Battery Sizing and NAC Voltage Drop on a 24 VDC System
A 24 VDC addressable fire alarm control panel is supplied by two 7 Ah sealed lead-acid batteries connected in series. The calculated standby load is 0.30 A for 24 hours; the alarm load is 3.8 A for 5 minutes, including all notification appliance circuits. The batteries are in a 90 degrees F electrical room and are two years old. The contractor says the 7 Ah batteries are adequate because the alarm time is short. A separate NAC circuit has 18 horn/strobes at 0.10 A each, fed from the panel through 500 ft of No. 14 AWG. The panel terminal voltage is 24.0 V, and the minimum listed device operating voltage is 20.4 V. The conductor resistance is 3.07 ohms per 1,000 ft.
1. Calculate the required battery capacity and determine whether the existing 7 Ah batteries are adequate.
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Two 7 Ah batteries in series produce 24 V but still have a capacity of 7 Ah; they do not add to 14 Ah. Standby energy: 0.30 A x 24 h = 7.2 Ah. Alarm energy: 3.8 A x (5 min / 60 min/h) = 3.8 x 0.0833 = 0.317 Ah. Total = 7.517 Ah. Applying the 20 percent reserve required for secondary power gives 7.517 x 1.25 = 9.40 Ah. The 7 Ah battery is therefore below the calculated requirement before any temperature or aging derating. At 90 degrees F, lead-acid battery capacity and life are reduced, and two-year-old batteries may already have lost capacity, so the actual available capacity is even less favorable. The contractor's 'short alarm time' argument ignores the 24-hour standby load, which is the dominant term. The correct fix is to install a larger battery, such as a 12 Ah or 18 Ah set as allowed by the panel listing and enclosure, or reduce the standby/alarm load by adding a remote power supply. A new battery calculation must be submitted.
2. Calculate the voltage drop on the 18-device NAC and determine whether the circuit meets the 20.4 V minimum at the last device.
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Total NAC current = 18 x 0.10 A = 1.8 A. For a 500 ft one-way run, the current travels 500 ft out and 500 ft back, so the conductor length is 1,000 ft. Resistance = 3.07 ohms per 1,000 ft x 1.0 = 3.07 ohms. Voltage drop = I x R = 1.8 A x 3.07 ohms = 5.53 V. Voltage at the last device = 24.0 V - 5.53 V = 18.47 V, which is below the 20.4 V minimum. The circuit is not acceptable as designed. Changing to No. 12 AWG at 1.93 ohms per 1,000 ft would give a drop of 1.8 x 1.93 = 3.47 V and an end voltage of 20.53 V, which is only marginally above the minimum and leaves little margin for connection resistance or low panel voltage. A better remedy is No. 10 AWG, a remote notification power supply located near the load, or splitting the devices between two circuits. The voltage drop must be recalculated at the actual panel output voltage and with the device current at the minimum operating voltage.
3. What documentation should accompany the corrected battery and NAC design?
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The submittal should include a battery calculation using the panel manufacturer's standby and alarm current figures, the 24-hour standby plus 5-minute alarm duty, the 20 percent reserve, and any temperature or aging derating required by the manufacturer. It should include a point-by-point NAC voltage-drop calculation showing the panel terminal voltage, conductor size and length, device current, resistance, and voltage at the last device. Include the battery and device listing sheets, the panel NAC rating, and the enclosure capacity for the larger batteries. The as-built drawings should show the revised circuit and power-supply locations. At acceptance, the contractor should document the actual measured battery voltage under load, the actual voltage at the end of the NAC, and the completed record of completion. If a remote power supply is added, its supervision, trouble annunciation, and battery calculation must also be documented.