NAC Expansion and Battery Capacity in a Medical Office
A 14,000 ft² medical office building has a 24 V dc addressable FACP with four notification appliance circuits. The owner adds a 3,000 ft² suite and wants to extend NAC 2, which currently serves 18 horn/strobes. The new suite needs eight horn/strobes. Nameplate data: existing devices draw 0.075 A each; new devices draw 0.115 A each. The panel NAC is rated 2.5 A. The proposed circuit is 300 ft one-way from the panel to the last device using 14 AWG stranded cable. The panel is powered by two 7 Ah, 24 V batteries. Connected standby load is 0.30 A; calculated alarm load after the addition is 3.6 A. The contractor proposes to reuse the 7 Ah batteries, set the panel for a 3-minute alarm period to save battery capacity, and skip a voltage-drop calculation because “the panel is 24 V and the devices are 24 V.” The AHJ has adopted NFPA 72 and NEC 2020. The owner wants the suite open in two weeks.
1. Calculate the voltage drop at the end of the proposed NAC circuit and determine whether the 14 AWG proposal is acceptable. Show your assumptions and state what you would change.
Read the worked answer
First calculate the connected load: 18 existing devices × 0.075 A = 1.35 A; 8 new devices × 0.115 A = 0.92 A; total = 2.27 A. The panel NAC is rated 2.5 A, so the circuit is within the rating, but only by 0.23 A - little margin for inrush or future additions.
Use the loop resistance method. 14 AWG stranded copper is approximately 3.07 Ω per 1,000 ft. For a 300 ft one-way run, the loop is 600 ft: 600 × 3.07 / 1,000 = 1.84 Ω. Voltage drop = 2.27 A × 1.84 Ω = 4.18 V.
For a 24 V dc system, voltage-drop calculations should use the minimum usable battery voltage, not the nominal 24 V. At 20.4 V at the panel, the last device sees 20.4 − 4.18 = 16.22 V. If the appliance is listed with a minimum operating voltage of 16 V, this is marginal and not acceptable for a design with tolerance, aging, and connection resistance. At nominal 24 V it looks like 19.82 V, which is why the contractor’s “24 V in, 24 V out” assumption is wrong.
I would not accept the 14 AWG proposal. Use 12 AWG (approximately 1.93 Ω/1,000 ft) or split the load between two NACs. With 12 AWG: 600 × 1.93 / 1,000 = 1.16 Ω; drop = 2.27 × 1.16 = 2.63 V; end voltage at 20.4 V = 17.77 V, which provides reasonable margin if the appliance is listed for 16 V minimum. Verify the actual appliance listing and panel synchronization/current limits.
2. Determine whether the existing 7 Ah batteries are adequate for 24-hour standby plus 5-minute alarm after the addition. Use a 20% aging/depletion margin and show the calculation.
Read the worked answer
Battery capacity is calculated in ampere-hours for each phase, then summed and multiplied by a margin.
Standby: 0.30 A × 24 h = 7.2 Ah. Alarm: 3.6 A × (5 min / 60 min/h) = 3.6 × 0.0833 = 0.30 Ah. Subtotal = 7.2 + 0.30 = 7.50 Ah. With 20% margin: 7.50 × 1.20 = 9.00 Ah required.
The existing 7 Ah batteries are therefore undersized. A 12 Ah battery (or a pair of batteries configured to provide the required capacity at 24 V) would be a practical minimum, and the charger must be listed and rated to recharge the larger battery within the code-required time. The panel’s battery calculation worksheet should be updated with the actual alarm load, including all notification appliances and any auxiliary loads.
The contractor’s proposal to set a 3-minute alarm period is not an acceptable substitute for proper battery sizing unless the adopted code and the panel listing expressly permit that shorter duration for this occupancy. The design should meet the 24-hour standby/5-minute alarm baseline, and any deviation should be approved by the AHJ and documented.
3. What acceptance checks and documentation should be completed before the new suite is turned over to the owner?
Read the worked answer
Before turnover, verify the design basis and the installed system, not just that the panel powers up. At minimum:
- Confirm the NAC load calculation and voltage drop for each modified circuit, using the actual device listings and the minimum battery voltage.
- Confirm the battery calculation and charger capacity, and record the battery date and model.
- Test every new and existing appliance on the affected NAC for audibility/visibility, synchronization, and correct operation.
- Verify that the panel annunciates the new suite devices with correct addresses and text, and that the added devices do not exceed panel or power-supply limits.
- Perform the required acceptance test and document results, including any deficiencies and their correction.
- Update the as-built drawings, device list, sequence of operations, and inspection/testing records.
The AHJ should receive the required submittals and test documentation before final acceptance. The owner should also receive a written statement of the battery/NAC design basis so future additions are not made without recalculating capacity.