6.6 Primary & Secondary Power Supply Calculations

Primary & Secondary Power Supply Calculations

A fire alarm system must remain fully operational even when catastrophic events, severe weather, or structural fires sever utility electrical power to a building. Under NFPA 72 Chapter 10 and NEC Article 760, every fire alarm system must be equipped with at least two independent, reliable power sources: a Primary Power Supply and a Secondary (Standby) Power Supply.

For Texas fire alarm licensees, understanding the mechanical requirements of dedicated power circuits and mastering the mathematical calculations for secondary battery banks are essential design and testing competencies tested on the SFMO licensing exam.

---

1. Primary Power Supply Requirements (NFPA 72 § 10.6.5)

The primary power supply provides continuous electrical energy for normal system operation, battery charging, and all supervisory monitoring functions. It must meet rigid installation criteria:

+-----------------------------------------------------------------------------+
|                   PRIMARY POWER SUPPLY CODE REQUIREMENTS                    |
|                                                                             |
|   [UTILITY / GENERATOR]                                                     |
|            |                                                                |
|            v                                                                |
|   +-----------------------------------------------------------------------+ |
|   | DEDICATED BRANCH CIRCUIT                                              | |
|   | - Supplies ONLY the fire alarm system and dedicated secondary power.  | |
|   | - Dedicated neutral conductor (no shared multi-wire branch neutral).  | |
|   | - Prohibited from supplying general lighting or wall receptacles.     | |
|   +-----------------------------------------------------------------------+ |
|            |                                                                |
|            v                                                                |
|   +-----------------------------------------------------------------------+ |
|   | CIRCUIT DISCONNECT & BREAKER LOCKING                                  | |
|   | - Circuit breaker painted RED or marked with red identifier.          | |
|   | - Mechanical lock-on device permanently attached (breaker lock-out).  | |
|   | - Labeled 'FIRE ALARM CIRCUIT' in durable, permanent lettering.       | |
|   | - Location of breaker documented inside FACP deadfront door.          | |
|   +-----------------------------------------------------------------------+ |
|            |                                                                |
|            v                                                                |
|   +-----------------------------------------------------------------------+ |
|   | OVERCURRENT PROTECTION & GFCI RESTRICTION                             | |
|   | - Sized per NEC Article 760 (typically 15A or 20A branch).            | |
|   | - Shall NOT be connected through a GFCI or AFCI device unless          | |
|   |   specifically required by the NEC for hazardous/wet areas.           | |
|   +-----------------------------------------------------------------------+ |
+-----------------------------------------------------------------------------+

Critical Primary Power Checklist:

1. Dedicated Branch Circuit: The branch circuit cannot feed any other loads (no convenience receptacles, water coolers, or room lights).
2. Mechanical Lock: A permanent mechanical lock-on clip must prevent accidental manual shutoff by building maintenance staff.
3. Red Marking & Identification: The breaker handle or face must be identified in red and marked "FIRE ALARM CIRCUIT."
4. Directory Labeling: The electrical distribution panel directory must clearly state the fire alarm circuit number, and the location of the electrical distribution panel must be posted permanently inside the FACP enclosure.
5. Dedicated Neutral: The branch circuit neutral cannot be shared across multi-wire branch circuits to avoid induced transient surges or unintended disconnects during neutral servicing.

---

2. Secondary (Standby) Power Supply Standards

The secondary power supply must automatically, bumplessly transfer to supply operating power within 10 seconds of primary AC power failure, without generating false alarms or dropping supervisory data.

+-----------------------------------------------------------------------------+
|                SECONDARY POWER DURATION STANDARDS BY SYSTEM TYPE            |
|                                                                             |
|   SYSTEM CLASSIFICATION                   STANDBY TIME    ALARM DURATION    |
|   =====================================   ============    ==============    |
|   1. Standard Protected Premises          24 Hours    +   5 Minutes         |
|      (Horns, Strobes, Bells)                                                |
|                                                                             |
|   2. Emergency Voice / Alarm (EVACS)      24 Hours    +   15 Minutes        |
|      (High-Rise / Mass Notification)                                        |
|                                                                             |
|   3. Supervising Station / Central Sta.   24 Hours    +   5 Minutes         |
|      (Without standby generator)                                            |
|                                                                             |
|   4. Supervising Station / Central Sta.   4 Hours     +   5 Minutes         |
|      (With auto-start generator per NFPA 110)                               |
|                                                                             |
|   5. Household Systems (NFPA 72 Ch 29)    24 Hours    +   4 Minutes         |
+-----------------------------------------------------------------------------+

[!IMPORTANT] Standard Systems vs. Voice (EVACS) Systems: Note the crucial distinction on licensing exams: Standard commercial systems with horn/strobe evacuation require 24 hours of standby followed by 5 minutes of alarm. Buildings equipped with an Emergency Voice/Alarm Communications System (EVACS) or High-Rise voice systems require 24 hours of standby followed by 15 minutes of full-load voice evacuation alarm.

---

3. Battery Chemistries & Operational Profile

Secondary power for protected premises systems is almost universally supplied by rechargeable secondary storage batteries housed inside the FACP enclosure or an adjacent battery cabinet:

+-----------------------------------------------------------------------------+
|                        BATTERY CHEMISTRIES COMPARISON                       |
|                                                                             |
|   1. VALVE-REGULATED LEAD-ACID (VRLA - AGM / GEL CELL)                      |
|   - Industry standard for commercial fire alarm systems.                   |
|   - Maintenance-free, non-spillable, sealed construction.                   |
|   - Float charge voltage: 2.25 - 2.30 Volts per cell (13.5 - 13.8V per 12V).|
|   - Typical operational service life: 3 to 5 years.                         |
|   - Must be replaced when capacity drops below 80% of rated nameplate.      |
|                                                                             |
|   2. NICKEL-CADMIUM (Ni-Cad)                                                |
|   - Higher initial cost; highly resistant to extreme temperature swings.    |
|   - Long service life (10-15 years), but subject to cell voltage memory.   |
|                                                                             |
|   3. LITHIUM-ION (Listed Energy Storage Systems)                            |
|   - High energy density, compact footprint.                                 |
|   - Requires specialized battery management systems (BMS) and listing       |
|     under UL 1973 / NFPA 855 for fire alarm control units.                  |
+-----------------------------------------------------------------------------+

Ambient Temperature Derating

Batteries are rated at a nominal laboratory temperature of 77°F (25°C).

• Cold Environments: Operating in unheated spaces (e.g., parking garages or exterior riser rooms at 32°F / 0°C) reduces available discharge capacity by 20% to 30%, requiring a derating factor (larger battery capacity).
• Hot Environments: Elevated temperatures above 95°F (35°C) accelerate internal chemical degradation and plate corrosion, reducing battery service life by approximately 50% for every 18°F (10°C) permanent increase above baseline.

---

4. The NFPA 72 Battery Sizing Formula & Mathematical Method

To size a battery bank accurately, the designer must calculate the total current drawn by all connected components in two distinct operational states:

1. Non-Alarm Quiescent Standby Current ($I_{\text{standby}}$): Current consumed by main processor, LCD displays, addressable SLC modules, smoke detector idle currents, and supervisory relays.
2. Full-Load Alarm Current ($I_{\text{alarm}}$): Current consumed by the panel during an active alarm, including all activated notification appliances (horns, strobes, chimes, audio amplifiers), auxiliary control relays, and communicator transmitters.

+-----------------------------------------------------------------------------+
|                      BATTERY LOAD PROFILE OVER TIME                         |
|                                                                             |
|   CURRENT (Amps)                                                            |
|      ^                                                                      |
|      |                                                    +---------------+ |
|      |                                                    | ALARM CURRENT | |
|      |                                                    | (e.g., 4.5A)  | |
|      |                                                    | (5 or 15 min) | |
|      |                                                    +---------------+ |
|      |                                                            |         |
|      |  +---------------------------------------------------------+         |
|      |  | STANDBY CURRENT (e.g., 0.65A)                           |         |
|      |  +---------------------------------------------------------+         |
|      +------------------------------------------------------------+----->   |
|         0 Hours                         24 Hours            End of Test     |
|                                   TIME DURATION                             |
+-----------------------------------------------------------------------------+

Step-by-Step Sizing Formulas

$\text{Standby Amp-Hours } (AH_{\text{sb}}) = I_{\text{standby}} \text{ (Amperes)} \times T_{\text{standby}} \text{ (Hours)}$

$\text{Alarm Amp-Hours } (AH_{\text{al}}) = I_{\text{alarm}} \text{ (Amperes)} \times T_{\text{alarm}} \text{ (Hours)}$

$\text{Subtotal Amp-Hours } (AH_{\text{sub}}) = AH_{\text{sb}} + AH_{\text{al}}$

$\text{Required Battery Capacity } (AH_{\text{total}}) = AH_{\text{sub}} \times 1.20 \text{ (20\% Safety / Aging Factor)}$

[!WARNING] The 1.20 Safety / Aging Factor: NFPA 72 § 10.6.7.2.1 mandates that calculated battery capacity must include a minimum 20% safety margin (multiplying by 1.20). This accounts for battery aging, cell degradation over time, internal resistance buildup, and manufacturing tolerances. Failing to apply the 1.20 multiplier is an automatic failure on the Texas licensing exam!

---

5. Comprehensive Worked Engineering Example

Project Specifications:

• System Type: Standard commercial office protected premises fire alarm.
• Standby Current ($I_{\text{standby}}$): 0.85 Amperes (850 mA).
• Alarm Current ($I_{\text{alarm}}$): 4.20 Amperes.
• Required Standby Time ($T_{\text{standby}}$): 24.0 Hours.
• Required Alarm Time ($T_{\text{alarm}}$): 5 Minutes ($5 / 60 = 0.0833$ Hours).

Step-by-Step Solution:

Step 1: Calculate Standby Amp-Hours $AH_{\text{sb}} = 0.85 \text{ A} \times 24.0 \text{ hr} = 20.40 \text{ Amp-Hours}$

Step 2: Calculate Alarm Amp-Hours $AH_{\text{al}} = 4.20 \text{ A} \times \left(\frac{5}{60}\right) \text{ hr} = 4.20 \times 0.0833 = 0.35 \text{ Amp-Hours}$

Step 3: Calculate Subtotal Capacity $AH_{\text{sub}} = 20.40 \text{ AH} + 0.35 \text{ AH} = 20.75 \text{ Amp-Hours}$

Step 4: Apply 20% Aging / Safety Factor (1.20) $AH_{\text{total}} = 20.75 \text{ AH} \times 1.20 = 24.90 \text{ Amp-Hours}$

Step 5: Select Commercial Battery Bank Standard sealed lead-acid batteries are manufactured in nominal ratings: 7 AH, 12 AH, 18 AH, 26 AH, 33 AH, 55 AH, etc. Since $24.90 \text{ AH} > 18 \text{ AH}$, the technician must install a minimum of two 12V 26 AH batteries in series (for a 24VDC system).

---

6. Battery Charger Performance & Trouble Annunciation

NFPA 72 § 10.6.7.3 establishes strict performance rules for the integral or external battery charging equipment:

1. Recharge Timeline: An automatic battery charger must be capable of fully recharging a completely discharged battery bank back to 100% capacity within 48 hours while the system continues to operate under maximum normal supervisory load.
2. Supervision of Power Supplies: The FACP must continuously monitor the secondary power source. The panel must transmit a distinct Trouble Signal within 200 seconds for any of the following conditions:
   • Total loss of primary AC power.
   • Physical disconnection of the battery bank.
   • Battery charger failure or blown battery fuse.
   • Low battery terminal voltage (falling below 85% of rated system operating voltage—i.e., below 20.4VDC on a nominal 24VDC system).