Third-Floor Addition Pushes a 24 V Panel Past Its Listed Battery and NAC Capacity
Your company is replacing the addressable panel in a 22,000 sq ft, two-story medical office building in Newark, Delaware. The new panel is a 24 V addressable unit with a built-in power supply, four NAC outputs, and a listed standby battery capacity of two 7 Ah, 12 V sealed lead-acid batteries connected in series. The contractor supplies these measured 24 V currents: panel with no field devices 0.110 A; 62 addressable devices total 0.036 A; two remote annunciators 0.024 A; NAC 1 (14 horn/strobes) 1.35 A; NAC 2 (nine strobes) 0.95 A. The plan reviewer at the Office of the State Fire Marshal requires 24 hours of standby plus 5 minutes of alarm with a 1.25 aging factor shown on the permit drawings. Late in review the tenant adds a 6,000 sq ft third-floor suite on the same panel: standby rises to 0.290 A and total alarm current to 3.40 A. The panel is listed at 2.5 A per NAC and 3.0 A total. The installer's answer: "We'll just put in two bigger batteries."
1. Compute the required secondary (battery) capacity before and after the third-floor addition, and state whether the listed battery configuration is acceptable in each case.
Read the worked answer
Two 7 Ah, 12 V batteries in series give 7 Ah at 24 V, not 14 Ah - series connection adds voltage, not capacity. That is the first place installers go wrong.
Before the addition
- Standby: 0.110 + 0.036 + 0.024 = 0.170 A × 24 h = 4.08 Ah
- Alarm: 1.35 + 0.95 = 2.30 A × (5/60) h = 0.19 Ah
- Subtotal 4.27 Ah × 1.25 = 5.34 Ah → within the listed 7 Ah. Acceptable.
After the addition
- Standby: 0.290 A × 24 h = 6.96 Ah
- Alarm: 3.40 A × 0.0833 h = 0.28 Ah
- Subtotal 7.24 Ah × 1.25 = 9.05 Ah → exceeds the listed 7 Ah. Not acceptable.
The 5-minute alarm term is small; the standby term dominates and is what the third floor broke. Note also that the alarm current of 3.40 A exceeds the panel's 3.0 A total NAC rating, so the battery is not the only problem.
2. The third floor is going forward. Identify every deficiency in the installer's "bigger batteries" proposal and describe a compliant solution, including what must go back to the AHJ.
Read the worked answer
Three separate failures, each with its own fix:
Battery capacity vs. listing. A panel is listed with a specific maximum battery and charger. Dropping in physically larger batteries that exceed the listed maximum is a listing violation and usually overloads the charger, which then cannot restore the battery in the required time. Fix: a listed, supervised auxiliary power supply with its own battery and charger for the added load, or a larger listed panel/enclosure combination.
NAC load. 3.40 A total exceeds the 3.0 A panel rating, and the split across circuits must also be checked against 2.5 A each. Fix: a listed NAC power extender (or auxiliary supply) for part of the notification load, with its own 24 V battery sized for its appliance load, supervised for AC failure, battery missing, and open circuit.
Documentation. Because the design basis changed, battery and voltage-drop calculations for the added supply must be revised and resubmitted with the permit drawings so the AHJ reviews what is actually installed.
Remember that a remote supply does not remove the load from the calculation - you must size each supply's battery for the load it actually carries at end-of-discharge voltage, not nominal 24 V.
3. The owner asks whether the alarm-duration term can be cut from 5 minutes to 1 minute, since the fire department responds quickly. How do you answer, and what else must the batteries do at the end of the standby period?
Read the worked answer
No. The 24-hour standby and 5-minute alarm duration are minimums in the design basis required by the AHJ; they are not negotiable downward to make a battery calculation close. The 5 minutes is not a response-time allowance - it is the window in which occupants must be notified and the system must still be signaling after the worst-case 24 hours of standby, which is precisely when the batteries are at their weakest.
Also insist on the two verification points the arithmetic alone does not prove:
- The alarm load must be carried at end-of-discharge voltage, not at float voltage. Appliances draw more current at lower voltage; confirm the panel's low-battery cutoff and the appliances' listed operating range still overlap.
- The charger must be able to restore the installed battery within the manufacturer's/code-required recharge time at its rated output. If the required capacity forces a battery the charger cannot properly recharge, the correct answer is a different supply, not a larger battery.
If the owner wants a smaller battery, the honest option is to reduce the connected load, not to shorten the code minimums.