The ABCs, D, and Ks of Fire Extinguishers

The ABCs, D, and Ks of Fire Extinguishers

Extinguishers should be located along normal paths of travel and near exits.

Fire extinguishers give us an opportunity to keep a small fire emergency from turning into a large fire emergency. Large fire emergencies may have significant economic impacts, or even cause loss of life or injury.  

Commercial occupancies are required to have fire extinguishers as part of their various code requirements. These codes can be found in Occupational Safety and Health Administration’s (OSHA) requirements, the International Code Council’s Building Code and Fire Code requirements (IBC & IFC), and the National Fire Protection Association’s (NFPA) requirements. Each of these code-making organizations have set requirements for the placement, training, and use of fire extinguishers. Not all fire extinguishers are the same and they even have specific uses. For this article, we will focus on portable fire extinguisher requirements for commercial use.  

There are three basic types of hazards that will classify all occupancies: light, ordinary and extra hazard (NFPA, 2022, The NFPA (2022) states that extinguishers should be listed and labeled and shall meet or exceed all requirements of UL 711, CAN/ULC-S508, Standard for the Rating and Fire Testing of Fire Extinguishers and at least one of the applicable standards designated by Underwriters Laboratory in Canada and the United States (4.1.1).  

Fire extinguishers are commonly placed in conspicuous locations so they are easy to locate in case of fire. Although the placement of fire extinguishers may seem obvious, there are guidelines and standards set forth by the OSHA, IFC, and the NFPA to provide the classification, labeling and location of portable fire extinguishers. 

Fire extinguishers are classified by the type of fire that they will extinguish.  

Class A fire extinguishers are used for ordinary combustibles such as wood, paper, some plastics and textiles. This class of fire requires the heat-absorbing effects of water or the coating effects of certain dry chemicals. Extinguishers that are suitable for Class A fires should be identified by a triangle containing the letter "A."  

Class B fire extinguishers are used for flammable liquid and gas fires such as oil, gasoline, etc. These fire extinguishers deprive the fire of oxygen and interrupt the fire chain by inhibiting the release of combustible vapors. Extinguishers that are suitable for Class B fires should be identified by a square containing the letter "B."  

Class C fire extinguishers are used on fires that involve live electrical equipment, which require the use of electrically non-conductive extinguishing agents. (Once the electrical equipment is de-energized, extinguishers for Class A or B fires may be used.) Extinguishers that are suitable for Class C fires should be identified by a circle containing the letter "C."  

Most fire extinguishers found in businesses are of the A, B and C classification of extinguisher.  

Class D fire extinguishers are used on combustible metals—such as magnesium, titanium, sodium, etc.—which require an extinguishing medium that does not react with the burning metal. Extinguishers that are suitable for Class D fires should be identified by a five-point star containing the letter "D."  

Class K fire extinguishers are used on fires involving cooking media—fats, greases, oils, etc.—in commercial cooking sites such as restaurants. These fire extinguishers work on the principle of saponification. Saponification takes place when alkaline mixtures—such as potassium acetate, potassium citrate, or potassium carbonate—are applied to burning cooking oil or fat. The alkaline mixture combined with the fatty acid creates a soapy foam on the surface that holds in the vapors and steam, extinguishing the fire. These extinguishers are identified by the letter K. 

Fire extinguishers are labeled so users can quickly identify the classes of fire on which the extinguisher will be effective. The marking system combines pictographs of both recommended and unacceptable extinguisher types on a single identification label.  

Also located on the fire extinguisher label is the UL rating. The UL rating is broken down into Class A and Class B:C ratings. Numerical ratings allow you to compare the relative extinguishing effectiveness of various fire extinguishers. For example, an extinguisher that is rated 4A:20B:C indicates the following:  

  1. The A rating is a water equivalency rating. Each A is equivalent to 1 1/4 gallons of water. 4A = 5 gallons of water.  
  2. The B:C rating is equivalent to the amount of square footage that the extinguisher can cover, handled by a professional. 20 B:C = 20 square feet of coverage.  
  3. C indicates it is suitable for use on electrically energized equipment.  

When analyzing these ratings, note there is not a numerical rating for Class C or Class D fires. Class C fires are essentially either a Class A or a Class B fire involving energized electrical equipment where the fire extinguishing media must be non-conductive. The fire extinguisher for a Class C fire should be based on the amount of the Class A or Class B component. For extinguishers used on a Class D fire, the relative effectiveness is detailed on the extinguisher nameplate for the specific combustible metal fire for which it is recommended. 

OSHA (2002) requires that employers select and distribute fire extinguishers based on the classes of anticipated workplace fires and also on the size and degree of the hazard that would affect their use. The following charts contain the IFC requirements for classes of fires and travel distance to an extinguisher. NFPA (2022) requires Class K extinguishers to be placed no more than 30 feet from hazard.  

Some local requirements may be stricter, so you should always check with the local fire marshal and underwriter for risk management. In some areas, the fire marshal will provide some assistance in locating fire extinguishers as well. 

IFC (2018) TABLE 906.3(1) 


LIGHT (Low) 


ORDINARY (Moderate) 


EXTRA (High) 


Minimum-rated single extinguisher 




Maximum floor area per unit of A 

3,000 square feet 

1,500 square feet 

1,000 square feet 

Maximum floor area for extinguisherb 

11,250 square feet 

11,250 square feet 

11,250 square feet 

Maximum distance of travel to extinguisher 

75 feet 

75 feet 

75 feet 

IFC (2018) TABLE 906.3(2) 





Light (Low) 





Ordinary (Moderate) 





Extra (High) 





Extinguishers need to be conspicuously located and available in the event of a fire. They also should be located along normal paths of travel and near exits. Portable fire extinguishers that are not wheeled should be installed on the hanger or bracket supplied or placed in cabinets or wall recesses.  

The locations of fire extinguishers must be identified so they are readily available to employees without subjecting them to injury. Height requirements for mounting extinguishers depend on the weight of the unit. If the unit weighs less than 40 pounds, it should be installed so the top of the extinguisher is no more than 5 feet above the floor. If the unit weighs more than 40 pounds, it should be installed so the top of the extinguisher is no more than 3.5 feet above the floor. At no point should the extinguisher be less than 4 inches from the floor.  

OSHA (n.d.) states that instead of Class A portable fire extinguishers, an employer may use uniformly spaced standpipe systems or hose stations connected to a sprinkler system installed for emergency use by employees. Such systems must meet the respective requirements of 29 CFR 1910.158 or 1910.159, that provide total coverage of the area to be protected, and that employees are trained at least annually in their use (1910.157(d)(3). While OSHA may have this exemption, it does not exempt the property for applicable adopted codes that the local jurisdiction is enforcing.  

OSHA (n.d.) states when the employer has provided fire extinguishers for employee use, the employer must provide an educational program to familiarize employees with the principles and use of the extinguishers. This educational program should be completed during the initial hiring and annually thereafter (1910.157(g)(1). Check with the local fire agency to see if it has a program that provides fire extinguisher training. There are also options for virtual training as well.  

Portable fire extinguishers must be visually inspected monthly and annually in accordance with NFPA 10 (NFPA, 2022). The inspection should ensure that:  

  • Fire extinguishers are in their assigned place.  
  • Fire extinguishers are visible and are not blocked or hidden.  
  • Fire extinguishers are mounted correctly and easily accessed.  
  • Pressure gauges show adequate pressure in the operating range.  
  • Fullness – check by weighing or heftiness.  
  • Pin and seals are in place.  
  • Fire extinguishers show no visual sign of damage or abuse.  

*Non-rechargeable extinguishers are maintained occurring to manufacture requirements.  

Maintenance, inspection and testing of an extinguisher are the responsibility of the owner or designee. Those individuals who are performing maintenance are required to have a certification to maintain fire extinguishers with the correct certification proof. Maintenance for internal inspection is based on the type of extinguisher that should be done at least annually or more often if conditions warrant. The employer shall record the annual maintenance date and keep these records for one year after the recorded date or the life of the shell of the extinguisher.  

Hydrostatic testing of portable fire extinguishers is done to protect against unexpected in-service failure. This can be caused by internal corrosion, external corrosion and damage from abuse, etc. Hydrostatic testing must be performed by trained personnel with proper test equipment and facilities. OSHA requires hydrostatic testing according to the schedule listed in 29 CFR 1910.157 Table L-1. In addition to the code and regulation requirements extinguishers also have to be maintained by manufacturer requirements as well.  

Fire extinguishers are a tool that can often be very effective in stopping or slowing a fire until the fire department arrives. It is important to remember that combustible products are very harmful and super-heated gases can cause ill effects on the human body. Extinguishers should only be used if you are comfortable in using them as well as being trained in using them properly. Using a fire extinguisher improperly will only cause the fire to become more dangerous and cause injury to the user.  

This article, written by Carl A. McMillan originally appeared in the August 2004 edition of Occupational Health & Safety. The article is one of the most popular on the OH&S website,, and was graciously updated by Tom Maloney in February 2021.  

This article originally appeared in the April 2022 issue of Occupational Health & Safety.

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