Battery Sizing and Voltage Drop on a Campus Panel with Remote Booster
A school campus has a main fire alarm control panel in the administration building and a remote booster power supply in a gymnasium 900 feet away via a 14 AWG circuit. The panel is rated 24 VDC, 6 A total, with 4 A available for notification. The gym booster drives 18 horn/strobes at 0.15 A each in alarm and has a 7 Ah battery. The contractor's calculation shows the booster draws 2.7 A in alarm and the 14 AWG run has a round-trip resistance that produces a 3.1 V drop at that current. The panel's low-battery trouble has been reported during monthly tests, and the booster has never had a load test. The AHJ wants the battery and voltage-drop calculations before the next inspection.
1. Evaluate the voltage drop on the 14 AWG feeder to the booster and its effect on the system.
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At 2.7 A, a 3.1 V drop on a 24 VDC circuit leaves about 20.9 V at the booster, which is below the typical operating range for many 24 VDC appliances and may cause the booster to drop out or the panel to report trouble. The designer should recalculate using the actual round-trip resistance of 14 AWG over 900 feet, confirm the booster's minimum operating voltage, and increase conductor size or relocate the booster if the drop exceeds the allowable percentage. The calculation should be based on worst-case alarm current, not standby current, and should be documented for the AHJ.
2. Is the 7 Ah battery adequate? Show the reasoning a reviewer should apply.
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The reviewer must calculate standby plus alarm load for the required duration. If the booster draws 2.7 A in alarm and has a standby load, the 7 Ah battery may be undersized once derating and the required alarm duration are applied. The calculation should use the manufacturer's battery derating factor, the required standby time (typically 24 hours) plus alarm time (typically 5 minutes for the booster's load), and the actual current draw. The contractor should provide the completed calculation; if the result exceeds the battery capacity, a larger battery or additional power supply is required.
3. What does the repeated low-battery trouble during monthly tests suggest, and what should be done?
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Repeated low-battery trouble suggests the battery is undersized, aged, or not being charged properly, or that the charger cannot recover the battery between tests. The technician should load-test the battery, verify charger output and charging current, check for loose connections, and confirm the battery calculation matches the installed battery. If the battery fails the load test, it should be replaced and the cause of the repeated trouble documented.