Fire Alarm NAC Calculator
Notification appliance circuit load and voltage drop.
About this calculator
This calculator checks whether a notification appliance circuit (NAC) — the wiring that powers horns, strobes, and combination horn/strobes on a fire alarm system — can deliver enough voltage to its farthest device, per NFPA 72, the National Fire Alarm and Signaling Code. It starts by summing current draw using typical UL-listed values for each device type (about 0.177A for a combination horn/strobe, 0.138A for a strobe alone, 0.060A for a horn alone), then computes round-trip wire resistance from your circuit length and wire gauge (2.58 Ω per 1,000 feet for 14 AWG copper, down to 1.02 Ω for 10 AWG), doubling it to account for the full loop out and back. Multiplying current by that round-trip resistance gives voltage drop, which is subtracted from supply voltage to get the end-of-line voltage — the calculator flags a problem if that falls below 16V, a common minimum operating threshold for notification devices.
It separately checks total current against a typical 2.5A NAC panel/booster rating and, if exceeded, calculates how many circuits you'd need to split the load across evenly. Finally, it sizes standby battery capacity for a 5-minute alarm condition with a 20% safety margin, though it doesn't include the much larger 24-hour standby current that NFPA 72 also requires — that's typically dominated by other panel loads, not the NAC itself, so don't mistake this battery figure for your full standby calculation.
Inputs
Results
Total circuit current (A)
2.46
Voltage drop (V)
6.35
Figures current as of 2026. Source: National Fire Protection Association, NFPA 72, National Fire Alarm and Signaling Code
How to Use This Calculator
- Enter Horn/strobes, Strobes only, and Horns only.
- Set Circuit length (ft), Wire gauge, and Supply voltage (VDC).
- Review Total circuit current (A) and Voltage drop (V).
- Use End-of-line voltage (V) and Voltage acceptable to inform your decision.
How the result changes with Horn/strobes
| Horn/strobes | Total circuit current (A) | Voltage drop (V) |
|---|---|---|
| 5 | 1.58 | 4.06 |
| 7.5 | 2.11 | 5.43 |
| 15 | 3.35 | 8.63 |
| 25 | 5.12 | 13.2 |
What each input means
- Horn/strobes
- Number of combination horn/strobe devices (typical draw ~0.177A each).
- Strobes only
- Number of strobe-only devices (typical draw ~0.138A each).
- Horns only
- Number of horn-only devices (typical draw ~0.060A each).
- Circuit length (ft)
- One-way wire run from panel/booster to the last device on the circuit.
- Wire gauge
- 1=14 AWG (2.58 Ω/kft), 2=12 AWG (1.62 Ω/kft), 3=10 AWG (1.02 Ω/kft).
- Supply voltage (VDC)
- NAC panel output voltage (typically 24 VDC nominal).
What each result means
- Total circuit current (A)
- Sum of all device current draws on the circuit.
- Voltage drop (V)
- Voltage lost in wire resistance (I × R × 2 for round trip).
- End-of-line voltage (V)
- Voltage available at the last device. Must be ≥16V for most devices.
- Voltage acceptable
- Whether end-of-line voltage is >=16V. If No, use heavier wire or split circuit.
- Current within rating
- Whether circuit is under 2.5A NAC limit. If No, split into multiple circuits.
- NAC circuits needed
- Number of circuits required to keep each under the 2.5A limit.
- Devices per circuit
- Recommended device count per circuit if splitting.
- Total devices
- Total notification appliances on the circuit.
- Total power (W)
- Total power consumption of the NAC.
- Battery for alarm (Ah)
- Battery capacity for 5-minute alarm duration with 20% safety factor.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersHorn/strobes = 10, Strobes only = 5, Horns only = 0, Circuit length (ft) = 500 = 6 input(s) provided
- Calculate Total circuit currentTotal circuit current = numHornStrobes * currentHornStrobe +2.46 = 2.46
- Calculate Voltage dropVoltage drop = totalCurrent * roundTripResistance6.35 = 6.35
- Calculate End-of-line voltageEnd-of-line voltage = supplyVoltage - voltageDrop17.65 = 17.65
- Calculate Voltage acceptableVoltage acceptableYes = Yes
Figures and sources
- NFPA 72 notification appliance circuit voltage/current constraints and standby battery requirements (2026) — National Fire Protection Association, NFPA 72, National Fire Alarm and Signaling Code
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator double the wire resistance when computing voltage drop?
Current has to travel from the panel out to the farthest device and then back again to complete the circuit, so the calculator doubles the one-way resistance (2 × circuit length / 1000 × Ω per 1,000 ft) to get the true round-trip resistance before multiplying by current. Using only the one-way resistance would understate voltage drop by half and could make an undersized circuit look like it passes.
How much difference does wire gauge actually make to whether my circuit passes?
The calculator's resistance values drop sharply as gauge increases — 2.58 Ω/1,000 ft for 14 AWG, 1.62 Ω/1,000 ft for 12 AWG, and 1.02 Ω/1,000 ft for 10 AWG — so moving from 14 AWG to 10 AWG cuts round-trip resistance, and therefore voltage drop, by more than half for the same circuit length. If End-of-Line Voltage is failing the 16V minimum, switching to a heavier gauge is often the fix without having to shorten the circuit or remove devices.
What should I do if Current Within Rating comes back as No?
That means your total device current exceeds the typical 2.5A NAC panel/booster rating the calculator checks against. The Circuits Needed output tells you how many separate NAC circuits it would take to keep each one under that limit (total current divided by 2.5A, rounded up), and Devices Per Circuit gives a rough even split of your total device count across that many circuits.
Why doesn't the Battery for Alarm figure cover the full 24-hour standby period NFPA 72 requires?
This calculator only sizes battery capacity for the 5-minute alarm condition on this specific NAC — total current times 5 minutes (as a fraction of an hour) with a 20% safety factor — because that's the load unique to notification appliances. The much larger 24-hour standby requirement is dominated by the fire alarm control panel's other continuous loads (detectors, panel electronics, other circuits), which aren't part of this NAC-specific calculation, so your full battery sizing needs to add that separately.
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