9.2 Secondary Power and Battery Calculations

Why battery math is a high-risk exam skill

Module 2 expects you to power the control unit after the branch circuit dies. Independent OpenExamPrep teaching here is the secondary-power calculation used on fire alarm jobs, not a claim that DOS prints this worksheet on the exam. Chapter 6 previewed watt-hours and ampere-hours on a 150 mA, 24-hour example and stopped before the safety factor. This section finishes the job: inventory every load, convert to ampere-hours (AH), apply ×1.20, and pick a battery that is not smaller than the result.

The written sitting is closed-book. You will not receive a manufacturer spreadsheet. You must keep the units straight: amperes, hours, ampere-hours. Five minutes is not 5 hours. Milliamperes are not amperes. Two 12 V batteries in series do not become “twice the AH.”

Primary power versus secondary power

Primary power is the dedicated alternating-current branch that feeds the FACU (line-voltage landing remains electrician work under 19 NYCRR 195.2; you still need to know that the panel is not meant to live on a lighting switched receptacle). Secondary power is the rechargeable battery (and charger) that must run the system when primary power is lost. The calculation answers: how many ampere-hours must those batteries store so the system can sit in standby and then still go into alarm for the required minutes.

Do not copy the fire-alarm hour pair onto every security control. Burglar panels follow their own listings. Use fire numbers on fire systems and the listing on security systems.

Durations you are expected to apply

As commonly taught from NFPA 72 secondary-power capacity (New York’s 2025 Uniform Code references NFPA 72—22; the DOS bulletin names Uniform Code plus NFPA standards, not a subsection number):

5 minutes = 5/60 hour = 1/12 hour ≈ 0.0833 h. 15 minutes = 15/60 hour = 0.25 h.

Special occupancies can require longer standby. If a problem states a different hour figure, use that figure. If it does not, use 24 h + 5 min for a horn-strobe fire alarm system and 24 h + 15 min for EVACS.

What current to put in each column

Use total current drawn from the batteries in each state:

• I_s (standby current) — FACU electronics, communicator idle, annunciator, SLC or detector power, NAC supervision, and any auxiliary load that remains energized in standby.
• I_a (alarm current) — everything that is on during a full alarm: FACU in alarm, communicator transmitting, annunciator in alarm, device power that remains, and every NAC or speaker circuit that will operate together.

These are not “standby plus extra NAC” unless the worksheet labels a column that way. The formula below uses full I_s for 24 hours and full I_a for the alarm minutes. You do not run 24 hours of alarm current.

Door holders that drop out on alarm are not an alarm-NAC load; if they remain powered from the FACU in standby, they are a standby load. Read the riser.

Required capacity before safety factor:

AH_req = (I_s × T_s) + (I_a × T_a)

• I_s, I_a in amperes
• T_s, T_a in hours
• Result in ampere-hours

With 20 percent safety factor (widely taught NFPA 72 practice):

AH_batt = AH_req × 1.20

Then select the next standard battery whose rating is ≥ AH_batt. Common sealed-lead-acid sizes in this trade include 7 AH, 8 AH, 12 AH, 18 AH, and 26 AH (12 V modules). A 24 VDC FACU typically uses two 12 V batteries in series. Series adds voltage; it does not add ampere-hours. Two 12 V, 8 AH batteries in series are 24 V, 8 AH, not 16 AH.

Full worked example — horn-strobe system (24 h + 5 min)

Teaching nameplates below continue Chapter 6’s 75 mA horn-strobe figure. They are not a particular manufacturer’s sheet. In the field, copy the battery-calculation current from the cut sheet, including strobe current at the voltage you are analyzing.

Standby current inventory (I_s)

Check: 125 + 40 + 35 + 40 + 10 = 250 mA = 0.250 A.

Alarm current inventory (I_a) — worst-case simultaneous NACs

Check: 0.200 + 0.070 + 0.040 + 0.040 + 0.900 + 0.750 + 0.400 = 2.400 A.

Convert to ampere-hours

Standby time T_s = 24 h. Alarm time T_a = 5 min = 5/60 h = 1/12 h.

1. Standby AH = I_s × T_s = 0.250 A × 24 h = 6.00 AH.
2. Alarm AH = I_a × T_a = 2.400 A × (5/60) h = 2.400 × 1/12 = 0.200 AH.
3. AH_req = 6.00 AH + 0.200 AH = 6.20 AH.
4. Apply 20% safety factor: AH_batt = 6.20 AH × 1.20 = 7.44 AH.

Units: amperes × hours = ampere-hours. The 0.200 AH alarm term is small because 5 minutes is a short time; the 24-hour standby term dominates. Omitting NAC current still wrecks the alarm term on a larger voice system, and omitting the communicator wrecks standby on a hungry cellular communicator.

Select the battery

7.44 AH > 7 AH, so a 7 AH battery is too small. 7.44 AH < 8 AH, so the next standard size that works is 8 AH.

For a 24 VDC panel: install two 12 V, 8 AH batteries in series. Label the calculation 8 AH at 24 V, not “16 AH.” If 8 AH is not on the truck and you only have 12 AH modules, two 12 V, 12 AH in series also satisfy 7.44 AH; you may not drop back to 7 AH.

EVACS comparison — 15 minutes and a heavier alarm load

Keep I_s = 0.250 A (standby AH still 6.00 AH). Voice amplifiers raise alarm current. Teaching EVACS alarm current: I_a = 3.000 A (the 2.400 A notification/control load plus 0.600 A of amplifier and speaker-circuit draw).

1. T_a = 15 min = 0.25 h.
2. Alarm AH = 3.000 A × 0.25 h = 0.750 AH.
3. AH_req = 6.00 + 0.750 = 6.75 AH.
4. AH_batt = 6.75 × 1.20 = 8.10 AH.

8.10 AH > 8 AH, so the 8 AH pair that worked for the 5-minute horn-strobe example is not enough. Select the next size, 12 AH (two 12 V, 12 AH in series for a 24 V panel).

That is why EVACS minutes and speaker-amplifier current belong in the same worksheet as the strobes. Using 5 minutes on a voice system under-sizes the bank.

Recharge and voltage, briefly

The FACU charger must restore the batteries after a discharge; NFPA 72 also addresses recharge capability. This section’s exam skill is capacity in AH, not charger design. After 24 hours the battery voltage is lower than float — that is the same reason Section 9.1 told you not to prove NAC drop against 27.6 V float alone.

Exam traps

• Treating 5 minutes as 5 hours: 2.400 A × 5 h = 12 AH of alarm alone, which is the wrong time unit.
• Forgetting to convert mA to A (250 mA × 24 h is 6.00 AH, not 6,000 AH).
• Skipping the 1.20 multiplier (stopping at 6.20 AH).
• Multiplying only standby by 1.20 and dropping the alarm term (0.250 × 24 × 1.20 = 7.20 AH — that is not this example’s answer).
• Using 15 minutes on a horn-strobe problem, or 5 minutes on EVACS.
• Adding AH because two batteries are in series (8 + 8 = 16 AH is wrong for series).
• Sizing from FACU standby only and ignoring NAC and communicator current.

Write I_s, I_a, hours, AH_req, ×1.20, then the next size that is not smaller. Section 9.3 puts that worksheet in the job folder the AHJ and 19 NYCRR 195.10 both expect to see.