For a hydraulically calculated light hazard area under a mezzanine constructed of 2 in. (50 mm) x 14 in. (350 mm) combustible solid wood members…

The construction described, with solid combustible wood members spaced 4 ft (1.2 m) on center, is classified as "obstructed construction" per NFPA 13. According to the standard, construction is considered obstructed when solid structural members are spaced at 7.5 ft (2.3 m) or less. For a hydraulically calculated sprinkler system in a Light Hazard occupancy with combustible obstructed construction, the maximum allowable protection area per standard spray sprinkler is 130 ft² (12.1 m²). This limitation ensures adequate water distribution around the structural obstructions to control a potential fire.

B. 168 ft² (15.6 m²): This protection area is typically associated with Ordinary Hazard occupancies under specific obstructed construction conditions, not Light Hazard.

C. 200 ft² (18.6 m²): This is the maximum protection area for a Light Hazard occupancy under unobstructed construction using a pipe schedule system, not a hydraulically calculated one.

D. 225 ft² (20.9 m²): This is the maximum protection area for a Light Hazard, hydraulically calculated system, but only under unobstructed construction, which does not apply to this scenario.

1. NFPA 13, Standard for the Installation of Sprinkler Systems (2019 Edition).

Table 10.2.4.2.1(a) "Protection Areas and Maximum Spacing for Standard Spray Sprinklers for Light Hazard Occupancies": This table explicitly lists the maximum protection area for a "Hydraulically calculated" system with "Combustible obstructed construction" as 130 ft².

Section 3.7.5, "Obstructed Construction": Defines construction where members impede heat flow or water distribution.

Section 3.7.6, "Unobstructed Construction": Defines unobstructed construction, in part, as having structural members spaced more than 7.5 ft (2.3 m) on center. The 4 ft spacing in the question falls below this threshold, classifying it as obstructed.

2. SFPE, Handbook of Fire Protection Engineering (5th Edition).

Chapter 52, "Automatic Sprinkler System Calculations," Section 52.3.2.1, "Area of Protection": This section explains that the maximum sprinkler protection area is a function of the hazard classification and the type of ceiling construction (obstructed or unobstructed), referencing the specific tables within NFPA 13. It reinforces that obstructed construction significantly reduces the allowable coverage area compared to unobstructed construction. (DOI: https://doi.org/10.1007/978-1-4939-2562-452)

3. Jensen, R. (Editor), Fire Protection Handbook (20th Edition), National Fire Protection Association.

Section 17, Chapter 2, "Design of Sprinkler Systems": This chapter details the design criteria from NFPA 13, including the tables for sprinkler spacing and protection area. It clarifies that these values are fundamental design parameters derived from extensive fire testing and are dependent on the combination of hazard, system type, and construction features. The distinction between obstructed and unobstructed construction is highlighted as a critical factor in determining these limits.