Types of Notification Appliances

NFPA 72 Chapter 18 governs notification appliances — the devices that alert building occupants to a fire alarm condition. The three primary types are:

  • Audible appliances (horns) — produce a temporal pattern (T3) sound: three pulses, pause, repeat. Required in all areas where occupants may not see a strobe.
  • Visual appliances (strobes) — produce synchronized flashes at 1 Hz. Required in areas where hearing-impaired occupants may be present, and now mandatory in most commercial occupancies under ADA and IBC.
  • Combination appliances (horn/strobes) — the most common device in commercial fire alarm design, combining both functions in one unit.

Speaker/strobes are used in emergency voice evacuation systems where pre-recorded or live voice messages replace or supplement the horn signal.

Strobe Candela Requirements

NFPA 72 Table 18.5.4.3.1 specifies minimum candela (cd) ratings based on room dimensions. Key values:

  • Room up to 20 ft × 20 ft: 15 cd minimum (wall-mounted)
  • Room up to 30 ft × 30 ft: 30 cd minimum
  • Room up to 40 ft × 40 ft: 60 cd minimum
  • Room up to 50 ft × 50 ft: 95 cd minimum
  • Corridors up to 20 ft wide × 100 ft long: 15 cd strobes, spaced no more than 100 ft apart, with the first strobe within 15 ft of a corridor end

Wall-mounted strobes must have their lens center between 80 and 96 inches above the finished floor (AFF). In sleeping areas (hotels, dormitories), ceiling-mounted strobes with 110 cd minimum are required within 16 feet of the pillow position.

Strobe Synchronization

NFPA 72 Section 18.5.4 requires that strobes within the same field of view be synchronized to flash within 200 milliseconds of each other. Unsynchronized strobes can trigger photosensitive seizures. Most manufacturers provide a synchronization module or use addressable protocol to achieve this. Each NAC circuit typically requires one sync module per power supply.

NAC Circuit Design

The Notification Appliance Circuit (NAC) is the wiring loop that connects the FACP to its horn/strobe devices. Key design considerations:

Current capacity: Each NAC output from the FACP has a maximum current rating — commonly 2.0 A or 3.0 A per circuit. Sum the alarm current draw of all devices on the circuit and ensure the total stays within this limit. Horn/strobes typically draw 65–150 mA each depending on strobe setting.

Voltage drop: NEC and NFPA 72 require that the end-of-line device receives adequate voltage for proper operation. Calculate voltage drop using V = I × R, where R is the total wire resistance (both conductors) for the circuit length. Most manufacturers specify a minimum operating voltage (e.g., 16V for a 24VDC nominal system).

Wire sizing: Fire alarm NAC circuits typically use 18 AWG or 16 AWG wire. Larger wire (16 AWG or 14 AWG) is used on longer runs or circuits with higher current loads to reduce voltage drop.

Class A vs. Class B: Class B (Style Y) NAC circuits require an End-of-Line (EOL) resistor at the last device. Class A circuits loop back to the panel and continue operating with a single wire break.

Placement Rules for Common Areas

In open office areas, every point of the room must be within the effective coverage area of at least one strobe. For wall-mounted devices, NFPA 72 provides room-based tables. For large open areas over 100 ft × 100 ft, ceiling-mounted strobes on a grid pattern are more practical — the effective radius depends on the candela rating and mounting height.

Restrooms and other areas where hearing protection may be worn require both audible and visual notification. Mechanical rooms with high ambient noise levels may require horn appliances with higher dB output (minimum 15 dB above average ambient noise level per NFPA 72 Section 18.4.3).