NAC Voltage Drop and Battery Sizing for a Warehouse Addition
A 24 VDC addressable fire alarm control unit serves a new 18,000 sq ft warehouse addition. NAC 3 is a single Class B notification circuit feeding 30 listed horn/strobes. Each appliance draws 0.090 A at 24 VDC in alarm. The farthest appliance is 1,000 ft of cable length from the panel, one-way, wired with 14 AWG stranded copper at 3.07 ohms per 1,000 ft. With all notification appliances operating, the panel NAC terminals measure 24.0 VDC. The appliances are listed for 16–33 VDC. The panel standby load is 0.250 A; the alarm load, including all NACs, is 3.20 A. The owner requires 24 hours of standby and 5 minutes of alarm. The existing battery is a single 24 V, 7 Ah sealed lead-acid unit. The panel is rated 3.0 A per NAC.
1. Calculate the voltage at the farthest horn/strobe using the conservative total-load-at-the-end method and then the evenly distributed load method. Does the circuit comply with the device listing?
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Total alarm current on NAC 3 is 30 × 0.090 A = 2.70 A, which is below the 3.0 A circuit rating, so the problem is voltage drop, not overload. Loop resistance is 2 × 1,000 ft × 3.07 Ω/1,000 ft = 6.14 Ω. Using the conservative total-load-at-the-end method: Vdrop = 2.70 A × 6.14 Ω = 16.6 V, so the farthest appliance sees 24.0 − 16.6 = 7.4 V. That is far below the 16 V listing minimum. If the load is treated as evenly distributed, the drop is roughly half, about 8.3 V, giving 15.7 V - still below 16 V and not acceptable with any margin. The circuit fails. In an actual layout, the farthest device sees the cumulative drop of all upstream segments, so the distributed result is the more realistic check, but it still fails here.
2. Size the battery per NFPA 72 and determine whether the existing 7 Ah battery is acceptable.
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NFPA 72 battery sizing uses the calculated standby load for 24 hours plus alarm load for 5 minutes, then a 1.2 safety factor. Standby: 0.250 A × 24 h = 6.00 Ah. Alarm: 3.20 A × (5/60) h = 0.267 Ah. Sum = 6.267 Ah; × 1.2 = 7.52 Ah. A 7 Ah battery is below the calculated minimum, so it is not acceptable even before considering aging, terminal voltage, or manufacturer derating. The next standard size is 8 Ah, but the charger must be rated to recharge it within the required time and the panel's battery circuit must accept it. If the specification requires 20% aging capacity, the required rating would be higher (about 9.0 Ah), so the designer should confirm the project basis. The existing 7 Ah unit must be replaced.
3. List and evaluate two remediation options for the voltage drop, including the effect on the battery calculation.
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Two defensible fixes: (1) split NAC 3 into three circuits of 10 appliances each. Each circuit draws 0.90 A; with the conservative end-load method the drop is 0.90 × 6.14 = 5.5 V, leaving 18.5 V, which is above 16 V. This requires three available NACs or a listed NAC extender, and the battery alarm load stays about the same, but any added power-supply standby load must be added to the 24-hour calculation. (2) Move the notification power source closer to the load with a listed NAC power extender or booster, or replace the 14 AWG run with 10 AWG. A 10 AWG loop at 1.24 Ω/1,000 ft gives 2.48 Ω; 2.70 × 2.48 = 6.7 V, leaving 17.3 V by the conservative method. Replacing 14 AWG with 12 AWG alone is not enough by the conservative method. Verify synchronization and that the booster is supervised.