NAC Voltage Drop and Mixed-Manufacturer Strobes
You are commissioning the notification appliance circuits on an addressable fire alarm system in a four-story assisted living facility. The panel's listed NAC output is 24 VDC at 2.0 A maximum per circuit. Circuit NAC-3 will carry 14 horn/strobes rated 0.032 A each at 24 V. The run is 300 ft one-way using 16 AWG stranded copper rated 4.016 ohms per 1,000 ft. The panel's low-battery voltage is 20.4 V, and each appliance is listed for a minimum operating voltage of 16 V. Because of a supply delay, the electrical contractor proposes substituting a different manufacturer's strobes on the last six devices; they are UL 1971 listed and visually identical. The facility opens in three weeks. The GC has asked you to "just make it work" and says the previous inspector never checked voltage drop.
1. Calculate the voltage drop on NAC-3 and determine whether the circuit complies. What else must you check before signing off?
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Load current = 14 × 0.032 A = 0.448 A. Conductor length = 2 × 300 ft = 600 ft. R = 600 ft × 4.016 Ω/1,000 ft = 2.41 Ω. Drop = 0.448 A × 2.41 Ω = 1.08 V. Worst-case source is the panel's low-battery voltage, 20.4 V, so end-of-line voltage = 20.4 − 1.08 = 19.32 V, above the 16 V minimum. On paper the circuit passes.
I would not sign off on the hand calculation alone. Notification appliances have inrush, and current at reduced voltage may be higher than the 24 V rating; the listed appliance data and the manufacturer's voltage-drop calculator account for that. I would also check the panel's 2.0 A NAC rating (0.448 A is fine), synchronization, and actual tap/connector losses. If the manufacturer's calculator shows the end voltage below the appliance's listed minimum, the circuit fails and needs 14 AWG, a shorter run, or a booster/power supply.
2. Evaluate the proposal to substitute six UL 1971-listed strobes from another manufacturer. What codes, listings, and performance issues govern this?
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The substitution is not automatically acceptable. UL 1971 listing only says the strobe meets the candela/performance standard; it does not mean the device is compatible with the panel or with the other appliances on the NAC. NFPA 72 requires visible appliances in the same area to flash in synchronization, and mixing manufacturers or product families on one NAC commonly causes loss of synchronization or unacceptable inrush. The panel is also listed with specific compatible appliances; using an appliance not on the manufacturer's compatibility list can void the listing and fail the AHJ inspection. I would require a compatibility letter from the panel manufacturer, a synchronization test of all 14 devices, and AHJ approval before installing the substitutes. If they cannot sync or are not listed, the options are to wait for the specified devices or add a listed sync module - not to improvise in the field.
3. The GC asks to add four more 0.032 A horn/strobes to NAC-3 for a corridor. What is your answer, and what do you check?
Read the worked answer
New current = 18 × 0.032 A = 0.576 A; drop = 0.576 × 2.41 Ω = 1.39 V; end voltage = 20.4 − 1.39 = 19.01 V, still above 16 V. Voltage drop alone probably allows the addition, but that is only one check. I would verify: (1) the panel's 2.0 A per-circuit rating and total NAC capacity; (2) the battery standby calculation with the added supervisory and alarm current; (3) synchronization and candela coverage for the corridor; (4) whether the added devices are compatible and on the same circuit; and (5) the manufacturer's calculator including inrush. If all pass, I would document the revised load and get the change approved. If the panel is near its 2.0 A limit or the battery margin is thin, I would add a booster supply rather than exceed the listing.