คู่มือแรงดันตก NAC ระบบแจ้งเหตุเพลิงไหม้. คำนวณแรงดันตกวงจร NAC 24V สำหรับฮอร์น สโตรบ และแหล่งจ่ายระยะไกลตาม NEC, NFPA 72 และ IEC

This localized guide keeps the same engineering workflow: list the real alarm current, calculate the farthest or segmented route, then compare the ending voltage with NEC, NFPA 72, IEC, equipment listing, and AHJ requirements.

สรุป

Use the listed current of every appliance at the selected candela and tone setting.
Calculate from the worst-case source voltage, not from a casual 24.0V assumption.
Enter one-way route length for the farthest device or each segment when loads are distributed.
Compare the ending voltage with the device listing, panel manual, NFPA 72 documentation, and NEC Article 760 wiring rules.

Key Definitions for NAC Voltage Drop

A notification appliance circuit is a circuit that supplies horns, strobes, bells, speakers, or combination appliances used to notify occupants.

Voltage drop is the voltage lost in the pair of circuit conductors as appliance current flows through wire resistance.

End-of-line voltage is the voltage available at the farthest or worst-case appliance after conductor loss is subtracted.

A power-limited fire alarm circuit is a NEC Article 760 circuit supplied by a listed power-limited source with installation and separation rules.

Design Workflow Before You Submit the NAC

Step 1

List every appliance on the circuit with its exact candela, horn tone, sync module, and alarm current. For example, twelve 75 cd horn-strobes at 0.135A each produce 1.62A of alarm load before panel limits are checked.

Step 2

Confirm the usable source voltage from the fire alarm control unit, NAC booster, battery calculation, and manufacturer instructions. A 24V nominal system may be evaluated at a lower worst-case voltage after standby operation.

Step 3

Measure the real cable path, including vertical drops, slack, back boxes, raceway offsets, and the return path assumed by the calculator. Do not use a straight-line floor-plan distance.

Step 4

Calculate the farthest-device drop for a quick conservative check. For long corridors, calculate segment-by-segment so upstream current is not incorrectly applied to the entire run.

Step 5

Verify the ending voltage against every device listing and submit the voltage drop calculation with battery calculations, appliance settings, conductor size, and circuit class.

Code and Standard Checkpoints

This guide uses public background on fire alarm notification appliances , the NEC , and IEC 60364 while leaving final acceptance to the listed equipment, NFPA 72 edition adopted locally, and the AHJ.

NFPA 72 requires fire alarm documentation to include voltage drop calculations where required by the authority having jurisdiction; the 2025 edition added clearer design language for notification appliance circuit voltage drop.
NEC Article 760 governs fire alarm circuit wiring methods, power-limited and non-power-limited separation, conductor use, and installation rules. Voltage drop does not replace those wiring checks.
NEC 110.3(B) requires listed equipment to be installed and used in accordance with its instructions, so the panel and appliance voltage range controls the pass/fail decision.
IEC 60364-5-56 covers safety services such as fire detection and alarm supplies, while IEC 60364-5-52 keeps conductor sizing tied to installation method, current capacity, and voltage drop.
Many AHJs expect the last appliance to remain above the manufacturer minimum, often 16V on 24V appliances, after worst-case source voltage and conductor loss are considered.

Typical NAC Planning Checks

These examples are planning numbers. Always use the actual appliance data sheet, selected candela, panel output rating, cable material, temperature, and local AHJ requirements.

NAC scenario	Trial conductor	Voltage-drop check	Design decision
8 horn-strobes, 0.110A each, 220 ft farthest route	18 AWG copper	About 6.0V round-trip drop at 0.88A	Likely too tight if the worst-case source is near 20.4V and devices need 16V
12 horn-strobes, 0.135A each, 180 ft route	16 AWG copper	About 4.7V at 1.62A	Split the NAC or use 14 AWG if the end voltage margin is below 2V
6 strobes, 0.090A each, 300 ft corridor	14 AWG copper	About 3.1V at 0.54A	Usually workable when source voltage and sync module loss leave enough margin
Remote booster to last floor device, 2.0A, 120 ft	12 AWG copper	About 3.8V at full load	Check booster terminal rating and consider two shorter NACs
Class A return path, same 1.0A load, 250 ft out and back	16 AWG copper	Treat the alarm current path per the manufacturer method	Do not assume Class A routing halves the voltage drop unless the listed design method says so
24V speaker-strobes with audio power plus strobe load	14 AWG copper plus speaker cable	Calculate speaker line loss separately from strobe DC drop	Keep NAC voltage drop and speaker wattage loss as separate submittal calculations

Worked Calculator Examples

Small retail NAC: 8 horn-strobes at 0.110A, 220 ft

Total alarm current is 8 x 0.110A = 0.88A. Enter DC, 24V nominal, copper, 18 AWG, and 220 ft one-way. The approximate round-trip drop is 6.0V, leaving only 18.0V from a 24.0V source and about 14.4V from a 20.4V worst-case source. If the appliances are listed for 16V minimum, 18 AWG fails the conservative check.

Office corridor NAC: 12 horn-strobes at 0.135A, 180 ft

Total alarm current is 1.62A. With 16 AWG copper over 180 ft, voltage drop is roughly 4.7V. From a 20.4V source, the last device sees about 15.7V before sync module or terminal losses, so the designer should split the circuit, move the booster, or test 14 AWG in the calculator.

Warehouse strobe run: 6 strobes at 0.090A, 300 ft

Total current is 0.54A. With 14 AWG copper over 300 ft, voltage drop is near 3.1V. From 20.4V, the end voltage is about 17.3V, which may pass a 16V appliance listing if the panel manual allows the method and all actual device currents match the submittal.

Plan Review Checklist

Show the circuit source, panel or booster model, available alarm current, battery standby assumptions, and worst-case source voltage.
Use the listed alarm current for each selected candela, not a catalog family average.
Document conductor size, copper or aluminum material, route length, class, and whether the calculation is lumped-end or segment-by-segment.
Confirm the route complies with NEC Article 760 separation, support, plenum/riser cable, and power-limited circuit requirements.
Compare the final voltage with the device listing, NEC 110.3(B), NFPA 72 documentation expectations, and local AHJ submittal forms.

Common Questions

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

Use the source voltage required by the panel, booster, battery, and appliance instructions. Many submittals check a 24V nominal NAC at a lower worst-case value such as 20.4V, then verify the last appliance remains above its listed minimum.

Is 10% voltage drop always allowed on notification appliance circuits?

No. A percentage rule is only a planning shortcut. The pass/fail point is whether every appliance operates within its listed voltage range, the panel instructions are followed under NEC 110.3(B), and the AHJ accepts the NFPA 72 documentation.

Should I calculate NAC loads as lumped at the end or by each segment?

A lumped-end calculation is conservative and fast. Segment-by-segment calculation is more accurate on long runs because only downstream load current flows through each upstream segment.

Does Class A fire alarm wiring reduce voltage drop?

Not automatically. Class A improves survivability by returning to the panel, but voltage drop must follow the listed equipment method and the actual current path during alarm and fault conditions.

Can I use the breaker or NAC rating as the load current?

No. Use the sum of actual appliance alarm currents. A 2.0A NAC loaded to 1.62A should be calculated at 1.62A, then separately checked against the 2.0A source rating.

How do IEC projects treat fire alarm voltage drop?

IEC designs tie fire detection and alarm supplies to IEC 60364-5-56 safety-service requirements and conductor selection to IEC 60364-5-52. The equipment listing and local fire code still set the final operating-voltage requirement.

Check the NAC Before the Cable Is Pulled

Run 18, 16, 14, and 12 AWG options with the actual horn-strobe current and route length before a failed inspection turns into rework.

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