Which class of extinguishers requires a rating number preceding the classification letter?

Class A and Class B fire extinguishers require a rating number preceding the classification letter to indicate their effectiveness in suppressing fires. This rating is essential as it provides an idea of the amount of firefighting capability based on the extinguishing agent and the specific types of materials involved in a fire. For instance, Class A extinguishers are rated based on the amount of water equivalent they provide in terms of extinguishing ordinary combustibles, while Class B extinguishers are rated according to their effectiveness on flammable liquids.

In contrast, Class K extinguishers, which are specific to cooking oils and fats, and Class D extinguishers, which deal with combustible metals, do not utilize a numeric rating system. Class C extinguishers, being designations for electrical fires, also do not require a numeric rating. Therefore, the necessity for a numeric rating is a specific characteristic of Class A and Class B extinguishers, distinguishing them from other classes.