13.1 Standby & Alarm Load Basis (24 h + 5/15 min)

13.1 Standby & Alarm Load Basis (24 h + 5/15 min)

Quick Answer: Size secondary power from two currents and two times: standby current × 24 hours, plus alarm current × alarm duration. Alarm duration is 5 minutes for most protected-premises systems and 15 minutes for emergency voice/alarm communications systems (EVACS) and many voice/releasing applications. Convert mA to A before multiplying. Domain 2.3.4 (power supply and loading calculations) tests this inventory math—not vague “put in a big battery.”

Chapter 10 put battery calculations on the shop-drawing package list. Chapter 12 locked drawing notes for voltage drop and fill. This chapter is the numeric engine: load basis (13.1), aging derating and bank selection (13.2), voltage drop (13.3), and conduit fill (13.4). On NICET Fire Alarm Systems Level II, wrong duration (5 vs 15 minutes) and botched unit conversion are classic traps.

Primary vs Secondary Power

When AC is present, the charger maintains the batteries and the system runs from primary. When AC fails, the system must transfer to secondary without losing required functions, and the secondary supply must be sized for the required standby period plus the required alarm period that follows.

Level II submittal work documents what the secondary must support, not only that “batteries are provided.” AHJs and plan reviewers expect a worksheet tied to the device schedule and cut sheets.

What “Standby Load” Means

Standby current (also called quiescent or supervisory current) is the continuous current drawn while the system is normal: monitoring initiating devices, supervising pathways, powering the control unit logic, remote annunciators, network cards, and any other loads that stay on before an alarm.

Build a standby current inventory:

1. Control unit base (main board, display, network, dialer/communicator as applicable).
2. Every initiating device and module on the SLC (or conventional zone loads).
3. Remote power supplies and booster panels in standby (including their own electronics).
4. Annunciators, printers, and other continuous accessories on battery-backed power.
5. Any continuously energized auxiliary loads the secondary is required to support.

Use manufacturer data at the system voltage (for example 24 VDC). Do not invent milliamp values. If a device draws different currents in different modes, use the standby-mode figure for the standby column.

Exam discipline: Notification appliances usually draw near-zero or negligible current in standby if they are silent and unlit. Their big draw belongs in the alarm column when the NAC energizes them.

What “Alarm Load” Means

Alarm current is the current under the design alarm condition used for secondary-power sizing—typically all notification appliances required to operate on battery-backed power, plus control-unit alarm-mode current, plus initiating/module currents that still apply, plus any other loads that run in alarm (door holders released may drop off; amplifiers and speakers on EVACS surge).

Build an alarm current inventory:

1. Control unit in alarm (often higher than standby).
2. Initiating devices/modules still powered.
3. Full notification load on each battery-backed NAC or amplifier channel (sum appliance currents from cut sheets at the candela/setting used).
4. Voice amplifiers, digital message repeaters, and speaker circuits for EVACS.
5. Releasing, fan-control, or other alarm-mode outputs if the secondary must power them for the alarm period.

If a remote NAC power supply has its own batteries, size that supply’s secondary separately from the FACU battery—but the load basis method is the same: standby + alarm with the correct duration.

Required Durations (Know Cold)

These numbers are the load-basis times used in secondary-supply calculations for exam and submittal work. Always confirm the adopted NFPA 72 edition and any special system chapter requirements for the occupancy—Level II is open-book, but you must know where the 5-minute vs 15-minute split lives and that voice systems are not the 5-minute default.

Trap: Using 5 minutes for a voice-evac system understates alarm amp-hours and produces an undersized battery. Using 15 minutes for a plain horn/strobe protected-premises system oversizes (safer but wrong if the question asks for the required basis).

Unit Conversion: mA → A → Ah

Battery capacity is in amp-hours (Ah).

1. Sum currents in mA if that is how the cut sheets read.
2. Convert: I (A) = I (mA) ÷ 1000.
3. Multiply by time in hours:
   • Standby Ah = I_standby (A) × 24
   • Alarm Ah = I_alarm (A) × (5/60) for 5 minutes, or × (15/60 = 0.25) for 15 minutes

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

Never multiply milliamp totals by 24 and call the result “Ah” without dividing by 1000. That error is off by a factor of one thousand.

Preliminary load Ah (before aging derating in 13.2):

Calculated Ah = Standby Ah + Alarm Ah

Section 13.2 multiplies that total by 1.20 (20% aging). Do not skip straight to catalog battery size without both steps unless the question already baked them in.

Worked Example — Device Table (Protected Premises, 5 min Alarm)

A small addressable protected-premises system (no EVACS) is powered from a 24 V control unit with onboard NAC capacity. Manufacturer currents (illustrative for method):

Convert:

• I_standby = 209 mA ÷ 1000 = 0.209 A
• I_alarm = 2919 mA ÷ 1000 = 2.919 A

Standby Ah = 0.209 A × 24 h = 5.016 Ah
Alarm Ah (5 min) = 2.919 A × (5/60) h = 2.919 × 0.08333 ≈ 0.243 Ah
Calculated load Ah = 5.016 + 0.243 = 5.259 Ah

(Section 13.2 will apply × 1.20 → about 6.31 Ah required before selecting the next standard battery size.)

Same Loads as EVACS (15 min Alarm) — Compare

If the same currents applied to a voice system requiring 15 minutes of alarm:

Alarm Ah = 2.919 × 0.25 = 0.730 Ah
Calculated load Ah = 5.016 + 0.730 = 5.746 Ah
(After 20% in 13.2 ≈ 6.90 Ah.)

The standby term dominates on many systems; the alarm duration still moves the final size and is a favorite exam differentiator.

Inventory Quality Checks

Before you submit or answer:

• Every NAC appliance on battery-backed power appears in the alarm column at the candela/setting used on the drawings.
• Remote supplies are either included or calculated as separate secondary systems—never double-counted and never forgotten.
• Communicators, cellular, network cards are on the correct column (usually continuous standby + any alarm-mode surge if documented).
• EOL resistors and supervision are not a substitute for appliance alarm current.
• Currents match cut sheets cited in the package (Chapter 10.2), not memory from a different project.

Level II Role

You assemble the load basis from schedules, risers, and product data; you verify someone else’s spreadsheet against the same sources; and you flag when the drawing set shows EVACS speakers but the battery sheet used 5 minutes. You do not invent sealed-engineer authority—you make the math and basis traceable.

Bridge to 13.2

5.259 Ah (this example, 5 min) is not yet the battery you order. Next: apply the 20% aging factor, consider temperature and manufacturer minimums, and select series/parallel bank configuration for system voltage.