Battery Sizing and NAC Voltage Drop at a Cold-Weather Panel
A 24 V addressable panel serves an unsprinklered 6,000 sq ft office building in Bangor. The panel is mounted in an unheated vestibule where the winter design temperature reaches 0 °C, and the battery manufacturer's capacity correction table shows a 0.90 factor at that temperature. The panel's published standby current is 0.240 A, which already includes the DACT. Total alarm current is 3.10 A, which includes the one notification appliance circuit in alarm. The design uses 5 minutes of alarm time. The installer furnished two 7 Ah, 12 V sealed lead-acid batteries in series and signed the record of completion. The single NAC is 16 AWG THHN in conduit, 420 ft from the panel to the last appliance. It carries 14 appliances drawing 0.115 A each at 24 V. Panel terminals measure 23.0 V under alarm load. The appliances are listed for 16–33 V. The panel's NAC output is rated 2.0 A. The owner now asks to add six identical appliances to the same circuit. No battery calculation was submitted with the permit.
1. Calculate the required battery capacity and determine whether the installed 7 Ah batteries are acceptable.
Read the worked answer
Standby: 0.240 A × 24 h = 5.76 Ah.
Alarm: 5 min = 0.0833 h; 3.10 A × 0.0833 h = 0.258 Ah.
Subtotal: 5.76 + 0.258 = 6.02 Ah.
Safety margin: 6.02 × 1.2 = 7.22 Ah.
Temperature correction: 7.22 ÷ 0.90 = 8.03 Ah of nameplate capacity, rated at the 20-hour rate, in the as-installed location.
Two 7 Ah batteries in series produce 24 V at 7 Ah - about 1 Ah below the calculated minimum before any allowance for aging. The installation does not comply; the record of completion was signed on an unsupported calculation. Install 12 Ah batteries (the next standard size at or above 8.03 Ah), confirm that the enclosure and charger are rated for them, and rerun the calculation whenever a load is added - for example, a cellular communicator's standby current must be included. Keep the calculation with the submittal so the AHJ can check the 1.2 margin and the temperature derating rather than taking the battery label on faith.
2. Evaluate the notification circuit's voltage drop and state whether it will operate reliably at end of battery discharge.
Read the worked answer
Use the conservative method: all load lumped at the far end.
- Conductor length = 2 × 420 ft = 840 ft.
- 16 AWG ≈ 4.89 Ω/1000 ft → 4.11 Ω.
- I = 14 × 0.115 A = 1.61 A.
- Drop = 1.61 × 4.11 = 6.6 V, so the last appliance sees 23.0 − 6.6 = 16.4 V.
That is 0.4 V above the 16 V listing floor while the batteries are healthy. At the end of the 24-hour standby discharge the battery bus may sit near 21–22 V, leaving roughly 14.5–15.5 V at the last device - below its listed range, so strobe intensity and horn dBA are not guaranteed. Field measurements at the panel with fresh batteries do not prove the circuit works on day two of an outage.
Acceptable fixes: rewire in 12 AWG (1.93 Ω/1000 ft → 1.62 Ω → 2.6 V drop → 20.4 V at the end), split the circuit into two runs, or add a listed remote power supply closer to the load. Recheck using appliance current at minimum rated voltage, not at nominal 24 V.
3. The owner wants six more appliances tapped onto the same NAC. What is your answer and what else must be recalculated?
Read the worked answer
Existing 1.61 A + (6 × 0.115 = 0.69 A) = 2.30 A against a 2.0 A circuit rating. The circuit is overloaded before voltage drop is even considered; the panel will current-limit or the devices will underperform, and the voltage drop from the previous task worsens with the added current.
Options:
- Use a spare NAC if the panel has one and total panel alarm current stays within rating.
- Install a listed NAC extender with its own 24-hour standby calculation and a monitored trouble connection to the FACP.
- Split the appliances across two circuits with separate home runs.
Do not simply tap the circuit. Adding 0.69 A also changes the battery calculation: alarm current becomes 3.10 + 0.69 = 3.79 A, so alarm Ah rises and the required battery may jump another size. Any new power supply needs its own battery, charger, and voltage-drop documentation, plus a revised record of completion and as-built. Explain to the owner that the added appliances are a design change, not a service call.