Fire alarm notification appliance circuits are low-voltage circuits, but they are not low-consequence circuits. A horn-strobe circuit can pass a quick continuity check and still fail the design if the last strobe sees too little voltage during alarm on standby batteries. For electricians, fire alarm designers, engineers, and careful DIYers working under permit, the practical question is not simply “is this 18 AWG cable allowed?” It is “will every listed notification appliance still operate at the end of the circuit when the panel output is at its worst-case voltage?”

TL;DR

Use listed appliance voltage range, not a generic 3% rule alone.
Class B NAC voltage drop uses the full out-and-back conductor path.
High-candela strobes can control wire size before circuit ampacity does.
Document panel voltage, alarm current, conductor size, length, and spare capacity.

A notification appliance circuit, or NAC, is a supervised fire alarm output circuit that powers horns, strobes, horn-strobes, chimes, speakers, and related notification appliances during alarm. Voltage drop is the voltage lost in the copper conductors and in-line devices while alarm current flows. A listed operating voltage range is the voltage window in which the manufacturer has evaluated the appliance. Those three definitions matter because a nominal 24 V fire alarm appliance is often listed to operate over a range such as 16 V to 33 V, while a battery-backed panel output may not be 24.0 V at the end of standby.

The code framework comes from several places. The National Electrical Code Article 760 covers fire alarm wiring installation in NEC projects. Fire alarm performance, documentation, notification appliance layout, and survivability decisions are commonly coordinated with NFPA 72. For international projects, the IEC family of standards and local fire alarm rules fill a similar role: equipment must be installed as listed, cable must suit the environment, and the circuit must deliver enough voltage to the load.

“For a 24 volt NAC, the real design limit is not 3 percent. It is the gap between the lowest fire alarm power-supply output you allow and the lowest listed operating voltage of the last appliance.”
— Hommer Zhao, Technical Director

Start With The Lowest Usable Voltage

Many voltage-drop mistakes happen because the calculation starts at 24.0 V. That is convenient, but it can be optimistic. A fire alarm control unit or remote NAC power supply may provide a regulated output in normal AC condition, then a lower output while running from standby batteries near the end of the required standby period. If the appliance is listed for 16 V minimum and the panel schedule allows 20.4 V at the source under battery alarm, the available drop budget is 4.4 V. If the same circuit is calculated from 24 V, the apparent budget becomes 8 V, which hides a weak design.

Use the equipment manual for the source voltage and each notification appliance current. Strobe current is especially sensitive to candela setting, temporal pattern, synchronization method, and model family. A single 15 cd wall strobe might draw less than 0.08 A, while a 110 cd or 177 cd unit can draw two to three times as much. Speaker-strobes add another layer because the speaker circuit and strobe circuit may be separate, shared, or controlled by different modules.

If you are doing preliminary routing, use the voltage drop calculator to compare conductor sizes, then confirm the final schedule against submittal data. The wire resistance calculator is useful when you already know the cable length and want to test 18, 16, and 14 AWG options quickly.

Worked Example: Class B Horn-Strobe Circuit

Suppose a Class B NAC leaves a remote power supply, runs down a corridor, and terminates at the end-of-line device after the last horn-strobe. The one-way cable route to the last appliance is 300 ft. The alarm current on the circuit is 0.92 A after adding six horn-strobes at their selected candela settings plus a synchronization module. The installer plans 18 AWG copper fire alarm cable with a resistance near 6.385 ohms per 1,000 ft.

18 AWG copper at 0.92 A

Round-trip length = 300 ft x 2 = 600 ft

Resistance = 6.385 ohms/kft x 0.600 kft = 3.831 ohms

Voltage drop = 0.92 A x 3.831 ohms = 3.52 V

If source under battery alarm is 20.4 V, last-device voltage is 16.88 V

In this example, 18 AWG barely clears a 16 V appliance minimum. It may be acceptable on paper, but it leaves less than 0.9 V margin before field changes, added devices, temperature, splices, or measurement uncertainty. Upsizing to 16 AWG copper at about 4.016 ohms per 1,000 ft reduces the drop to about 2.22 V and raises the last-device voltage to 18.18 V. Splitting the NAC into two shorter circuits can be even better if the power supply has available outputs and the riser diagram remains clean.

Comparison Table: NAC Wire Size And Layout Choices

The table below uses the same 0.92 A alarm load and a 300 ft one-way Class B route. Values are approximate copper conductor checks for early design comparison; final values should use the cable data and appliance submittals approved for the project.

Option	Resistance used	Voltage drop	Last device at 20.4 V source	Design decision
18 AWG, one circuit	6.385 ohms/kft	3.52 V	16.88 V	Works only with tight margin
16 AWG, one circuit	4.016 ohms/kft	2.22 V	18.18 V	Better field margin
14 AWG, one circuit	2.525 ohms/kft	1.39 V	19.01 V	Strong but harder to terminate
Two 18 AWG circuits	0.46 A each	1.76 V	18.64 V	Good if outputs are available
Move power supply midpoint	150 ft one-way	1.76 V	18.64 V	Often best for long corridors

Count The Right Current, Not Just The Device Count

A device count is not a load calculation. Two circuits with twelve devices can have very different voltage drop if one uses 15 cd strobes and the other uses 110 cd strobes in sleeping rooms, high ceilings, or noisy industrial spaces. Start with the appliance schedule, not the floor plan symbol count. For each NAC, list the device type, candela, selected tone, current draw, sequence on the circuit, and cumulative current after each point. This shows which segment carries the most current and helps catch the common mistake of applying full circuit current to the whole route when a more detailed segment calculation is needed.

Segment calculations matter most on long daisy-chained circuits. The first 40 ft may carry all six devices, the next 60 ft may carry five, and the last 30 ft may carry only one. A simple full-load round-trip calculation is conservative and fast, which is useful for planning. A segmented calculation is more accurate and can save a circuit that looks weak under a rough check. For plan review and commissioning, the final voltage-drop schedule should be easy to trace from the riser diagram and floor plan.

“On NAC circuits, the highest-risk line item is usually not cable ampacity. It is a high-candela strobe added near the end of a long corridor after the original voltage-drop schedule was already close to 16 volts.”
— Hommer Zhao, Technical Director

NEC 760, NFPA 72, And IEC Design Discipline

NEC 760 is mostly about installation rules: power-limited versus non-power-limited fire alarm circuits, cable types, separation from other conductors, mechanical protection, and permitted wiring methods. NFPA 72 is where designers usually coordinate system performance, audibility, visibility, synchronization, inspection, testing, and documentation. IEC-based projects may use different article numbers and product standards, but the engineering discipline is the same: source voltage, conductor resistance, appliance current, and listed operating range must agree.

Do not confuse a fire alarm NAC with a general-purpose 24 V DC control circuit. The math resembles a control circuit voltage drop check, but the documentation expectations are stricter because the system is life safety equipment. Also avoid treating ampacity as the only wire-size check. The same lesson appears in the voltage drop versus ampacity guide: a conductor can be electrically safe for current yet still too small for delivered voltage.

Design documents should show

Source output voltage used for battery alarm condition
Each appliance current at selected candela and mode
One-way route length, wire size, and conductor resistance
Calculated last-device voltage and spare current capacity

Field changes to recheck

Changing 15 cd strobes to 75 cd or 110 cd devices
Adding synchronization modules or isolators in the path
Rerouting cable around rated walls, shafts, or ceilings
Combining two planned NACs to save one output

A Field Scenario With Real Numbers

In a tenant improvement review, we checked a 24 V NAC serving a long open-office loop with eight wall horn-strobes. The first drawing used 18 AWG and showed 1.48 A on a 2.0 A output, which looked acceptable by current alone. The route length changed after coordination with a rated corridor, adding 85 ft one-way. The revised worst-case drop was 4.9 V from a 20.4 V battery alarm source, leaving only 15.5 V at the last high-candela appliance. The fix was not complicated: split the loop into two NACs, keep each below 0.82 A, and hold the last device above 18.2 V. The cost was one extra output and a cleaner schedule; the benefit was a design that the installer, engineer, and inspector could all verify.

“When a NAC calculation is close, I would rather split a 1.5 amp circuit into two 0.75 amp circuits than defend a last-device voltage that depends on perfect routing and no future candela changes.”
— Hommer Zhao, Technical Director

FAQ: Fire Alarm NAC Voltage Drop

What voltage drop limit should I use for a 24 V fire alarm NAC?

Do not use a generic 3% rule by itself. Use the fire alarm control unit output voltage under battery conditions and the listed minimum operating voltage of the notification appliances, often 16 V for nominal 24 V devices, then document the margin required by NFPA 72 design practice and NEC 760 wiring rules.

How do I calculate voltage drop on a Class B NAC circuit?

Use round-trip conductor length, total alarm current on the circuit, and conductor resistance. For example, 18 AWG copper at about 6.385 ohms per 1,000 ft, 0.92 A, and a 300 ft one-way Class B run gives about 3.52 V drop.

Does the end-of-line resistor current count in alarm voltage drop?

Usually the end-of-line resistor supervises the circuit in normal condition and is not the main alarm current. For voltage drop during alarm, count horns, strobes, speaker-strobes, synchronization modules, and any listed in-line device current that remains in the alarm path.

Can I mix candela settings on the same NAC calculation?

Yes, but each device current must match its selected candela and mode. A 15 cd strobe may draw around 75 mA while a 110 cd setting can draw more than 180 mA depending on model, so the voltage-drop schedule should list every setting.

What wire size is common for fire alarm NACs?

Many NACs use 18 AWG or 16 AWG fire alarm cable, but long corridors, high-candela strobes, or synchronized circuits may need 14 AWG or split circuits. NEC 760 installation rules, listing instructions, and local fire alarm specifications still control the final wiring method.

How much spare capacity should I leave on a NAC power supply?

Many designers keep at least 20% spare current capacity and verify voltage drop at the lowest battery-backed output voltage. If a 2.0 A NAC is already at 1.82 A, one future 177 mA strobe can erase the margin.

Calculate Before The Submittal Is Locked

Fire alarm voltage drop is easiest to fix while the riser, power supply locations, and appliance settings are still flexible. Before cable is pulled, compare 18 AWG, 16 AWG, 14 AWG, circuit splits, and remote power-supply locations with the maximum circuit length calculator. Then open the full calculator and document the source voltage, alarm current, route length, and last-device voltage for the project file.

Run The NAC Calculation Contact The Team

Fire Alarm NAC Voltage Drop: 24 V Circuits, Strobes, Horns, and Code Checks