NAC Voltage Drop Calculator
Check a notification appliance circuit the way plan reviewers expect: start at the 20.4 V UL 864 depleted-battery voltage, use NEC Chapter 9 Table 8 wire resistance, and confirm the farthest appliance still sees its 16 V minimum at full alarm load. Both accepted methods — conservative end-of-line and accurate point-to-point — run side by side on the same inputs, with Class A loop support, max-run and minimum-gauge solves, and a printable AHJ submittal sheet. Free, no login.
Circuit & appliances
“Wire ft” is the wire run from the previous row (first row: from the panel). Rows in circuit order, panel outward. Total run: 200 ft.
Submittal sheet header (project, panel, circuit, preparer)
Voltage drop
End-of-line
conservative · AHJ default
18.24 V
−2.16 V drop
Point-to-point
distributed · accurate
18.78 V
−1.62 V drop
Method delta: 0.54 V — distributed loads always fare better point-to-point; a circuit that fails lump-sum may still pass PTP.
Voltage at each tap (point-to-point)
Vdrop = 2 × I × L × R/1000 — NEC Ch. 9 Table 8 (75°C). Source 20.4 V per UL 864 (85% of 24 V); 16 V floor per UL 464/1971 regulated listing. Class A worst case = full loop. No T-taps on Class B NACs; speaker circuits are wattage-tap math, not DC drop.
NAC Voltage Drop Calculation
Notification appliance circuit voltage drop — prepared with the NORDIX NAC Voltage Drop Calculator (nordixhq.com/tools/nac-voltage-drop-calculator)
| Project: — | Panel model / ID: — |
| Circuit: — (Class B) | Prepared by: — Date: — |
| Source voltage: 20.40 V (UL 864 depleted battery — 85% of 24 V) | Minimum appliance voltage: 16.0 V |
| Appliances (circuit order) | Qty | mA each | mA total | Wire ft from previous |
|---|---|---|---|---|
| Horn/strobe 75 cd | 5 | 176 | 880 | 100 |
| Horn/strobe 75 cd | 5 | 176 | 880 | 100 |
| Total alarm load | 1,760 mA = 1.760 A | 200 ft | ||
| Wire type | Ω/1000 ft | Length 1-way | Circuit Ω (round trip) | Load (A) | Volts @ EOL | Min volts req’d | Result |
|---|---|---|---|---|---|---|---|
| #14 solid | 3.07 | 200 ft | 1.228 | 1.760 | 18.24 | 16.0 | PASS |
Method: end-of-line (lump-sum) — all appliance load assumed at the end of the governing wire length. Most conservative method per the NFPA 72 NAC task group.
| Point-to-point (supplemental) | Distance from panel | Segment carries (A) | Volts at tap |
|---|---|---|---|
| 5× Horn/strobe 75 cd | 100 ft | 1.760 | 19.32 |
| 5× Horn/strobe 75 cd | 200 ft | 0.880 | 18.78 |
Basis: Vdrop = 2 × I × L × R/1000; conductor resistance per NEC Chapter 9 Table 8 (uncoated copper, 75°C). Source voltage 20.40 V; appliance minimum 16.0 V per the regulated 16–33 V listing window (UL 464 / UL 1971) — sync-module configurations may list 16.5–17 V. Appliance currents are manufacturer UL-max RMS across the 16–33 V range (UL 1971, May 2004 rev.) — verify against installed-device datasheets and the DC vs FWR supply type. This calculation is a design aid; the panel’s listed installation instructions and the AHJ govern.
How this calculator works
A NAC is a 2-wire DC circuit, so current makes a round trip — the drop over a run of L feet at I amps is:
Vdrop = 2 × I × L × R / 1000 (R in Ω per 1000 ft, NEC Ch. 9 Table 8)
The end-of-line (lump-sum) method assumes every appliance sits at the far end of the circuit and all activate at once — the NFPA 72 NAC task group calls it “the most conservative method and the recommended design method for all systems.” The point-to-point method walks the circuit segment by segment: each stretch of wire only carries the current of the appliances at and beyond it, so distributed loads see less drop — up to ~30% less, which is why a circuit that fails lump-sum can still pass point-to-point. The trade-off: point-to-point is only as good as the as-built distances, so field changes can invalidate it. A third approach, load centering (half the load at the midpoint), is explicitly not recommended by the task group and this tool omits it. For Class A circuits the worst case is a single open that forces supply the long way around, so the governing length is the entire loop — which is why manufacturer tables list Class A maximum distances at exactly half their Class B values.
Worked example
The example from System Sensor’s voltage-drop white paper (AVWP004): five 110 cd strobes at 0.202 A each on 250 ft of 12 AWG, Class B:
- Load: 5 × 0.202 A = 1.01 A.
- Start at the UL 864 depleted-battery voltage: 24 × 0.85 = 20.4 V.
- Drop: 2 × 1.01 × 250 × 1.93/1000 = 0.97 V (the paper rounds 12 AWG to 2.0 Ω/1000 ft and gets 1.01 V — same conclusion).
- End of line: 20.4 − 0.97 = 19.43 V ≥ 16 V — pass, with room to grow the circuit.
Enter the same circuit above (one tap: 5 × strobe 110 cd, 250 ft, 12 AWG solid) to reproduce it.
Why 20.4 V, not 24 V
| Starting voltage | What it represents | Use it for |
|---|---|---|
| 24.0 V | Nominal system voltage — batteries healthy, AC on | Nothing — a documented AHJ rejection cause |
| 20.4 V | UL 864 depleted-battery cutoff — 85% of 24 V (2 × 10.2 V per battery) | The submittal default |
| ≈19–20 V | Panel terminal cut-off: 20.4 V minus the panel’s internal drop (0.5–2.5 V by model) | The refinement careful designers use — enter your panel’s figure |
The alarm must still work at the end of the 24-hour standby period, when the batteries are at their UL 864 cutoff — that is the moment the voltage-drop calculation models. Some panel manuals skip voltages entirely and publish an equivalent fixed budget instead (Potter: keep the drop under 3 V; Fire-Lite: a maximum-loop-resistance table); the custom source-voltage and minimum-voltage inputs let you reproduce those conventions exactly.
Reference: wire resistance (NEC Chapter 9, Table 8)
Ohms per 1000 ft, uncoated copper at 75°C — the conservative, code-aligned basis fire alarm worksheets use (wire-manufacturer 20–25°C values run ~20% lower; using them flatters the result). FPL/FPLR/FPLP fire alarm cable is typically solid conductor.
| AWG | Solid | Stranded (7-str) |
|---|---|---|
| 18 | 7.77 | 7.95 |
| 16 | 4.89 | 4.99 |
| 14 | 3.07 | 3.14 |
| 12 | 1.93 | 1.98 |
| 10 | 1.21 | 1.24 |
Why a generic voltage drop calculator fails NACs
- Wrong starting voltage — generic tools assume nominal supply. A NAC calc starts at 20.4 V because it models the end of battery standby.
- Wrong pass criterion — the NEC 3%/5% figures are branch-circuit recommendations. A NAC passes only if the farthest appliance sees its listed minimum (16 V regulated) — an absolute 4.4 V budget, not a percentage.
- Wrong currents — appliance draw must be the UL-max RMS across the 16–33 V window (strobes draw more at lower voltage), not a nominal 24 V figure. The device presets here carry UL-max values.
- No distributed loads, no Class A — one lumped load, no per-tap voltages, no worst-case loop analysis, and nothing you can print for the fire marshal.
Frequently asked questions
How do you calculate voltage drop on a fire alarm NAC?
Start at 20.4 volts — the UL 864 depleted-battery voltage, 85% of 24 V nominal — total the UL-max alarm current of every appliance on the circuit, and compute the 2-wire round-trip drop: Vdrop = 2 × amps × one-way feet × (ohms per 1000 ft ÷ 1000), with wire resistance from NEC Chapter 9 Table 8. The circuit passes if the voltage at the farthest appliance stays at or above its listed minimum — 16 V for regulated 24 V appliances. This calculator runs that end-of-line method and the point-to-point method side by side on the same inputs.
Why do fire alarm voltage drop calculations start at 20.4 volts instead of 24?
Because the calculation models the worst moment the system must survive: the end of the 24-hour battery standby period, when the batteries are at the UL 864 cutoff of 10.2 V each — 20.4 V for the series pair, 85% of nominal. A circuit that passes at 24 V can fail exactly when the building needs it, which is why starting at nominal voltage is a documented plan-review rejection. Careful designers go one step further and subtract the panel's internal drop (0.5–2.5 V by model) to the terminal cut-off voltage — this calculator has an input for it.
What is the difference between the end-of-line and point-to-point methods?
End-of-line (lump-sum) assumes every appliance sits at the far end of the circuit — the most conservative method and the one the NFPA 72 NAC task group recommends for all systems. Point-to-point walks the circuit segment by segment, so each stretch of wire carries only the current of the appliances beyond it; distributed loads can show up to about 30% less drop, meaning a circuit that fails lump-sum may still pass point-to-point. The trade-off is that point-to-point depends on exact as-built distances, so field changes can quietly invalidate it. A third method, load centering, is explicitly not recommended and this tool omits it.
What is the minimum voltage a fire alarm horn strobe needs?
16 volts for standard regulated 24 V notification appliances — UL defines their listed operating window as 16–33 V. That floor is a listing spec (UL 464 / UL 1971), not an NFPA 72 number, and it is not universal: System Sensor's MDL3 sync module lists 16.5–33 V and some sync configurations list 17 V, so a synchronized circuit's floor is the highest minimum on it. The minimum-voltage input is editable for exactly that reason — and note the drop budget from 20.4 V to 16 V is only 4.4 volts.
How does a Class A circuit change the voltage drop calculation?
Class A wiring loops back to the panel, and its worst case is a single open at the point that forces power to feed the long way around — so the calculation must use the entire loop length, not the distance to the farthest appliance. That is why manufacturer wiring tables list Class A maximum distances at exactly half their Class B values for the same load and gauge. Select Class A in this calculator and it adds the return leg and checks the full loop lump-sum.
What wire size should I use for a NAC circuit?
14 AWG solid FPL/FPLR/FPLP is the common default, but the right answer comes from the math: enter your appliances and run length and the calculator reports whether the gauge passes and the thinnest gauge that would. Rules of thumb: 18 AWG has four times the resistance of 12 AWG, so long runs and high-candela loads move you to 14 or 12 quickly; if even 10 AWG fails, the fix is splitting the circuit or adding a NAC power extender closer to the load, not more copper. Resistance values are NEC Chapter 9 Table 8 at 75°C — the conservative basis; wire-catalog 20°C values run about 20% lower and flatter the result.
Do appliance currents change at lower voltage?
Yes — strobes are constant-energy devices, so they draw more current as voltage falls. UL 1971 (May 2004 revision) requires published currents to be the maximum RMS draw across the entire 16–33 V window, which may not occur at either endpoint. That makes datasheet UL-max figures the correct input for voltage drop — using a nominal-24 V current understates the load. Also match the column to the supply: filtered DC and full-wave-rectified (FWR) outputs have different current tables, and regulated appliances are not listed for special-application FWR circuits unless the panel's compatibility document says so.
Can I use a regular voltage drop calculator for fire alarm circuits?
No — generic tools get all four fundamentals wrong. They start at nominal voltage instead of the 20.4 V depleted-battery cutoff; they judge against the NEC's 3%/5% branch-circuit recommendations instead of the absolute 16 V appliance floor; they take one lumped load instead of appliances distributed along the circuit (and have no Class A loop analysis); and they know nothing about UL-max appliance currents. This calculator exists because the fire marshal will notice each of those. Submit it together with the battery calculation — most AHJs require both in one package per power supply.
Method: 2-wire round-trip drop with NEC Chapter 9 Table 8 conductor resistance (uncoated copper, 75°C); 20.4 V starting voltage per the UL 864 depleted-battery cutoff; 16 V appliance floor per the regulated 16–33 V listing window (UL 464 / UL 1971) — sync-module configurations may list 16.5–17 V, and special-application appliances differ. End-of-line and point-to-point methods per the NFPA 72 NAC task group definitions; Class A worst case uses the full loop. Speaker audio circuits (25/70.7 V wattage taps) are a separate calculation — only a speaker-strobe’s strobe leads belong here. Verify appliance currents (DC vs FWR columns) against installed-device datasheets; the panel’s listed installation instructions and the AHJ govern.
Pair with the Fire Alarm Battery Calculator and NAC Circuit Loading Calculator (AHJs want the full package), or the NAC voltage drop chart, NAC current draw chart, and fire alarm wire size chart for the underlying tables.