Fire alarm voltage drop calculator (NAC circuits)

How to use the voltage drop calculator

Run it when you lay out a notification appliance circuit, again whenever appliances are added or candela settings change on an existing one, and before the plans go to the AHJ.

1. Enter the one-way cable length from the NAC terminals to the last appliance, measured along the real route with risers and drops. The calculator doubles it for the return conductor.
2. Pick the wire gauge and solid or stranded copper. Compare our calculated resistance in the table below with your cable's data sheet.
3. Enter the alarm load as one total current or as a device list. Use each appliance's maximum listed current for the candela, tone and voltage it will run at.
4. Enter the voltage at the NAC terminals and the highest listed minimum among the appliances on the circuit. Tick the battery check and enter the end-of-discharge voltage from your panel manual.
5. Read the last-device voltage, the PASS or FAIL against your minimum and the longest one-way run that still passes. Print the worksheet or download the Excel workbook for the submittal.

The fire alarm voltage drop formula guide derives every step and works a segment-by-segment example. New to the circuit itself? Start with the NAC circuit guide.

How the calculation works

First establish the voltage budget: subtract the device's listed minimum from the voltage available at the NAC terminals in the supply condition being checked. Obtain the maximum alarm current for the selected appliance settings and waveform from its installation instructions. Repeat the check for the battery condition; nominal panel voltage alone does not describe that condition.

Let L be one-way length in feet, r the resistance of one conductor in Ω per 1,000 ft at 75 °C, I the total alarm load in amperes, Vₛ the voltage available at the circuit source, and Vₘᵢₙ the device minimum from its listing. Convert meters to feet using L(ft) = L(m) ÷ 0.3048.

• Loop resistance: Rₗₒₒₚ = 2 × L × r ÷ 1,000.
• Cable voltage drop: ΔV = I × Rₗₒₒₚ.
• Drop percentage: 100 × ΔV ÷ Vₛ.
• Voltage at the last device: Vₗₐₛₜ = Vₛ − ΔV.
• Pass for the entered minimum: Vₗₐₛₜ ≥ Vₘᵢₙ.
• Maximum one-way length: Lₘₐₓ = 1,000 × (Vₛ − Vₘᵢₙ) ÷ (2 × I × r), for I > 0 and Vₛ ≥ Vₘᵢₙ.

These equations apply Ohm's law as applied in System Sensor's voltage-drop example, p. 4 to the outgoing and return conductors. If the source is already below the device minimum, no length passes. At zero current there is no finite voltage-drop length limit in this model.

Optional distributed-load estimate

For N equal-current devices equally spaced at L/N, 2L/N, …, L, each segment carries the current of the devices downstream. Summing those segment drops gives:

ΔVdistributed = (2 × r × L / 1,000) × (I / N²) × (1 + 2 + … + N) = ΔVlumped × (N + 1) / (2N)

The maximum length is Lₘₐₓ from the lumped formula divided by (N + 1)/(2N). One device gives the lumped result; ten give 55% of the lumped drop. The factor approaches one-half as N increases, but simply halving the drop is inaccurate for a small device count.

This is an algebraic extension of the same I × R cable-loss method: calculate each segment with its downstream current, then add the losses. The optional estimate assumes equal currents and equal spacing; use an actual segment-by-segment design for different spacing, mixed currents, or branches.

Worked example

For 14 AWG solid uncoated copper, a 300 ft one-way run, and 1.5 A total load, the calculated resistance at 75 °C is approximately 3.071293 Ω/1,000 ft. The loop resistance is 2 × 300 × 3.071293 ÷ 1,000 = 1.842776 Ω. Drop is 1.5 × 1.842776 = 2.764164 V.

At a 24 V source, the last device receives approximately 21.236 V, an 11.517% drop, which passes an entered 16 V minimum. Maximum one-way length is approximately 868.255 ft. At a 20.4 V battery-condition source, the same load leaves 17.636 V at the last device and limits length to approximately 477.540 ft.

For ten equal-current devices evenly spaced over that same 300 ft, the optional estimate gives 2.764164 × 11/20 = 1.520290 V drop. These are fictional calculation inputs, not an equipment specification or exam question.

Worked example: passes on 24 V, fails on battery

An invented hallway NAC has 8 horn-strobes at 0.12 A and 4 strobes at 0.09 A, all maximum listed currents for the chosen settings. Enter them as a device list: 0.96 A + 0.36 A = 1.32 A. The run is 400 ft one way on 16 AWG stranded copper, the highest listed minimum is 16 V, and the panel manual gives 20.4 V at end of battery discharge.

1. Resistance: the calculator gives 4.9812 Ω per 1,000 ft for 16 AWG stranded, so the loop is 2 × 400 × 4.9812 ÷ 1,000 = 3.985 Ω.
2. Drop: 1.32 A × 3.985 Ω = 5.260 V.
3. On 24 V the last appliance gets 18.740 V. PASS.
4. On 20.4 V it gets 15.140 V, a 25.8% drop. FAIL. The longest passing run on battery is 334.59 ft.

Fix one: move to 14 AWG stranded. The drop falls to 3.308 V, the last appliance gets 17.092 V on battery, and the run could reach 532.02 ft. Fix two: keep 16 AWG and split the devices over two NACs of 4 horn-strobes and 2 strobes each, 0.66 A per circuit. If both runs are still 400 ft, each drops 2.630 V and the last appliance gets 17.770 V on battery. Every figure above is the calculator's own output for these inputs. The fire alarm wire size calculator compares all five gauges in one pass.

Copper reference data at 75 °C

Calculated from NBS Handbook 100, Copper Wire Tables: ρ₂₀ = 1/58 Ω·mm²/m and α₂₀ = 0.00393/°C (appendix 5, printed p. 40); AWG geometry (§2.2, p. 6); stranded allowance (appendix 4, p. 38).

d(mm) = 0.127 × 92^((36 − AWG)/39); A = πd²/4; r₂₀ = ρ₂₀ × 304.8/A; r₇₅ = r₂₀ × [1 + 0.00393 × (75 − 20)].

Multiply by 1.02 for the handbook's concentric-lay stranded estimate. These calculated values use ideal annealed, uncoated copper; they are not a cable manufacturer's maximum resistance specification. Actual stranding, tolerances and construction can change resistance. Confirm against your cable's published data. The 75 °C assumption does not establish cable suitability.

For a code-book comparison, open NFPA 70-2020, Chapter 9, Table 8 and its notes in your own copy. Match material, coating, construction, units and temperature before comparing resistance. This tool retains the NBS calculation; it does not import or reproduce NEC table entries. Use the cable manufacturer's resistance for a project calculation when specified.

The coefficient above starts at 20 °C. The 20-to-75 °C multiplier is 1 + 0.00393 × 55 = 1.21615. Apply it once to a 20 °C resistance. Do not apply it again to a resistance already stated at 75 °C. The 2% stranding allowance is a concentric-lay estimate, not a bound for every cable construction.

These are our computed NBS-based values, not NEC table entries. Display values are rounded; circuit calculations retain full precision.

Design assumptions and limits

System Sensor's historical example starts at 20.4 V. The editable input lets you check that example condition; it is not a universal NAC output rating. Confirm it for your equipment and end-of-discharge condition. This tool subtracts cable loss only; enter the available NAC terminal voltage after any internal panel losses. A 24 V nominal source is not a worst-case supply guarantee.

The lumped result is conservative for placement of the entered fixed current on the stated radial run. It does not account for voltage-dependent current, inrush, synchronization modules, connection losses, power-supply regulation, return-fed Class A operation, or additional branches. Use the device's listed maximum current for the applicable settings and waveform.

Check against your project's specifications and the AHJ / your procedure. A pass means only that this simplified voltage estimate meets your entered minimum. Confirm circuit current limits, maximum device voltage, conductor suitability, equipment compatibility, manufacturer instructions and the required design method separately.

For related circuit concepts, read the NAC vs SLC guide. Use the fire alarm battery calculator for the separate capacity calculation.

The ×1.02 stranded estimate can be slightly non-conservative: it can understate resistance, understate voltage drop and overstate maximum length. The 14 AWG comparison is about 0.3% below the matching NFPA 70 Chapter 9, Table 8 value. Use the specified cable resistance for design; a near-boundary pass in this tool is insufficient.

Common voltage drop mistakes

• Entering the round trip. The calculator already doubles the one-way length. An out-and-back figure doubles the drop again.
• Checking 24 V only. Nominal voltage is not the battery condition. Run the battery check with the end-of-discharge voltage from your panel or power supply manual.
• Using typical appliance current. Strobes draw more as voltage falls. Use the maximum listed current for the candela, tone and waveform you will install.
• Judging by percentage. The pass test is volts at the last appliance against its listed minimum. On a mixed circuit, use the highest minimum; a sync module can raise it.
• Measuring on the floor plan. Straight-line distance misses risers, drops to each appliance and routing around obstructions. Use the cable route.
• Using the even-spacing shortcut on an uneven circuit. The distributed estimate assumes equal currents at equal spacing. Otherwise use the lumped result or a segment-by-segment calculation.
• Assuming Class A halves the drop. After an open, the far appliances can be fed from the return end over the whole route. Check the panel manual's method; this calculator models one radial run.

Voltage drop is one of three NAC checks. Size the standby batteries with the battery calculation method, and pick the cable rating from the FPL, FPLR and FPLP cable types guide.

Fire alarm voltage drop FAQ