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Fire alarm battery backup requirements (NFPA 72)

Alarm Ladder editorial teamUpdated 3 min read

Fire alarm battery backup requirements come down to three questions: how long the system must run on batteries, what load it must carry for that time, and how the backup is proven to work. NFPA 72 sets the framework, and the edition your authority having jurisdiction (AHJ) enforces, plus any local amendments, sets the project basis. This page explains the questions and where to look. It does not reproduce code text, tables or values.

What the requirements cover

  1. Run time. The batteries carry the system through a standby period and then an alarm period. The durations depend on the system arrangement, for example an ordinary notification system compared with an in-building voice system. Take the numbers from your own copy of NFPA 72, which covers power supplies in Chapter 10.
  2. Load basis. Standby time is multiplied by the standby current of every connected device, and alarm time by the alarm current of everything that is on in alarm. Both loads come from the equipment data sheets.
  3. Capacity correction. A margin and any correction for battery age or temperature are applied before a battery size is chosen. The equipment manual says which corrections it expects.
  4. Charger. It must restore the batteries after an outage and hold them at float, and a charger fault must show as a trouble.
  5. Supervision and checks. The panel reports loss of AC power, a low battery and a missing battery. Service visits measure battery voltage and check age and connections.

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A supplied module drawing labels COM, NO and NC. The door interface needs a switched contact that is open while the relay is at rest and closes when the relay operates. Which pair matches that instruction?

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From requirement to battery size

Once the run times and loads are known, the arithmetic is short: standby amperes times standby hours, plus alarm amperes times alarm hours, then the margin. A fictional panel at 0.4 amperes standby over 24 hours is 9.6 amp-hours before alarm load or margin. The full step order, including how margin differs from derating, is in the battery calculation method. To run your own numbers, use the fire alarm battery calculator.

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Every answer comes with a full explanation.

Common mix-ups

  • Requirement versus calculation. The requirement says how long and what must be supported. The calculation turns that into amp-hours.
  • Edition. An exam reference edition and the edition enforced on a job are separate decisions. Record which one the calculation uses.
  • Remote supplies. A remote power supply with its own batteries needs its own worksheet.
  • Growth. Added devices add current, so redo the sum when the system changes.

The secondary power supply term page defines the backup and how it works. Trouble signals shows how power faults appear at the panel, and the fire alarm terms glossary lists the other power terms.

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