6.3 Battery Sizing and Secondary Power

Battery Sizing and Secondary Power

When primary power is lost, the secondary battery must carry the system. The NFPA 72 sizing equation combines load, time, and a safety margin:

Required capacity (Ah) = [ (Standby amps × Standby hours) + (Alarm amps × Alarm hours) ] × Derating factor

Each term:

• Standby hours = 24 for protected premises.
• Alarm hours = 0.0833 (5 min, general) or 0.25 (15 min, voice/ECS).
• Derating factor = 1.25 under NFPA 72-2022 (10.6.10.1); 1.20 under NFPA 72-2019 and earlier. The factor accounts for battery aging, temperature, and capacity loss over the battery's service life. (This is a correction to flag: older study material — including the prior draft of this guide — used 1.20; the 2022 edition the FAS exam references uses 1.25.)

After computing required amp-hours, you select the next standard battery rating at or above the result, never below. Common sealed lead-acid (SLA) sizes are 4, 7, 7.2, 10, 12, 18, 26, 40, and 55 Ah.

Fully Worked Battery Calculation

Using the schedule from 6.2: total standby = 0.355 A, total alarm = 2.295 A, general fire alarm (5-minute alarm), NFPA 72-2022 in force.

Battery calculation worksheet

Step-by-step in words:

1. Standby leg: 0.355 A × 24 h = 8.520 Ah (the standby load dominates because it runs for a full day).
2. Alarm leg: 2.295 A × 0.0833 h = 0.191 Ah (large current, tiny time, so a small contribution).
3. Subtotal = 8.520 + 0.191 = 8.711 Ah.
4. Apply derating: 8.711 × 1.25 = 10.89 Ah required.
5. Select the next standard size ≥ 10.89 Ah → a 12 Ah battery set.

Note the result is 24 V at 12 Ah, normally two 12 V/12 Ah batteries in series. Series adds voltage, not capacity, so the set is 24 V/12 Ah — not 24 Ah.

Trap 1 — series capacity: assuming two 12 V batteries double the amp-hours. They double voltage; capacity stays the same. Trap 2 — charger vs battery: the question may give a charger output; battery sizing compares required Ah to battery Ah, while the charger must independently be able to recharge the battery within the NFPA 72 recharge time (commonly 48 hours to full). Trap 3 — wrong derating: using 1.20 when the 2022 edition (1.25) governs, which under-sizes the battery. If a battery passes at 1.20 but fails at 1.25, the 1.25 answer is the safe choice under the listed edition.

A shop-drawing review may embed this calc: the exhibit shows battery size, standby and alarm currents, NAC load, supply model, and wire distance. If the stem asks what is missing, the answer is often the calculation/worksheet or the derating basis — not a new device.

Secondary-Power Sources, Recharge, and the Charger

The secondary supply does not have to be a battery alone. NFPA 72 recognizes (a) a storage battery with an automatic charger, or (b) an engine-driven generator with a battery dedicated to starting/operating the system until the generator carries the load. In practice, the battery-plus-charger arrangement is the norm for protected-premises systems, so understand both halves:

• The battery must carry the full 24 h + alarm demand (the calculation above).
• The charger must independently be able to recharge a fully discharged battery to its full rated capacity within the time the standard allows (commonly 48 hours). A charger that is sized only for float maintenance, not recharge, is a design defect even if the battery itself is large enough.

This is why "battery sizing" and "charger sizing" are two questions. A stem that gives you a charger output current and asks for battery capacity is testing whether you compare required amp-hours to battery amp-hours, not to the charger.

Worked Variation: Voice/ECS System (15-Minute Alarm)

Re-run the same loads as a voice/alarm communication system, where the alarm leg becomes 15 minutes = 0.25 h:

• Standby leg: 0.355 A × 24 h = 8.520 Ah (unchanged).
• Alarm leg: 2.295 A × 0.25 h = 0.574 Ah (about three times the 5-minute value).
• Subtotal = 8.520 + 0.574 = 9.094 Ah.
• × 1.25 derating = 11.37 Ah required → still a 12 Ah battery here, but a heavier system could be pushed to the next size.

The takeaway: switching from a 5-minute to a 15-minute alarm increases only the alarm leg, and because the standby leg dominates, the change is often modest — but you must still use the correct duration the stem specifies. Trap 4 — wrong alarm duration: applying 5 minutes to a voice/ECS system (or 15 minutes to a general system) is a common error that the exam deliberately sets up by describing the system type in the stem and expecting you to choose the matching duration.