Fire Alarm NAC Voltage Drop After a Tenant Renovation
An existing 24 V dc fire alarm notification appliance circuit in a 30,000 ft² school serves 14 horns/strobes on a Class B loop. The panel is rated 4 A per NAC and is located in the main electrical room; the farthest device is 260 ft of 16 AWG copper from the panel. A summer renovation adds four new strobes at the end of the same NAC, each drawing 0.115 A continuous and 0.300 A inrush. After the work, the last device chimes but the strobe occasionally fails to flash during alarm, and the panel reports a NAC trouble after 10 minutes. Measured voltage at the last device is 19.4 V dc steady and 15.1 V dc during inrush. Devices are listed 16-33 V dc. The contractor proposes to raise the panel's NAC output to 28 V dc and leave the 16 AWG wiring. The fire marshal has not signed off.
1. Evaluate the voltage drop and determine whether raising the panel output to 28 V dc is an acceptable fix.
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The measured 15.1 V dc during inrush is below the devices' listed minimum of 16 V dc, so the circuit fails the listing requirement under worst-case alarm conditions. Raising the panel to 28 V dc may appear to add margin, but it is not a substitute for a point-to-point voltage-drop calculation. The panel may not be listed for field adjustment, and increasing voltage affects battery calculations, other devices, and synchronization. The correct approach is to calculate drop using the manufacturer's method, including inrush and continuous current, and then reduce drop by using 14 AWG conductors, splitting the NAC, or relocating the panel. A rough check shows why 16 AWG is marginal: 16 AWG copper is about 4.0 ohms per 1,000 ft, and the loop length and inrush current produce several volts of drop. The installer must show at least 16 V at the last device during inrush and not exceed 33 V at any device.
2. What fire alarm code and listing issues arise from adding four strobes to an existing NAC without recalculating?
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Adding notification appliances changes the NAC load, voltage drop, battery standby, and synchronization requirements. NFPA 72 and the panel listing require the designer to verify that the NAC does not exceed its rated output, that all appliances operate within their listed voltage range, and that the battery can support the required standby and alarm times. The panel trouble after 10 minutes suggests the NAC may be overloaded or the circuit is not properly supervised. Class B wiring must be supervised, and a trouble condition cannot be ignored. Strobes on the same circuit must be synchronized if required by the listing or the AHJ. The contractor cannot simply add devices and raise voltage; the submittal should include a load calculation, voltage-drop calculation, battery calculation, and device compatibility listing. The fire marshal will likely require a revised design before acceptance.
3. What licensing and inspection steps should be followed in Michigan before this system is placed back in service?
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Fire alarm work is regulated electrical work, and the contractor must hold the appropriate license or specialty qualification recognized in Michigan and must obtain the required permit from the local enforcing agency. The local fire marshal or AHJ will inspect the installation, including the NAC calculations, device listing, synchronization, and supervision. The contractor should not energize the modified circuit for acceptance until the calculations are corrected and the installation is inspected. The panel trouble must be investigated and cleared, not bypassed. The owner should receive documentation of the revised calculations and test results. Because licensing categories and local permit rules can vary, the contractor should confirm the exact fire alarm specialty requirement with the local building department and fire marshal before starting the corrective work.