Most fire extinguishers are the ABC type, so having an appropriate extinguisher available for a minor rubbing alcohol spill in a business occupancy is not really an issue. The question is at what point do the Class B requirements take precedence over the Class A requirements for extinguisher rating and distance of travel?
I typically look at the characteristics of the fire load within an area to be covered by the extinguisher. If the fire load consists mainly of Class A fire hazard materials, then follow the Class A extinguisher requirements. Similarly, if Class B fire hazard materials are predominant, then follow the Class B extinguisher requirements for rating and distance, but follow the Class A requirements for the same type of hazard to determine the A rating.
For example, in a business occupancy, the conditions would be considered a light hazard, and 3,000 sq. ft. per unit of A would be used. A 9,000 sq. ft. building would require a 3-A extinguisher with a distance of travel set at a maximum of 75 feet. Because the fire load in an office is predominantly of Class A fire hazard materials (wood, paper, fabric, plastics, etc.), the requirements for a Class B fire hazard are secondary, and any B rating would be acceptable (e.g., a 5-pound 3-A:40-B:C extinguisher).
However, an automobile maintenance shop with plenty of flammable/combustible liquids around would then require the Class B fire hazards to take precedence. Thus, the Class B rating of the fire extinguisher would be based on the distance of travel you are willing to provide. An automobile maintenance shop would be considered an ordinary hazard, so either a 10-B extinguisher at 30 feet or a 20-B extinguisher at 50 feet would be required. If the building has an area of 9,000 sq. ft., the required Class A requirements would be based on 1,500 sq. ft. per unit of A. Thus, the extinguisher in the previous example would not be sufficient (9,000 sq. ft./1,500 sq. ft. per unit of A = 6-A) unless you provided two extinguishers, which may be necessary anyway because of the limited distance of travel for a Class B fire extinguisher. If you used a 3-A:40-B:C extinguisher, the distance of travel would need to be 50 feet or less since the 40-B rating exceeds the 20-B minimum rating for that type of hazard.
