Adding Horn/Strobes to an Existing Fire Alarm NAC
A Connecticut low-voltage contractor is adding six ceiling horn/strobes to an existing 24 VDC notification appliance circuit (NAC) in a 120-unit assisted-living facility. The addressable fire alarm panel has a single NAC rated 24 VDC at 2.5 A. Eighteen appliances are already on the circuit. The contractor proposes to tap the six new devices at the end of the existing circuit and says, 'The panel has plenty of capacity, so it will work.' Manufacturer data show each new horn/strobe draws 85 mA at 24 V and has a minimum operating voltage of 20 V. With the existing load, the end-of-line voltage measured 21.8 V. The facility's fire marshal has scheduled the final inspection in 10 days. The existing cable is 16 AWG FPLR installed in a ceiling plenum; the contractor wants to extend it with 18 AWG FPLR because that is what the truck stocks. The panel is due for its annual test and the owner wants no disruption.
1. What calculations and field checks are required before adding the six devices, and why is panel ampacity not enough?
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Panel ampacity is only one limit. First, total NAC current: 18 existing appliances plus six new at 85 mA equals 0.51 A added; the existing load must be known. If the existing load is under 2.0 A, the 2.5 A panel rating may be okay, but the circuit must not exceed the panel's listed NAC rating after adding the devices. Second, voltage drop: the measured end-of-line voltage is 21.8 V with the existing load and a 20 V minimum, leaving only 1.8 V margin. Adding 0.51 A at the end increases the drop; using the manufacturer's voltage-drop table and the actual conductor length and gauge, the end voltage will likely fall below 20 V. Third, battery standby and alarm calculations must be redone. Also check synchronization, inrush, and whether the panel's NAC is shared. The correct fix is to calculate first, then use larger conductors, split the circuit, or add a listed NAC booster power supply; do not just tap the end.
2. The contractor proposes 18 AWG FPLR cable in the ceiling plenum and a tap at the end of the circuit. Identify the problems.
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The proposed cable has two problems. FPLR is a riser-rated fire alarm cable, not a plenum-rated cable. Because the cable is in a ceiling plenum, Article 300.22(C) and Article 760 require a plenum-listed fire alarm cable, such as FPLP, unless the installation is in a complete raceway acceptable for the space. Second, changing from 16 AWG to 18 AWG increases resistance and makes the voltage-drop problem worse; the panel/device manufacturer may also require a minimum conductor size. Tapping the new devices at the end is not automatically prohibited, but the circuit must remain supervised and listed for the arrangement; some panels limit T-taps or require a specific Class B wiring method. I would require the contractor to verify the panel listing and NFPA 72 supervision requirements before accepting a tap. The 10-day schedule does not justify an unlisted or undercalculated installation.
3. What should the contractor do before the 10-day inspection?
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Before the inspection, the contractor should stop the proposed installation and perform the load and voltage-drop calculations using the panel manufacturer's published data. If the existing NAC cannot support the added devices, the contractor should propose a listed booster power supply or a new NAC circuit, not a field tap. The revised design should be submitted to the AHJ/fire marshal and, if required, the panel manufacturer. Order FPLP cable for the plenum portion, install it with listed supports, and update the record of completion, battery calculations, and as-built drawings. After installation, perform the acceptance test required by NFPA 72, including alarm and trouble signals, and document the end-of-line voltage. Do not cover the ceiling or schedule the final until the revised design is accepted.