Installation, Wiring, and Circuit Integrity
Choosing the wiring method
First determine whether the circuit is power-limited fire alarm (PLFA) or non-power-limited (NPLFA) - the Article 760 requirements differ. PLFA cable is typically secured within 12 in. of boxes and at intervals not exceeding 4 1/2 ft, following the listing and the applicable article. Fire alarm conductors must not share a raceway, box, or enclosure with power, Class 1, or non-power-limited fire alarm conductors unless a barrier separates them or the combination is listed; where they run near power conductors, maintain the required separation (commonly 2 in.). Ground-fault detection is required, and grounding must follow the manufacturer and the NEC.
Supervision and circuit classes
Class B circuits lose everything beyond a single open; Class A circuits continue to operate past a single open and are preferred where survivability matters. Know the style designations for initiating device circuits and notification appliance circuits, and remember that end-of-line devices supervise the circuit - a missing or wrong-value EOL resistor reports a trouble immediately at commissioning.
Pathway survivability
NFPA 72 assigns pathway survivability levels (0 through 4). Level 0 is no added protection; the higher levels are met with listed circuit-integrity (CI) cable, a fire-rated enclosure or raceway, or a combination - verify the exact level definitions in the edition you are tested against.
Worked example: voltage drop
Formula: Vd = 2 × L × I × R ÷ 1000, with R in ohms per 1000 ft. A 24 V NAC drawing 2 A through 200 ft one-way of 14 AWG (R ≈ 3.07 Ω/1000 ft): Vd = 2 × 200 × 2 × 3.07 ÷ 1000 ≈ 2.46 V, leaving about 21.5 V at the last appliance - acceptable for most listed 24 V appliances (typically 20.4 V minimum). Always check the last device's listed minimum, not just a percentage.