NAC Loading and Battery Sizing for an Assisted-Living Fire Alarm
A four-story assisted-living building has an addressable fire alarm panel with four notification appliance circuits rated 2.5 A each at 24 VDC. On NAC 3, the contractor plans 18 wall-mounted horn/strobes, each drawing 65 mA at 24 VDC, wired on a single Class B path of 14 AWG stranded copper. The farthest device is 240 ft of cable length from the panel. The panel charger is rated 0.75 A, and the standby load is 150 mA. The alarm load on NAC 3 is the only significant added load. A 7 Ah sealed lead-acid battery is on the shelf and the installer wants to use it. Device ratings are 16–33 V, and the manufacturer requires at least 20.4 V at the end of the circuit with all devices operating. The electrical inspector will witness the acceptance test and review battery calculations. The owner asks whether the existing battery is adequate for 24 hours standby plus 5 minutes alarm.
1. Calculate the end-of-line voltage on NAC 3 and determine whether the circuit meets the manufacturer's 20.4 V requirement. Address the panel's 2.5 A rating as part of your answer.
Read the worked answer
Total NAC current:
18 devices × 0.065 A = 1.17 A
Voltage drop with 14 AWG (4,110 CM), 240 ft one-way:
VD = 2 × 12.9 × 1.17 × 240 ÷ 4,110 VD = 7,245.6 ÷ 4,110 = 1.76 V
End-of-line voltage = 24 − 1.76 = 22.24 V
This is above the manufacturer's 20.4 V minimum, so the circuit is acceptable on voltage drop. The calculation is conservative because it lumps all 18 devices at the far end; a distributed load produces slightly less drop, so the real end voltage will be a little higher.
Circuit loading is also acceptable: 1.17 A is 47 percent of the 2.5 A NAC rating, well under the rating and under a common 80 percent design limit. Confirm that inrush/sync surge does not trip the NAC and that the panel's overall alarm current, including other NACs and the panel itself, remains within the panel and battery ratings. The 20.4 V criterion comes from the manufacturer's listing and instructions, which are enforceable under 110.3(B), not from a generic NEC voltage-drop table.
2. Perform the 24-hour standby plus 5-minute alarm battery calculation and determine whether the 7 Ah battery is adequate. State the assumptions and any follow-up checks.
Read the worked answer
Standby: 0.150 A × 24 h = 3.60 Ah
Alarm: assume the panel alarm current equals the 150 mA standby figure plus the NAC load: 0.150 + 1.17 = 1.32 A. For 5 minutes: 1.32 A × (5 ÷ 60) h = 0.11 Ah
Total: 3.60 + 0.11 = 3.71 Ah
Apply a 20 percent capacity margin, as most manufacturers and NFPA 72 guidance require for battery aging and terminal voltage: 3.71 × 1.20 = 4.45 Ah.
A 7 Ah battery has about 5.6 Ah usable at 80 percent depth of discharge, so it is adequate for the calculated load, with roughly 1.1 Ah of margin. Do not stop at the arithmetic: verify the panel's actual alarm current from its published tables, confirm the charger can recharge the battery within the required time (7 Ah × 1.2 ÷ 24 h ≈ 0.35 A, well within the 0.75 A charger), check battery date and terminal voltage, and confirm the required duration for this occupancy. If the system is a voice-evacuation system, the alarm duration is longer than 5 minutes and the calculation must be redone; a 12 Ah battery may then be the better choice.