NAC Voltage Drop and Mixed-Manufacturer Synchronization
A 24 VDC addressable fire alarm control unit serves a new elementary school. NAC 1 is a Class B circuit wired with 14 AWG stranded copper, 300 ft from the panel to the last appliance. The circuit has 18 horn/strobes, each drawing 0.10 A at 24 V, for a total of 1.8 A. The panel's NAC 1 is rated 2.5 A maximum, and its terminal voltage is 24.0 V with the circuit connected. The appliances are listed for 16-33 V. The circuit serves the classroom wing and a portable classroom at the far end. The contractor wants to add six more horn/strobes at 0.15 A each on the same circuit because 'the voltage drop calculation still works.' One existing appliance is from a different manufacturer and has no sync input; during activation, the strobes flash out of synchronization. The contractor proposes to leave the mixed appliances in place and tell the staff that flashing is 'cosmetic.'
1. Calculate the voltage drop for the existing circuit and for the proposed addition. Does the addition comply with the circuit rating and appliance operating range?
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Use the two-conductor voltage-drop formula Vd = 2 x L x R x I, with 14 AWG copper taken as approximately 3.1 ohms per 1000 ft. For the existing load: Vd = 2 x 300 ft x 0.0031 ohm/ft x 1.8 A = 3.35 V. The voltage at the end of the circuit is approximately 24.0 - 3.35 = 20.65 V, which is above the 16 V minimum, so the existing circuit is acceptable on voltage drop alone. For the proposed addition, the new load is 6 x 0.15 A = 0.9 A, making the total 2.7 A. The new drop would be 2 x 300 x 0.0031 x 2.7 = 5.02 V, leaving about 18.98 V, which is still above 16 V. However, the total current of 2.7 A exceeds the NAC's 2.5 A maximum rating. The addition therefore cannot be approved on that circuit even though the end voltage appears acceptable. The contractor must split the load onto another NAC, reduce the load, or use a listed power supply with adequate capacity. The calculation should use the panel's actual loaded terminal voltage and the appliance's worst-case current, not just nominal values.
2. Explain why the mixed-manufacturer appliances flash out of synchronization and what must be done.
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Strobe synchronization is a system function, not a cosmetic feature. Strobes must flash within the manufacturer's listed synchronization tolerance so that the combined light output does not create confusing or reduced warning, and so that the installation matches the listing. Mixing appliances from different manufacturers on the same NAC is a common cause of out-of-sync flashing because each product may use a different sync protocol or no sync input at all. The appliance with no sync input cannot follow the panel's synchronization signal, so it will free-run. The fix is to replace that appliance with a compatible listed device from the same manufacturer as the rest of the circuit, or to use a listed synchronization module that is compatible with all appliances on the circuit. The contractor should not leave the mixed arrangement and describe it as cosmetic. The revised design should be checked against the panel's synchronization compatibility list, and the acceptance test should verify that all visible appliances on the circuit flash in synchronization.
3. What design and documentation steps should the contractor complete before the added devices are approved?
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The contractor should submit a revised NAC calculation showing the actual device currents, the panel's loaded terminal voltage, the conductor size and length, and the end-of-line voltage for each circuit. Because the existing NAC is at its 2.5 A limit, the plan should redistribute the added horn/strobes to another NAC or a listed remote power supply with enough capacity. The submittal should also identify the replacement synchronized appliance or sync module, include the manufacturer's compatibility listing, and show that all devices on the circuit will flash in synchronization. The contractor should update the as-built drawings and the record of completion to reflect the final device list, circuit assignments, and power calculations. Before final acceptance, the technician should measure the actual current on each NAC, verify the end voltage under alarm load, and test synchronization visually across the circuit. Any deviation from the submitted design should be documented and reviewed, not left as a field change.