Secondary Power Sizing on a Voice Evacuation Panel in a Cold Closet
A 12 V addressable panel serves a 42-unit assisted living wing and is configured as a voice evacuation system, so the secondary power calculation must use a 15-minute alarm duration. On site the technician measures 0.480 A of standby current with every initiating device and the panel in normal, and 2.60 A of alarm current with the notification and voice circuits fully driven. The panel and its single 12 V, 18 Ah AGM battery sit in an unheated utility closet that read 55 degrees F that morning; the battery data sheet gives a capacity correction of 1 percent per degree below 77 degrees F. The battery is dated four years ago. The panel's listed charger output is 0.75 A, and the manufacturer permits a maximum battery of 33 Ah in the enclosure.
1. Does the installed battery satisfy the secondary power requirement? Show the calculation and state your recommendation.
Read the worked answer
Standby energy: 0.480 A x 24 h = 11.52 Ah
Alarm energy (voice system, 15 min): 2.60 A x 0.25 h = 0.65 Ah
Subtotal: 12.17 Ah
With the manufacturer's 1.20 aging/spare factor: 12.17 x 1.20 = 14.6 Ah required at 77 degrees F
Available capacity at 55 degrees F: 22 degrees below 77 x 1 percent = 22 percent loss, so 18 Ah x 0.78 = 14.0 Ah
14.0 Ah available is less than 14.6 Ah required, and that figure assumes a new battery. At four years old the battery has already lost capacity, so treat the install as non-compliant and replace it. Confirm with a load test before removal so the record shows the as-found condition, and re-measure the standby and alarm currents at the time of replacement rather than reusing nameplate values.
2. The owner asks whether a 26 Ah battery can simply be dropped in for more margin. What must you verify before agreeing?
Read the worked answer
Three things, in this order.
Physical and listing limits. 26 Ah is under the 33 Ah maximum, but confirm it fits the enclosure and that the panel's battery cable and terminals are rated for it.
Recharge capability. The charger makes 0.75 A while the panel draws 0.480 A in standby, leaving about 0.27 A net. The energy removed in a full 24-hour plus 15-minute event is 12.17 Ah, which needs roughly 0.25 A to restore within 48 hours. That passes, but only barely, and it will not pass at all if the standby load grows. Get the manufacturer's recharge data rather than assuming a straight amp-hour division.
Temperature. A bigger battery does not fix a 55 degree closet. Charging voltage must be temperature-compensated or the battery will never reach full charge, and a 26 Ah battery in a cold closet may perform worse over time than a correctly sized one in conditioned space.
A 20 Ah battery (15.6 Ah derated) would satisfy the load with less charger strain; the more durable fix is to reduce standby load or move the panel and battery into conditioned space.
3. What changes if the authority having jurisdiction accepts the 5-minute alarm duration used for non-voice systems?
Read the worked answer
Compute it rather than argue it:
- Alarm energy at 5 minutes: 2.60 A x 0.0833 h = 0.22 Ah
- Subtotal: 11.52 + 0.22 = 11.74 Ah
- With the 1.20 factor: 14.1 Ah required
- Available in the cold closet: 14.0 Ah
The result is still short, so the duration argument does not rescue the installation. That is worth saying out loud to the owner, because it removes the temptation to negotiate the duration instead of replacing the battery. It also means your recommendation does not depend on how the AHJ reads the duration rule, which is a stronger position if the decision is questioned later. Note that for a voice system the 15-minute duration is not optional in the first place.