According to NFPA 72, National Fire Alarm and Signaling Code, the maximum nominal spacing for spot-type smoke detectors on smooth, flat ceilings is 30 ft (9.1 m). This spacing serves as a baseline for design, ensuring that any point on the ceiling is within an acceptable distance of a detector. Specifically, all points on the ceiling must be within a distance of 0.7 times the selected spacing. For a 30 ft nominal spacing, this means no point on the ceiling can be more than 21 ft (6.4 m) from the nearest detector, which provides effective coverage for early detection in a standard room configuration.

A. 15 ft (4.6 m): This spacing is significantly less than the maximum nominal spacing allowed for smoke detectors and would result in an overly dense, inefficient, and unnecessarily costly installation.

B. 20 ft (6.1 m): While a designer could choose to use a 20 ft spacing, it is not the maximum nominal spacing permitted by the code for standard applications.

C. 25 ft (7.6 m): This is also a permissible spacing but is not the maximum nominal value allowed by NFPA 72 for smoke detectors on smooth ceilings.

1. NFPA 72, National Fire Alarm and Signaling Code (2022 Edition). Chapter 17, Initiating Devices, Section 17.6.3.1.1.1 states: "The distance between smoke detectors shall not exceed a nominal spacing of 30 ft (9.1 m)..." This section explicitly establishes the 30 ft nominal spacing as the fundamental rule for smooth ceilings in the absence of a specific performance-based design.

2. SFPE Handbook of Fire Protection Engineering (5th Edition). Chapter 51, "Detection of Fires and Special Hazards," Section 51.5.2, "Spacing and Location of Spot-Type Smoke Detectors." This section discusses the prescriptive requirements of NFPA 72, stating, "For smooth ceilings, the distance between adjacent smoke detectors is not to exceed a nominal spacing of 9.1 m (30 ft)." (DOI: 10.1007/978-1-4939-2565-051).

3. University of Maryland, Department of Fire Protection Engineering. Course materials for ENFP 411: Fire Alarm and Detection Systems. Lecture notes on "Initiating Devices" consistently reference NFPA 72, Chapter 17, to establish the 30 ft (9.1 m) nominal spacing for spot-type smoke detectors as a foundational principle for system design on smooth ceilings.