NAC Voltage Drop and Secondary Supply on a Medical Office Building

Run the loop math.

• Circuit current: 14 x 0.105 A = 1.47 A, comfortably under the 2.5 A circuit rating.
• Loop resistance: 420 ft one-way means 840 ft of conductor. 18 AWG copper is about 6.51 ohms per 1,000 ft at typical operating temperature, so 840 ft x 0.00651 = about 5.47 ohms.
• Voltage drop: 1.47 A x 5.47 ohms = about 8.0 V.
• End-of-line voltage: 24.0 - 8.0 = about 16.0 V. At the panel's 20.4 V brownout condition it is roughly 12.4 V.

Most notification appliances are listed for roughly 16 to 33 V, so the design lands exactly on the bottom of the range at full battery - and far below it at low battery. That is a fail, not a pass, because the fire condition that matters most often coincides with a degraded supply.

Fix options, cheapest first: repull in 16 AWG (about 4.09 ohms per 1,000 ft, giving roughly 3.4 ohms, a 5.1 V drop, and about 18.9 V at the end); split the circuit across two NACs; or add a listed NAC extender or booster supply near the third floor. Recalculate using the appliance current at the actual end-of-line voltage, since strobe current is not perfectly constant. Keep the calculation with the as-builts - the AHJ may ask for it.

Sizing is a two-part sum, then a margin.

• Standby. Use the measured 24-hour load, not a guess: panel electronics, initiating circuits, annunciators, and any transmission equipment. At 0.185 A, 24 hours = 4.44 Ah.
• Alarm. The alarm load is NAC-1 at 1.47 A plus panel alarm draw of roughly 0.30 A, about 1.77 A. For a non-voice system the alarm period is typically 5 minutes: 1.77 A x (5/60) h = about 0.15 Ah. If voice evacuation were added, the alarm period lengthens and the number grows.
• Total and margin. 4.44 + 0.15 = 4.59 Ah, plus roughly 20 to 25 percent for aging and temperature per the manufacturer's instructions: about 5.7 Ah. A 7 Ah battery is the minimum correct size; the 5 Ah on the truck is inadequate even before margin.

On the reused battery: it has been in service since 2020, likely in a warm closet, and it was sized for a different panel and load. Do not carry it over. Perform the manufacturer's capacity/load test, and replace the battery if it fails or if it cannot demonstrate the required capacity. Mark the installation or replacement date on the battery, and record the test result in the inspection record. Old batteries that pass a no-load voltage check routinely fail under alarm load - that is the failure the fire will find.

Tell him the number and the reason, in writing, before he signs.

Four more appliances at 0.105 A add 0.42 A, taking NAC-1 to about 1.89 A - still under the 2.5 A circuit rating, so the circuit looks fine by current alone. The voltage drop is the problem: with 16 AWG, 1.89 A x 3.44 ohms is about 6.5 V of drop, leaving roughly 17.5 V at the end of the line, which is inside the listing but with no room for a low-battery condition or a panel that regulates low.

The honest answer is that spare capacity is a design decision, not an afterthought:

• Document the spare capacity of each NAC - current headroom and voltage headroom - on the Record of Completion and in the as-builts, so the next contractor does not guess.
• Leave at least one NAC position or an extender panel available for future tenant work rather than tapping the end of the longest run.
• Require a recalculated voltage drop for any added load before the devices are ordered.
• If the third-floor corridors echo, the real issue may be intelligibility or appliance placement, not quantity; a voice system or better device layout may serve the owner better than four more horns.

Getting this into the proposal prevents an argument later about who pays to repull the corridor.