Calculate end-of-line voltage for notification appliance circuits per NFPA 72 §10.18. Verify all devices receive adequate voltage at full alarm load.
This calculator verifies that every notification appliance on a fire alarm NAC circuit receives adequate operating voltage under full alarm load, following the wire resistance method referenced in NFPA 72 §10.18. Engineers use it to select wire gauge and circuit topology so that the last device on the longest run meets the manufacturer's minimum voltage requirement.
The voltage at the end-of-line device equals the supply voltage minus the total resistive voltage drop in the circuit conductors. Wire resistance is R = (ρ/1000) × L × 2, where ρ is the conductor resistance in ohms per 1,000 feet (from NFPA 70 Chapter 9 Table 9), L is the one-way circuit length in feet, and the factor of 2 accounts for both conductors (hot and return). Voltage drop is V_drop = I_total × R, and voltage at the end-of-line is V_EOL = V_supply − V_drop.
For Class A (loop-return) wiring, the effective wire length doubles again because the circuit travels to the last device and returns to the panel, so the 2× factor becomes 4×. Total current is the sum of all devices' alarm currents, taken at their worst-case (maximum) values. Most horn/strobe combination devices draw 80–175 mA each at 24 VDC in alarm; datasheets must be consulted for the specific listed device.
NFPA 72 §10.18.1 requires that all initiating and notification appliance circuits be designed so that all devices receive sufficient voltage to operate under maximum load. Device manufacturers publish minimum operating voltage ranges (typically 16–33 VDC for 24 VDC nominal systems), and NFPA 72 requires the calculated end-of-line voltage to remain within this listed range. NFPA 70 (NEC) Table 9 provides wire resistance values used for conductor sizing. UL 1481 lists power supplies for fire protective signaling systems and specifies the regulated output voltage tolerance at the panel terminals.
Using heavier wire gauge is the most effective way to reduce voltage drop on long circuits — increasing from 18 AWG (7.95 Ω/kft) to 14 AWG (3.14 Ω/kft) reduces resistance by 60%. An alternative approach is to split the circuit into shorter branches by adding a secondary power extender panel or NAC expander closer to the devices, effectively reducing L.
Class A wiring doubles the wire run length compared to Class B, which can cause circuits that pass under Class B to fail under Class A. Always calculate for the wiring class specified in the project documents. Verify the voltage at each device location, not just the last device — devices early on the circuit will have slightly higher voltage, but the worst-case (last device) determines compliance. Temperature also affects copper resistance: at 75°C, resistance is higher than at 20°C; most tables use 75°C values, which is conservative and code-compliant.
Set the supply voltage (12 or 24 VDC), the minimum device operating voltage from the manufacturer's data sheet, the wire gauge, and the wiring class (A or B). Then enter each NAC device with its alarm current in mA, quantity, and one-way distance from the panel in feet. The calculator computes voltage at each device location and flags any device that falls below the minimum voltage as a FAIL. Adjust wire gauge or split the circuit until all devices show PASS. The Summary panel shows total current, maximum distance, wire resistance, voltage drop, and end-of-line voltage.
Most UL-listed 24 VDC notification appliances have a minimum operating voltage of 16 VDC and a maximum of 33 VDC, per the device's listing data sheet. This 16–33 VDC window is the compliance target for NFPA 72 voltage drop calculations. Some devices, particularly high-intensity strobes, may have a narrower window; always verify with the specific manufacturer's submittal data.
Yes, for the same circuit. Class A wiring requires conductors to loop back from the last device to the panel, effectively doubling the total wire length and therefore doubling the resistance compared to an equivalent Class B circuit. However, Class A provides survivability through a single open fault, which is the design trade-off. Some designers use heavier gauge on Class A circuits to compensate.
NFPA 72 does not prohibit mixing wire gauges on a circuit, but it significantly complicates voltage drop calculations — you must calculate resistance for each segment separately using its own gauge. In practice, most designers use a single consistent gauge throughout a circuit to simplify calculations and inspection.
In a series-wired circuit, all devices share the same supply current path. The total current from all devices flows through the first segment of wire from the panel, producing the full voltage drop on that segment. Each successive segment carries the current of all remaining devices. The voltage at any device equals the supply minus the cumulative drop through all preceding wire segments.
18 AWG (7.95 Ω/kft) is the most common wire gauge for NAC circuits in commercial fire alarm systems and is the minimum gauge recognized by NFPA 70 Article 760 for fire alarm wiring. For longer circuits with higher device loads, 16 AWG or 14 AWG is used. Addressable SLC loops often use 18 AWG as well, but low-current SLC circuits are less sensitive to voltage drop than high-current NAC circuits.
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