test fires
    • • Class C Rating: - Electrical conductivity tests during agent discharge (no fires)
    • • Class D Rating: - Combustable metal test fires
    • • Class K Rating: - Cooking appliances with cooking oil test fire

    Although each of the UL ratings relates to a letter classification of a fire, UL also uses numbers that relate to fire sizes for Class A and B. For example, a rating of 4-A:20-B:C means–

    • • It will extinguish twice as much Class A fire as a 2-A rated extinguisher
    • • It will extinguish twenty times as much surface area as a 1-B rated extinguisher
    • • It is suitable for use on energized electrical equipment

    Class A Fire Tests

    The Class A laboratory tests use special kiln-dried wood that will burn the same for each test.

    UL 771 Fire Tests for 2-A and 4-A

    Rating Pieces Size/Length* Configuration
    2-A 78 2" x 2" x 255/8" 13 layers of 6 pieces
    4-A 120 2" x 2" x 333/8" 15 layers of 8 pieces
    *Lumber dimensions are nominal (2" x 2" nominal is 11/2" x 11/2" actual)

    Class B Fire Tests

    The experienced laboratory fire technicians are expected to extinguish Class B fires that are 21/2 times as large as a novice user. The UL 711 test fires are 21/2 ft2 for 1-B and 50 ft2 for 20-B. So an extinguisher with a rating of 1-B means the average person can extinguish one square foot of Class B fire.

    With this information, you should feel confident that you understand extinguisher ratings. And with this knowledge and an understanding of NFPA 10, your customers can rest assured that you have installed the correct extinguishers for building protection and hazards in each area. That means they can grab the nearest extinguisher and use it with confidence in the event of a fire.

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    Mark Conroy is an engineer in our Boston, MA office and a member of the NFPA 10 Technical Committee. © 2017 Brooks Equipment.

    Disclaimer: The opinions expressed in the above Tech Series article are the author’s only and provide limited information. Although the information is believed to be reliable, Brooks Equipment Company, LLC expressly disclaims any warranty with respect to the information and any liability for errors or omissions. The user of this article or the product(s) is responsible for verifying the information’s accuracy from all available sources, including the product manufacturer. The authority having jurisdiction should be contacted for code interpretations.