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NFPA 72
Validated against the standard.
PDF export
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$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NFPA 72 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Fire alarm panels must run on battery when utility power fails. This app does the NFPA 72 secondary-power (battery) calculation and checks notification-appliance circuit voltage drop, the two numbers that decide battery size and wire gauge.
What it computes
- Standby current × required standby time (commonly 24 hours), plus alarm current × alarm time (5 or 15 minutes), with a sizing safety factor.
- NAC voltage drop — confirms the last horn/strobe still has its minimum rated voltage at the end of the circuit.
Worked example
Standby 0.25 A for 24 h = 6.0 Ah; alarm 1.5 A for 15 min = 0.375 Ah; total 6.375 Ah × 1.2 safety = 7.65 Ah, so a 7.65 Ah or larger battery set is required.
Related calculators
For the suppression side, see the sprinkler hydraulic calculator and the NFPA reference.
Fire alarm FAQ
How is battery size determined?
NFPA 72 requires the secondary supply to carry the system in standby (often 24 hours) plus a period in alarm (5 or 15 minutes). Sum those amp-hours and add a safety factor to size the battery.
What standby time should I use?
Twenty-four hours is common for many systems, but the required time depends on the system type and the authority having jurisdiction. The app lets you set it.
Why check NAC voltage drop?
Horns and strobes have a minimum operating voltage. Voltage drop on a long notification circuit can starve the last device, so the calculation confirms it still works.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
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