NFPA 13 Occupancy Classifications for Fire Sprinkler Systems

The amount of water needed for a fire sprinkler system to control a potential fire is directly related to the potential fire size from the quantity and combustibility of the contents the system is designed to protect. This concept is known as the occupancy hazard design approach for fire sprinkler systems.

The idea that an assessment of the potential fire severity is an important part of fire sprinkler design dates back to the first edition of NFPA 13, published in 1896. This first edition focused heavily on the protection of mills and manufacturing facilities. 

Although the importance of the severity of the hazard being protected was known, a proposed solution for more severe hazards would not be incorporated into the standard until 1928. In that year, the idea that open sprinklers activated by heat detectors could be used to control more severe fires was included in the same section as the concept that fire severity was important. 

In 1931, the idea that a system could be overdesigned was first addressed when the concept of separating sprinkler systems into Class A and Class B systems was introduced. 

In this concept, Class A systems contained installation requirements for sprinkler systems installed in mills and manufacturing facilities. Class B systems modified the requirements of Class A systems for occupancies where the potential fire hazard was less severe than mills, such as apartment buildings, hospitals and schools. 

This concept took its more modern form in the 1940 edition when the light hazard, ordinary hazard and extra hazard occupancies were established. Unlike today’s definitions that quantify combustibility and quantity of combustibles, the first definitions called out specific occupancies. 

Today, the occupancy hazard design approach is the more common of the non-storage design approaches. It relies on the concept that all spaces are given an occupancy classification based on the expected fire size (heat release rate) of what is contained in the space. 

These classifications account for things such as stockpile height and the presence of flammable and combustible liquids. 

Classifying each space individually ensures that buildings that would typically be considered lower hazards (for example, a school building) are protected appropriately when it may include a laboratory that represents a higher-hazard space. 

Occupancy classifications

The latest edition of NFPA 13, Standard for the Installation of Sprinkler Systems (https://bit.ly/42jokDU), contains three occupancy classifications: light hazard, ordinary hazard and extra hazard. The ordinary and extra hazard classifications are each broken down further into two groups. 

It’s important to note that the occupancy hazard classifications used for sprinkler system design in NFPA 13 are not the same as the occupancies used in the building, fire or life safety codes. The occupancies in NFPA 13 are based on the quantity and combustibility of the contents, whereas those used in fire or life safety codes are based on the unique life safety needs for the occupants. 

The annex material associated with the definitions found in NFPA 13 provides some example occupancy types. However, it is important for the designer/engineer of record to conduct a review of the occupancy and its contents to appropriately classify each space within the building. 

Once the occupancy hazard classification has been determined, the engineer/designer can apply the appropriate design criteria, often in the form of a density of water applied over a specific area. 

1. Light hazard occupancies

Light hazard occupancies represent the lowest fire severity potential of the occupancies in NFPA 13 sprinkler systems. They are defined in NFPA 13 as “spaces with low quantity and combustibility of contents.”

2. Ordinary hazard group 1 occupancies

Ordinary hazard group 1 occupancies represent a moderate level of fire severity as the quantity is greater than the light hazard; however, combustibility is still low. The definition includes a height limitation for stockpiles of 8 feet (2.4 meters). 

3. Ordinary hazard group 2 occupancies

Similar to the ordinary hazard group 1 classification, the ordinary hazard group 2 occupancies represent areas with a moderate level of fire severity. However, in the group 2 occupancy classification, the quantity is greater than both light hazard and ordinary hazard group 1. The combustibility of the contents is more severe than in ordinary hazard group 1. 

The height of stockpiles is taller than ordinary hazard group 1 for the same combustibility, with a maximum height of 12 feet (3.7 meters). However, it maintains the 8-foot (2.4 meters) maximum for contents with higher heat release rates. 

4. Extra hazard group 1 occupancies

Extra hazard group 1 occupancies represent spaces nearing the top level of fire severity. The quantity of combustibles is now classified as very high. This includes spaces where the potential for rapidly spreading fires exists, such as areas where dust and lint are present. 

5. Extra hazard group 2 occupancies

Extra hazard group 2 occupancies represent the highest hazard classifications and include spaces where the quantity of combustibles is very high, spaces that possess substantial amounts of combustible or flammable liquids and spaces where shielding of combustibles is extensive. 

The occupancy classifications found in NFPA 13 are used to support the selection of the appropriate design criteria for the sprinkler system. Instead of specifically calling out occupancy types from the building code, the engineer/designer is tasked with identifying how a particular space is used to classify the potential fire severity and assign the appropriate occupancy hazard category. 

For more information on the occupancy hazard classifications found in NFPA 13, check out the NFPA 13 Online Learning Series (https://bit.ly/4jthPEt).

[Editor’s note: This was originally published as a blog post on the National Fire Protection Association’s website. It is published here with permission and some light editing.]

Robin Zevotek, PE, is a principal fire protection engineer with the Technical Services Department at the National Fire Protection Association. His background includes engineering design, research and emergency services. At NFPA, Zevotek is a subject matter expert in fire protection systems, building and life safety, and emergency response and responder safety.