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DE Fire Alarm study materials
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DE Fire Alarm: Power Supplies, Batteries, and Secondary Power Calculations

DE Fire Alarm · Work through your exam, one topic at a time.

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Power Supplies, Batteries, and Secondary Power Calculations

This domain tests whether the system keeps working when the utility fails. You need the primary supply rules, the standby-plus-alarm duration, and the arithmetic of a battery calculation including the derating factor.

Primary and secondary power

  • Primary power is a dedicated branch circuit, protected by overcurrent device, marked for fire alarm use, and accessible only to authorized personnel.
  • Secondary power is typically sealed lead-acid or nickel-cadmium batteries, or a generator. Secondary supply must carry the system under maximum normal load for 24 hours and then operate it under alarm conditions for 5 minutes. Voice evacuation systems require 15 minutes of alarm operation instead of 5.
  • Special occupancies or AHJ conditions can extend standby to 60 hours.
  • The charging means must be listed and sized to restore the battery in the time the manufacturer and code allow (commonly 4 hours).

Battery calculation method

  1. Sum standby current for all equipment (panel, annunciators, modules, transponders) and multiply by 24 hours.
  2. Sum alarm current for all equipment, including notification appliances and the panel itself, and multiply by 5/60 hour (0.0833) or 15/60 (0.25) for voice.
  3. Add the two results.
  4. Multiply by 1.20 for battery aging and discharge-rate derating.
  5. Select the next standard battery size above the result (4.5, 5, 7, 7.5, 8, 12, 18, 26 Ah).

Worked example

Standby load: panel 0.12 A + three NAC extenders at 0.045 A = 0.135 A + annunciator 0.035 A, total 0.29 A. Standby capacity = 0.29 × 24 = 6.96 Ah. Alarm load: panel 0.20 A + 40 addressable devices at 0.3 mA = 0.012 A + notification appliances 1.64 A, total 1.85 A. Alarm capacity = 1.85 × 0.0833 = 0.154 Ah. Sum = 7.114 Ah; × 1.20 = 8.54 Ah. The next standard size is 12 Ah, so a 12 Ah battery is specified. Then verify the charger can recharge 12 Ah within the allowed time and that battery-lead voltage drop at 1.85 A does not pull panel terminals below their minimum.

Traps

  • Omitting the 1.20 derating factor.
  • Using the 5-minute alarm duration on a voice evacuation system that needs 15 minutes.
  • Leaving the panel's own alarm current out of the alarm total.
  • Choosing a battery that equals the calculated value instead of the next size above.