Battery and Charger Capacity After Adding Door Holders and a New NAC
A 24 V dc addressable panel protects a 60,000 sq ft light manufacturing plant. The panel is fed from a 20 A branch circuit and carries two 12 Ah sealed lead-acid batteries installed four years ago. The present calculated standby load is 0.42 A (panel, annunciators, six duct detectors) and the alarm load is 1.65 A. The owner wants to add four magnetic door holders that are energized continuously to hold egress doors open, each drawing 0.25 A at 24 V dc, plus a new notification circuit that adds 0.55 A in alarm. The contractor states that the existing 12 Ah batteries were sized with a 1.2 safety factor and will still be adequate. The fire official requires 24 hours of standby followed by 5 minutes of alarm. The panel charger is rated 1.0 A.
1. Recalculate the required battery capacity with the new loads. State the required ampere-hour rating and explain how the series connection of two 12 V batteries affects the calculation.
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Standby load: 0.42 A + (4 x 0.25 A) = 0.42 + 1.00 = 1.42 A. Door holders are energized continuously, so they belong in standby even though they release during alarm. Standby energy = 1.42 A x 24 h = 34.08 Ah.
Alarm load: 1.65 A + 0.55 A = 2.20 A for 5 minutes = 2.20 x (5/60) = 0.18 Ah.
Subtotal: 34.08 + 0.18 = 34.26 Ah. Apply the 1.2 safety factor: 34.26 x 1.2 = 41.1 Ah required.
Series connection matters: two 12 V batteries in series produce 24 V, but the capacity remains the rating of one battery. A pair of 12 Ah batteries is a 24 V, 12 Ah battery set, not 24 Ah. The existing set provides about 29 percent of the required capacity. The replacement must be a pair of 12 V batteries of at least 41 Ah each, so a standard 55 Ah pair (or the next listed size the panel manufacturer permits) is appropriate. Confirm the selected batteries are listed for fire alarm use and that the enclosure has space and the panel's charging profile matches the battery type.
2. Evaluate the 1.0 A charger against the new load and state what must change.
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The charger must do two things at once: carry the standby load and recharge the battery set within the time the panel listing and NFPA 72 require, which is typically 24 hours.
- Standby load alone is 1.42 A, which already exceeds the 1.0 A charger rating. The charger cannot even hold the system up while the batteries are connected.
- Recharge current for a 41 Ah set over 24 hours is roughly 41/24 = 1.7 A, and that is on top of the standby load, for a total of about 3.1 A.
A 1.0 A charger is inadequate on both counts. The panel's power supply and charger must be upgraded to a unit rated for the connected load plus recharge current, and the calculation must be redone using the manufacturer's published charger and battery data. Also verify the float voltage is correct for sealed lead-acid batteries (commonly about 27.6 V at 25 C with temperature compensation), that the charger is supervised for failure, and that the replacement batteries carry the correct listing. Finally, four-year-old batteries should be load tested before any reuse decision; even if the capacity math had worked, aging and plate degradation would require replacement or a documented load test.
3. The contractor proposes to power the four door holders from the new notification appliance circuit. Identify the operational problem and the correct design approach.
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Putting door holders on a notification circuit creates a life-safety conflict. Notification circuits are silenced at the panel, either automatically or by the responding fire department. When the NAC is silenced, the 24 V dc disappears and every door holder releases, which may be acceptable during an alarm but also happens on any NAC trouble, on a silenced signal for a non-fire event, or during testing. Worse, if the holders are on a NAC that can be silenced before occupants have evacuated, doors that are supposed to hold open on a signal may release at the wrong time, and nuisance releases lead owners to defeat the holders with wedges.
The correct approach is a dedicated, supervised 24 V dc door-holder circuit from the panel's power supply or a listed door-holding power supply, arranged so that the holders release on fire alarm, on loss of the fire alarm power supply, and on manual release, but not merely because the NAC was silenced. The circuit must be electrically supervised and monitored for open and short conditions, and the release function must be coordinated with the building's fire alarm sequence and with the door hardware fail-safe requirements. Recalculate standby load with the holders on that circuit (it is still continuous), and verify that the added load does not push the power supply beyond its rating.