By Fire Alarm BT, Founder, Fire Alarm Blueprint Trainer · Creator of the FireAlarmBT YouTube channel · reviewed against NFPA 72 & NFPA 70 principles
Fire Alarm Panel AC Fail and Low Battery Trouble: Secondary Power Troubleshooting
A panel that has been running on batteries all weekend is a secondary-power story with two chapters: why the primary power went away, and whether the batteries were ever sized and maintained to carry the load. This case study covers both, including the NEC 760 branch-circuit rules and the NFPA 72 standby requirements.
The setting
Retail strip center, conventional FACP in the electrical room. Monday 7:15 AM service call from the monitoring company.
TROUBLE: AC POWER FAIL
LOW BATTERY · 23.1 VDC · ON SECONDARY
What the technician found on arrival
The panel shows AC POWER FAIL and LOW BATTERY. The store manager says the lights are on, so 'it can't be the power.' The monitoring log shows AC FAIL at Saturday 02:10 followed by LOW BATTERY Sunday 04:45.
The question
Two troubles, one panel, and a building that clearly has power. What do you do?
Step-by-step: what the technician did and why
- 1
Acknowledge and read the timeline
AC fail followed ~26 hours later by low battery is the signature of a healthy battery set carrying the system through a real outage — the panel did exactly what it should.
Finding26.5 hours from AC fail to low battery — just past the 24-hour standby requirement.
- 2
Check the dedicated fire alarm branch circuit
The FACP must be on its own branch circuit with a red, labeled, lockable breaker. Building lights being on proves nothing about that one breaker.
FindingBreaker 'FIRE ALARM — DO NOT TURN OFF' in Panel LP-2 is tripped. No lock-out device installed.
- 3
Find out why the breaker tripped before resetting it
A tripped breaker can mean a fault in the panel's transformer or power supply. Check for shorts before reapplying power.
FindingNo short at the panel primary. The manager confirms a brief utility outage Saturday night; the breaker nuisance-tripped on the surge.
- 4
Restore AC and verify the charger
With AC back, confirm the panel transfers off battery and the charger is charging at the correct float voltage.
FindingAC trouble clears. Charger output 27.3 VDC; batteries charging at 1.1 A.
- 5
Evaluate the batteries — do not just clear the trouble
Deep-discharged SLA batteries may not recover. Check date codes, load-test after recharge, and compare to the required standby + alarm capacity.
FindingBatteries dated 5 years 2 months ago — past service life. After a 4-hour charge, voltage sags to 20.8 V under alarm load. Replace.
- 6
Replace, recalculate, and secure the breaker
Install correctly sized batteries per the standby calculation, date-label them, and add a breaker lock so this doesn't repeat.
FindingInstalled 2× 12 V 18 Ah; the standby calculation requires 15.2 Ah with 20% margin. Breaker lock-out installed and labeled.
Key takeaways
- •AC fail → low battery about 24 h later means the batteries did their job.
- •Always find the dedicated FA breaker — 'the lights are on' is not evidence.
- •Find out why a breaker tripped before you reset it.
- •Any deep discharge on batteries past 4–5 years = replace and recalculate.
Code references
NFPA 72 · 10.6.7 — Secondary Power Supply
Secondary supply must carry the system for 24 h in standby followed by 5 min in alarm (15 min for EVAC systems).
NFPA 72 · 14.4 / Table 14.4.3.2 — Batteries
Charger test, discharge/load test, and voltage checks on schedule; replace when a test fails or per manufacturer.
NFPA 70 (NEC) · 760.41(B) / 760.121(B)
FACP branch circuit must be dedicated; disconnecting means identified in red, marked 'FIRE ALARM CIRCUIT', accessible only to qualified personnel.
Section numbers vary between editions. Always verify against the edition adopted by your Authority Having Jurisdiction (AHJ).
Sample service report
Found
07:20 FACP TROUBLE: AC POWER FAIL (Sat 02:10) and LOW BATTERY (Sun 04:45). Panel on secondary power on arrival, 23.1 VDC.
Cause
Dedicated FA breaker LP-2/17 tripped during utility surge; no lock-out. Batteries (2× 12 V 18 Ah, dated 5+ yrs) deep-discharged and failed load test after recharge.
Action taken
Reset breaker after verifying no fault; installed breaker lock-out and red label. Replaced batteries with 2× 12 V 18 Ah (standby calc 15.2 Ah incl. margin), date-labeled.
Verification
AC normal; charger 27.3 VDC. Battery load test passed (24.6 V under alarm load). Panel NORMAL 10:05. Recommend owner add FA breaker to post-outage checklist.
Practice this call in the Troubleshooting Lab
Work through the same scenario step by step, with wrong-move feedback and a downloadable completion certificate. Free, no account required.
Frequently asked questions
How long must fire alarm batteries carry the system?
For a typical protected premises system NFPA 72 requires 24 hours of standby followed by 5 minutes of alarm (15 minutes for emergency voice/alarm communications systems). Designers apply a derating factor to the calculated amp-hours and verify against the adopted edition.
Why can't the fire alarm panel share a branch circuit with other loads?
NEC Article 760 requires a dedicated, individual branch circuit for the fire alarm control unit, identified in red at the disconnect and not protected by GFCI or AFCI devices, so that unrelated loads or tripping can't remove primary power from the system.
Related guide: Fire Alarm Battery (Standby) Calculation Basics
This case study is educational. It does not replace the manufacturer's instructions, the currently adopted codes and standards, a licensed designer or engineer, or review by the Authority Having Jurisdiction.