Fire Alarm Primary and Secondary Power for the NICET Exam: 24-Hour Standby, 5-Minute Alarm, and Battery Rules (NFPA 72, 2026)
Fire alarm power supplies for the NICET exam: 24-hour standby plus 5-minute alarm, the longer voice-system load, generator options, and AC-loss trouble logic.
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This guide solves the next-step problem for Fire Alarm candidates: it explains what matters, then gives you a direct way to test that knowledge with practice questions instead of guessing what to study next.
Why Power Supplies Are Easy Points on the NICET Exam
Power questions are some of the most predictable on the NICET Fire Alarm Systems exams, because they test a short list of numbers and one idea: a fire alarm system has to keep working when the building loses power, and it has to tell someone when it has. NICET's current exam page says the FAS exams are based on **NFPA 72 (2022)**, and power supply requirements live in the chapter on fundamentals (Chapter 10 in that edition). Check the current NICET Fire Alarm Systems exam page for the edition your test is written to, because a change of edition is the place figures and section numbers shift. The idea to hold onto is two independent sources. The **primary supply** is normally the building's commercial light and power, on a dedicated branch circuit with a locked-out disconnect so nobody switches it off by accident. The **secondary supply** is what carries the system through an outage, usually rechargeable batteries inside or beside the control unit, though an engine-driven generator can play that role under the conditions below. When the primary fails, the panel must transfer to the secondary and report a trouble condition. It does not go quiet and it does not wait for the batteries to die before saying something.
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Fire Alarm · Question 1 of 5
What is the primary difference between a conventional and an addressable fire alarm system?
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The Numbers to Memorize: 24 Hours, 5 Minutes, 15 Minutes
The classic secondary-supply rule for a protected premises system is **24 hours of standby followed by 5 minutes of alarm**. Standby means the system sitting there quietly, supervising everything, with no alarm sounding. Alarm means every notification appliance the system is designed to operate, running at full load. The battery has to cover both, in that order, so the exam loves to ask what happens at hour 23 when a fire starts. The answer is that the system must still operate its alarm load for the required alarm period. Systems that carry **emergency voice/alarm communications** use a longer alarm period, commonly cited as **15 minutes**, because speakers and amplifiers draw far more current than horns and strobes and an evacuation message needs to run to completion. Supervising-station signaling (central station, remote station) can carry longer standby figures in the code, and those numbers differ by system type and edition. For those, confirm the figure in the edition your exam uses rather than trusting a flashcard. Generators are the other source of exam variation. Under the code's secondary-supply options, an engine-driven generator can serve as the secondary source in place of a full battery capacity, but the usual pairing is a **shorter battery backup, commonly cited as 4 hours, to bridge the generator's start-up and transfer**. The test point is that the generator is not free of conditions. It must be arranged to start and transfer, and the battery still covers the gap. Verify the exact generator alternatives in your edition before the exam.
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Sizing the Battery: Standby Load, Alarm Load, and Margin
Battery questions are arithmetic with a code wrapper. The sequence is the same every time. Add up the standby current of every device and the panel itself and multiply by 24 hours to get amp-hours. Add up the alarm current with every notification appliance on and multiply by the alarm period in hours (5 minutes is 5/60 of an hour; 15 minutes is a quarter of an hour). Add the two. Then apply the **derating or safety margin** your reference calls for, because batteries do not deliver their full nameplate capacity at the end of life and in cold rooms. Many study references use a 20 percent margin, but treat that as a manufacturer and reference convention to confirm, not a number to assume the exam gives you. A worked example makes the pattern stick. Say standby is 0.25 A and full alarm is 3.0 A. Standby is 0.25 x 24 = 6.0 Ah. Alarm for 5 minutes is 3.0 x (5/60) = 0.25 Ah. Total 6.25 Ah before margin. With a 20 percent margin that becomes 7.5 Ah, so you would select the next standard battery size up. If this were an emergency voice system with a 15-minute alarm period, the alarm term becomes 3.0 x 0.25 = 0.75 Ah and the total climbs to 6.75 Ah before margin. Notice how little the alarm term matters compared to 24 hours of standby: the standby current is where most of the capacity goes, which is why one extra always-on device is more costly than it looks. For deeper practice on this, our [circuit calculations guide](/blog/nicet-fire-alarm-circuit-calculations-battery-nac-voltage-drop-2026) covers battery sizing and voltage drop side by side.
Trouble Signals: What the Panel Must Tell You
The second half of the topic is signaling. Loss of the primary supply, a failed or disconnected battery, a low battery, and a charger failure are all **trouble** conditions, not alarms and not supervisory signals. The reason is the priority logic from signal types: trouble means the system's ability to do its job is impaired. A panel running on batteries during an outage is still protecting the building, but it has lost its safety margin, and the code wants a person to hear about it. The exam will often hand you a scenario and ask for the signal. Power company outage, panel on batteries, AC LED off: trouble. Battery disconnected during maintenance: trouble. Battery voltage sagging below the panel's low threshold: trouble. A smoke detector in alarm during an outage: alarm, and it must still be annunciated and still operate the notification appliances from the secondary supply. The panel does not suppress alarms because it is on batteries. Timing details, such as how quickly loss of primary power must produce a trouble indication and what the battery charger must restore after discharge, are stated in the code with specific figures that I have not independently verified for the 2022 edition for this overview. Look them up in section 10.6 of your edition and put them on your own flashcard with the section number so the note stays auditable.
Common Mistakes on Power Supply Questions
Candidates lose points in four predictable ways. First, they size for 24 hours of alarm instead of 24 hours of standby plus the short alarm period. Second, they forget the voice system rule and use 5 minutes for a panel that drives speakers. Third, they classify AC loss as supervisory because the word sounds right; it is trouble. Fourth, they treat a generator as removing the need for any battery, ignoring the bridging battery. A fifth, quieter error is mixing up minutes and hours in the arithmetic, so write the unit beside every number as you work. A last trap is the dedicated circuit. The primary supply should be on a dedicated branch circuit, protected from accidental shutoff, and the disconnect should be marked. If a scenario shows the panel fed from a shared lighting circuit with no marking, expect the answer to point out the deficiency.
Study Strategy and Next Steps
Build a one-page power card with five lines: primary and secondary definitions, 24 hours plus 5 minutes, 24 hours plus 15 minutes for voice, the generator-plus-battery idea, and a short table of trouble conditions. Then do ten battery-sizing problems with different loads until the arithmetic is automatic, writing units each time. Add the section locator from your edition to each line so you can defend every number. Practice in the [Fire Alarm Prep app](/apps/fire-alarm) and work through the [NICET fire alarm practice questions](/questions/fire-alarm) on VoltExam, which mix power, signaling, and circuit questions the way the real exam does. Download Fire Alarm Prep and make the power numbers automatic before exam day.
Sources
Checked 2026-10-17. - Dynafire: Fire Alarm System Battery Backup Requirements (24 hours standby, 5 minutes alarm, 15 minutes for emergency voice/alarm) - Security Sales & Integration: Understanding Secondary Power Supplies (generator and 4-hour battery option, cited from an older NFPA 72 edition) - Figures not independently verified for NFPA 72 (2022): 20 percent margin, AC-loss trouble timing, charger recovery time. Confirm in section 10.6 of your edition.
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