Battery Capacity and Dedicated Supply for an Expanded FACP
A three-story, 45,000 sq ft office building in Durham, North Carolina, has a 24 VDC addressable fire alarm control panel. The existing sealed lead-acid batteries are 12 Ah, 5 years old. The contractor’s load calculation shows 0.35 A standby and 2.40 A alarm. The owner wants to add four duct smoke detectors and two notification circuits, which will add 0.25 A standby and 0.80 A alarm. The contractor proposes to keep the existing 12 Ah batteries and install an 18 Ah battery if the existing one fails the load test. The panel charger is rated for 24 VDC at 1.5 A. The building has a central station connection and is fully occupied during weekdays. The contractor uses 24 h standby plus 5 min alarm for the battery calculation and applies a 20 percent derating factor for aging and end-of-life capacity.
1. Calculate the required battery capacity for the expanded system using 24 h standby plus 5 min alarm and a 20 percent derating factor. Is an 18 Ah battery acceptable?
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Required capacity equals standby load times 24 hours plus alarm load times 5/60 hour, then divided by 0.80. For the expanded system, standby is 0.35 + 0.25 = 0.60 A and alarm is 2.40 + 0.80 = 3.20 A. The ampere-hour total is (0.60 × 24) + (3.20 × 5/60) = 14.40 + 0.267 = 14.667 Ah. Dividing by 0.80 gives 18.33 Ah required. An 18 Ah nameplate battery is less than 18.33 Ah, so it fails the calculation even before considering that capacity at the 24-hour discharge rate may be lower than the 20-hour rating. The contractor should use a 24 Ah battery or larger, verify that it fits the panel enclosure, and verify that the charger can recharge it within the required time. The existing 5-year-old 12 Ah battery also cannot be retained merely because it passes a quick load test; it must be capacity-tested and likely replaced. Use the battery manufacturer’s data for the actual discharge rate and temperature.
2. The installer proposes to tap the FACP 120 VAC supply from a nearby convenience outlet circuit that also serves a janitorial closet GFCI. What is wrong and what is required?
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The fire alarm control panel must be supplied by a dedicated branch circuit that serves no other loads, with a means to lock the breaker in the on position and a permanent marking identifying the fire alarm circuit. A shared convenience circuit is not dedicated and creates a risk that a janitor or maintenance worker will trip the circuit while the fire alarm is needed. GFCI protection on that shared circuit is not a substitute for a dedicated supply and may introduce nuisance tripping. The circuit must be installed in accordance with NFPA 70 and NFPA 72, and the disconnect must be accessible only to authorized personnel. The contractor should correct the design before energizing the panel, not rely on a plug-in transformer or an extension cord. The panel’s secondary power and charger must also be verified after the supply is corrected.
3. After the battery replacement, what secondary power tests and records should the contractor provide?
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The contractor should verify charger voltage and current, perform a battery load test, and confirm that the panel transfers to battery on loss of AC and returns to normal without trouble. The test should demonstrate that the standby load is supported for the calculated duration and that the alarm load is supported for the required alarm time. Record battery make, model, rating, date installed, open-circuit voltage, load-test result, and charger readings. The record should also note the calculated standby and alarm loads and the derating factor. The central station should be notified before any AC interruption, and the account should be placed on test to avoid a false dispatch. The inspection report should be signed and kept with the system records and made available to the AHJ. Retain the records for the period required by NFPA 72 and the AHJ, and schedule re-testing.