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All About Fire Extinguishers Ratings
Classification of Fire Extinguishers
There are four general classifications of fires and extinguishing agents:
Class A
Class A fires are those that involve ordinary combustible materials such as wood, paper, cloth, rubber and plastics. The quenching and cooling effects of water or of solutions containing large amounts of water are the most effective means of extinguishing this type of fire.
Class B
Class B fires are those that occur in the vapor-air mixture over the surface of flammable liquids such as gasoline, oil, grease, paints and thinners. The limiting of air (oxygen) or the combustion inhibiting effect is the most effective measure against incipient fires of this class. Water must not be used on Class B fires because it tends to spread the fire.
Class C
Class C fires are those that occur in or near energized electrical equipment where non-conducting extinguishing agents must be used. Water must never be used on Class C fires.
Class D
Class D fires are those that involve combustible metals such as magnesium, titanium, zirconium and sodium. Water must never be used on class D fires. A Class D powdered extinguishing agent should be used.
Appendix 1-Fire Extinguisher Size and Placement for Class A Hazards
| Light(Low)Hazard Occupancy | Ordinary(Moderate)Hazard Occupancy | Extra(High)Hazard Occupancy | |
| Minimum rated single extinguisher | 2-A | 2-A | 4-A* |
| Maximum floor area per unit of A | 3,000 sq ft** | 1,500 sq ft. | 1,000 sq ft. |
| Maximum floor area for extinguisher | 11,250 sq ft** | 11,250 sq ft** | 11,250 sq ft** |
| Maximum travel distance to extinguisher | 75 ft. | 75 ft. | 75 ft. |
* Two 2 1/2 gallon (9.46 L) water type extinguishers can be used to fulfill the requirements of one 4-A rated extinguisher.
** See pamphlet 10, Portable Fire Extinguishers, National Fire Protection Association, 1981 Edition.
Appendix 2-Fire Extinguisher Size and Placement for Class B Fires Other than for Fires in Flammable Liquids of Appreciable Depth
| Type of Hazard (m2) | Basic Minimum Extinguisher Rating | Maximum Travel Distance to Extinguishers (ft.) | ||
| Light (Low) | ||||
| 5B | 30 | ||
| 10B | 50 | ||
| Ordinary (Moderate) | ||||
| 10B | 30 | ||
| 20B | 50 | ||
| Extra (High) | ||||
| 40B | 30 | ||
| 80B | 50 |
Appendix 3-Fire Extinguisher Size and Placement for Class C Hazards
Extinguishers with Class C rating shall be required where energized electrical equipment may be encountered which would require a non-conducting extinguishing medium. This will include fire either directly involving or surrounding electrical equipment. Since the fire itself is a Class A or Class B hazard, the extinguishers are sized and located on the basis of the anticipated Class A or B hazard.