NICET Fire Alarm Secondary Power: How to Size Standby Batteries Under NFPA 72 (2026)
Size fire alarm standby batteries for the NICET exam: 24-hour standby, 5-minute alarm, the derating factor, and a full worked amp-hour example under NFPA 72 Chapter 10.
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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 Battery Sizing Shows Up on Every NICET Exam
If you sit a NICET fire alarm exam and don't get at least one secondary-power battery calculation, count yourself lucky — because most candidates get two or three. It's the perfect exam question: it has a right answer you can defend to a code section, it rewards people who actually do the work in the field, and it punishes people who memorized a formula without understanding the two loads underneath it. Here's the concept the whole thing rests on. Every fire alarm system needs two power sources under NFPA 72 Chapter 10: a primary supply (usually the building's commercial AC) and a secondary supply (almost always a sealed lead-acid battery set, sometimes a generator). When the power goes out, the batteries have to keep the panel alive long enough for the building to be evacuated and for someone to notice the outage and respond. Sizing those batteries is a two-part current problem, and the exam tests whether you know that the panel draws a different amount of current when it's sitting quietly versus when it's in full alarm. Get that distinction and the math is mechanical. Miss it and every number you write down is wrong. You can drill this exact calculation style free in your browser at /questions/fire-alarm before you ever pay to sit the real thing.
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The Two Loads: Standby vs. Alarm
A fire alarm control unit lives in two states, and each has its own current draw that you'll find on the manufacturer's battery calculation sheet or spec data. Standby (supervisory) current is what the panel pulls doing nothing but watching — powering its own electronics, supervising every circuit, keeping detectors polled. It's a small number, often a fraction of an amp, but it runs for a long time. Alarm current is what the system pulls when it's actually in alarm — every notification appliance firing, strobes flashing, horns sounding, relays energized. It's a much bigger number, but it only has to run for a few minutes. The durations are the part the exam wants you to have cold. Under NFPA 72 (Section 10.6.7.2 in recent editions), a protected-premises system must carry 24 hours of standby, followed by 5 minutes of full alarm for a standard system. That 5 minutes jumps to 15 minutes of alarm for emergency voice/alarm communication (voice evacuation) systems. That single swap — 5 vs. 15 minutes — is one of the most common distractors on the exam, so read every question for the word 'voice.' Note also that some special cases (certain central-station and supervising-station arrangements, or systems without monitored primary power) carry different standby hours; the 24-hour figure is the standard protected-premises number and the one the exam defaults to.
The Calculation, Step by Step
Sizing the battery is just converting two currents into ampere-hours (Ah) and adding them. Ampere-hours is current multiplied by time in hours — that's the whole unit. Step 1, standby Ah: multiply the total standby current by 24 hours. Step 2, alarm Ah: multiply the total alarm current by the alarm time in hours. Five minutes is 5 ÷ 60 = 0.083 hours. Fifteen minutes is 0.25 hours. This is where people fumble — they multiply amps by '5' and get a number 12 times too big. Alarm time is always converted to hours. Step 3: add them for the raw total Ah. Step 4: apply the derating factor to get the required battery capacity. Here's a worked example. Say the panel's standby draw is 0.5 A and its alarm draw is 3.0 A, on a standard (non-voice) system. Standby: 0.5 A × 24 hr = 12.0 Ah. Alarm: 3.0 A × 0.083 hr = 0.25 Ah. Raw total: 12.0 + 0.25 = 12.25 Ah. Notice how the standby portion dominates. Because standby runs 24 hours and alarm runs 5 minutes, the standby number almost always drives the battery size. That's a useful sanity check: if your alarm Ah comes out bigger than your standby Ah on a standard system, you converted minutes wrong.
The Derating Factor Everyone Forgets
Twelve-point-two-five amp-hours is not the battery you order. A battery loses capacity as it ages, and its rated capacity assumes ideal temperature — neither of which describes a battery in year four inside a hot mechanical room. So you apply a safety/derating multiplier before selecting the battery. The common design practice is to multiply the total by 1.25 (a 25% margin) to cover aging, temperature, and code margin; many engineers use 1.20 (20%) as the aging allowance. Using 1.25 on the example above: 12.25 Ah × 1.25 = 15.3 Ah required. So you'd specify a battery rated at least 15.3 Ah — meaning a 16 Ah or 18 Ah battery, whatever the next standard size up is. You never round down to a battery smaller than the calculated requirement. On the exam, if the calculated value is 15.3 Ah and the choices are 12, 14, 18, and 20 Ah, the answer is 18 — the smallest battery that still meets or exceeds the number. Read each question carefully, because some NICET items ask for the raw calculated Ah (no derating) and some ask for the selected battery size (derating applied). The prompt tells you which. If a derating percentage is given in the question, use exactly that one rather than your habit.
Common Mistakes That Cost Points
The five that catch people every cycle. First, multiplying alarm current by minutes instead of hours — convert to hours first, 5 min = 0.083 hr. Second, using 5 minutes on a voice system — voice evacuation is 15 minutes, so scan for the word 'voice.' Third, forgetting the derating factor when the question asks for the selected battery, or applying it when the question asks for the raw calculated value. Fourth, rounding down to a battery below the required Ah — always round up to the next standard size. Fifth, mixing standby and alarm currents — using the alarm current for the 24-hour leg, or vice versa; label your two currents before you touch the calculator. This calculation is a close cousin of the notification-appliance voltage-drop and NAC-loading math tested alongside it. If secondary-power sizing feels shaky, the broader circuit-calculation set is worth a dedicated pass — those calculation items are where extra study time reliably converts into points on exam day.
The Bottom Line
Secondary-power battery sizing looks intimidating and is actually one of the most learnable points on the NICET exam, because it's the same four steps every single time: standby Ah (current × 24 hr), alarm Ah (current × time in hours), add, then derate and round up. Know the durations cold — 24-hour standby, 5-minute alarm, 15-minute voice — keep your minutes-to-hours conversion clean, and remember which number the question is actually asking for. Drill it until it's automatic. Try free NICET-style fire alarm practice questions on VoltExam at /questions/fire-alarm to build speed on the calculation items, work structured review by content area at /study/fire-alarm, and when you're ready to train daily on your phone, download Fire Alarm Prep at /apps/fire-alarm. The candidates who pass are the ones who've done this math a hundred times before it counted. Editions note: NFPA 72 section numbering for secondary power (Chapter 10) has shifted across the 2016, 2019, and 2022 editions. The 24-hr/5-min/15-min durations are stable, but confirm the exact section reference against the edition your NICET exam is written to — the exam is open-reference.
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