NFPA 13 Occupancies for Sprinkler Design

An assortment of building occupancy examples are classified in Sec. A.5.2 of the NFPA 13 Appendix. The following are noted as light hazard occupancies: offices, churches, schools, museums, auditoriums, library seating areas, restaurant seating areas, and unused attics. The maximum sprinkler spacing (protection area) for these is noted in Table 8.6.2.2.1(a) if standard sprinklers are used. Usually, the maximum limit is 225 square feet for light hazard on a hydraulically calculated system. However, when exposed construction is combustible, with structural members spaced less than 3 ft. apart, the maximum coverage limit shrinks to 130 square feet.

Ordinary hazard Group 1 occupancies include laundries, restaurant service areas, and automobile parking garages. Ordinary hazard Group 2 occupancies include the aforementioned dry cleaners, automobile repair and services areas, auditorium stages, woodworking plants, post offices, and stack room areas of libraries. Standard sprinklers protecting all ordinary hazard occupancies shall not cover an excess of 130 square feet per head (Table 8.6.2.2.1(b).

Extra hazard occupancy examples include printing plants, paint and varnish dipping operations, plywood manufacturing, solvent cleaning, and plastics processing. Maximum sprinkler spacing for these occupancies is limited to 100 square feet. However, where the required design density is less than 0.25 gpm/sf (and this goes for high-piled storage as well), a protection area of up to 130 square feet per sprinkler is allowable (Table 8.6.2.2.1-c). It should be noted that commercial insurance carriers and consultants develop their own literature containing more extensive listings of occupancy examples and classifications than does the NFPA 13 standard, data which often comes in handy when making an occupancy classification determination.

Design Density Criteria

The NFPA 13 Density / Area Curves are found in Fig. 11.2.3.1.1. When hydraulically calculating a light hazard sprinkler system, the design density utilized is typically 0.10 gpm/sf over a 1500 square foot (the most hydraulically demanding) area of operation. To begin a calculation, the designer starts with the end-sprinkler and works "backwards" to the water supply source. Suppose that the sprinklers are spaced 14 ft. apart on branch-lines that are 12 ft. apart. Our square foot coverage then, is (12 x 14) 168 square feet.

Q (in gpm) is determined by multiplying the density by the square foot coverage (.10 x 168), so we know that we'll need 16.8 gallons per minute (Q) discharging out of the end sprinkler.

The square root of the required end-head pressure is determined by "Q" divided by "K". If the design density is 0.10 and the K-factor of the sprinkler head is 5.5, we can ascertain our end-head pressure by dividing 16.8 by 5.5, and squaring the sum to obtain a 9.33 psi figure. 9.33 psi is the required end-head pressure. To double-check, we can simply plug in the numbers while performing the following equations to ensure that they match: Q= K times the square root of the pressure, K= Q divided by the square root of the pressure, and the design density equals Q divided by the square foot coverage. If our area of operation remains 1500 square feet, our design density will change to 0.15 for Ordinary hazard Group 1 occupancies and 0.20 for Ordinary hazard Group 2 occupancies.

Everything changes when extended-coverage sprinklers are employed. Let's suppose that we decide to extend our coverage to 324 square feet in a light hazard office, spacing sprinklers 18' x 18' apart. Now we must refer to the sprinkler manufacturer's data sheets for direction. If we choose to install Tyco EC-11 pendent sprinklers, the data sheets dictate that our end-sprinkler must discharge a minimum of 33 gpm at 8.7 psi. This means that our design density (Q divided by the square foot coverage) is still 0.10 gpm/sf. The K-factor of this particular sprinkler is 11.2, which we can validate by the equation K= Q divided by the square root of the pressure.

Extended-coverage sprinklers for ordinary hazard occupancies work the same way. For example, we could use the Tyco EC-14 extended-coverage pendent sprinkler (K=14.0) in a (Ordinary hazard group 1) restaurant service area to protect an 18' x 18' area, but here the data sheet parameters require a 49 gpm minimum discharge at 12.3 psi for the end-sprinkler. In other words, Q= 49, K= 14.0, the square root of the pressure is 3.51, and the coverage is 324 square feet. All the equations match, including the required design density (0.15) which is obtained by dividing Q by the 324 sq. feet. Of course, the local water supply must still be able to satisfy the resulting overall sprinkler system demand. In order for that to be accomplished, larger system piping is installed to deliver the additional gpm necessitated by the extended-coverage heads.

Sprinkler discharge characteristics are outlined in cogent form in Table 6.2.3.1- these outline the differing K-factors for sprinkler identification. One other handy table to reference for sprinklers in NFPA 13 is Table 6.2.5.1, which deals with classifications and temperature ratings.

To be absolutely certain of code compliance with respect to sprinkler elevations, we refer to Sec. 8.6.4.1 in NFPA 13. The allowable distances noted beneath roofs, beams, or ceilings are always measured to the sprinkler deflector. It is acceptable for designers to consult data sheets for appropriate distances below ceilings for specific sprinkler types, although the safe bet is to call for a distance between 1" and 12" beneath the underside of the roof deck. The closer sprinklers are to the ceiling, the faster they will operate. But caution must be exercised because often serious interferences to lateral water distribution can result from very close sprinkler placement to the ceiling. For all instances, the minimum of 1 inch (in the code) is to allow for the installation and removal of upright sprinklers. When sprinklers are installed beneath pitched roofs, the highest sprinkler deflector (Sec. 8.6.4.1.3.1) may extend 3 ft. down from the highest peak.