Fire Alarm Notification Circuit Voltage Drop on a Long Horn/Strobe Run
A fire alarm contractor is finishing a new 12,000 ft² office building. The notification appliance circuit (NAC) on the first floor serves 11 horn/strobes rated at 0.115 A each at 24 VDC, plus two remote strobes at 0.075 A each. The panel's NAC output is rated 24 VDC at 2.0 A and is listed for a maximum line resistance that yields a 20 percent voltage drop allowance. The run from the panel to the farthest device is approximately 410 ft of 16 AWG stranded cable (resistance about 4.02 ohms per 1000 ft per conductor). The panel's low-battery cutoff is 20.4 VDC. During the acceptance test the farthest horn/strobe sounds noticeably weaker than the others, and the contractor's helper suggests simply adding a booster at the end of the line.
1. Calculate the voltage at the farthest device and determine whether the circuit is within the panel manufacturer's allowance.
Read the worked answer
Total NAC load. 11 horn/strobes × 0.115 A = 1.265 A. Two remote strobes × 0.075 A = 0.150 A. Total = 1.415 A. That is below the 2.0 A panel rating, so the panel is not overloaded.
Loop resistance. The circuit is a two-wire loop: current goes out on one conductor and back on the other, so the resistance is doubled. 410 ft × 2 = 820 ft of conductor. At 4.02 ohms per 1000 ft, R = 820 × 4.02 / 1000 = 3.30 ohms.
Voltage drop. V_drop = I × R = 1.415 A × 3.30 ohms = 4.67 V. At a 24 VDC source, the farthest device sees 24 − 4.67 = 19.33 VDC.
Compare to cutoff and allowance. The panel's low-battery cutoff is 20.4 VDC. The farthest device would be at 19.33 VDC even with a healthy battery - below the cutoff. The manufacturer's 20 percent drop allowance permits a drop of 0.20 × 24 = 4.8 V, so 4.67 V is just inside the allowance on paper, but the resulting voltage (19.33 V) is below the panel's low-battery cutoff, which means the circuit will not function reliably on battery. The two criteria conflict, and the more restrictive one governs: the circuit fails.
Conclusion. The circuit is not acceptable. The drop is within the 20 percent rule but the end-of-line voltage is below the low-battery cutoff, so the design does not meet the reliable-operation intent. The helper's suggestion to add a booster at the end of the line is a field fix that changes the design and must be reviewed by the designer of record.
2. What corrective options should the contractor present, and what documentation is needed before changing the design?
Read the worked answer
Options, in order of preference:
- Reduce loop resistance. Replace the 16 AWG with 14 AWG (about 2.53 ohms per 1000 ft) or 12 AWG (about 1.59 ohms per 1000 ft). At 14 AWG: R = 820 × 2.53 / 1000 = 2.07 ohms, drop = 1.415 × 2.07 = 2.93 V, end voltage = 21.07 VDC - above the 20.4 V cutoff. At 12 AWG: drop ≈ 1.84 V, end voltage ≈ 22.16 VDC. Either works, but re-pulling is expensive.
- Split the NAC. Divide the 13 devices across two NACs so each carries less current. If the panel has a spare NAC, this is often the cheapest fix. Halving the current halves the drop to about 2.34 V, giving 21.66 VDC at the end.
- Add a listed NAC extender or booster near the middle of the run, not at the end. A booster at the end does not help the devices upstream of it and adds a new power source that must be supervised and listed for the purpose.
- Re-tap the circuit so the farthest devices are fed from a shorter branch, reducing the effective loop length.
Documentation. Any of these changes alters the design. The contractor should issue an RFI to the designer of record describing the measured voltage, the calculated drop, the panel cutoff, and the proposed fix. The designer must confirm the revised circuit is within the panel's listed NAC ratings and that the battery calculation still supports the standby and alarm periods. The contractor should also record the as-found voltage and the final acceptance test results, because the authority having jurisdiction will want to see that the notification appliances operate at their listed voltage range under battery-only conditions.