The fire alarm voltage drop formula is ΔV = I × Rloop. For a two-wire radial notification appliance circuit (NAC), placing all the load at the end gives ΔV = 2 × L × r × I ÷ 1,000, where L is the one-way length in feet and r is the resistance of one conductor in ohms per 1,000 ft. Subtract that drop from the voltage available at the NAC terminals to estimate the voltage at the last appliance.
Use the fire alarm voltage drop calculator to check the arithmetic or download an Excel workbook with live formulas. To compare gauges side by side, use the fire alarm wire size calculator. This guide explains the physics behind that fire alarm system voltage drop calculation, including the point-to-point (segment current) method most free calculators skip: on a real NAC each wire segment carries only the current of the devices beyond it, so the end-of-line shortcut overstates the drop. It covers a fixed-current, two-wire radial circuit; equipment instructions and project requirements still determine the design method.
Drafted by the TrueGrade Prep editorial team. Sources checked 17 Oct 2026. Pending expert review.
Start with Ohm's law and the complete wire loop
Fluke's explanation of Ohm's law relates voltage, current and resistance. Applying it to a cable gives a loss in volts equal to its current in amperes multiplied by its resistance in ohms.
Current travels through the outgoing conductor and back through the return conductor. For equal conductors, the resistance of that path is:
Rloop = 2 × L × r ÷ 1,000
Use the routed one-way length for L. The factor of two includes the return conductor. Entering the round-trip length and multiplying by two again doubles the loss incorrectly. If the resistance data already describes a complete pair, do not double it again. Convert meters to feet using L(ft) = L(m) ÷ 0.3048 before using resistance stated per 1,000 ft.
For a load concentrated at the end:
- Cable drop: ΔV = I × Rloop.
- Last-device voltage: Vlast = Vs − ΔV, where Vs is the available voltage at the NAC terminals.
- Drop percentage: 100 × ΔV ÷ Vs.
- Minimum-voltage comparison: Vlast ≥ Vmin, using the device's listed minimum.
A percentage describes the loss relative to the entered source. It does not establish the permitted loss for every device or project.
Copper resistance and temperature correction
The calculator derives ideal uncoated copper resistance from NBS Handbook 100, Copper Wire Tables, §2.2 and appendices 4–5. It does not copy an NEC conductor table. For AWG geometry:
d(mm) = 0.127 × 92^((36 − AWG)/39)
A(mm²) = π × d² ÷ 4
Using copper resistivity at 20 °C, ρ20 = 1/58 Ω·mm²/m, the resistance of 1,000 ft of one conductor is r20 = ρ20 × 304.8 ÷ A. Temperature changes that estimate:
rT = r20 × [1 + 0.00393 × (T − 20)]
For the web calculator's 75 °C assumption, the temperature multiplier is 1.21615. The workbook exposes temperature as an input within 10–100 °C. Its stranded option adds a factor of 1.02, the handbook's concentric-lay estimate. Actual cable construction and tolerances can change resistance; use the cable manufacturer's data for a project. A calculation temperature does not establish a cable's permitted operating temperature.
Worked example: all load at the end
These are fictional calculation inputs, not an equipment specification or an exam item: 14 AWG solid copper at 75 °C, 300 ft one way, 1.5 A alarm current, 24 V available at the source and a 16 V device minimum. Values below are rounded for display; comparisons use full precision.
The calculated one-conductor resistance is about 3.071293 Ω/1,000 ft. Therefore:
Rloop = 2 × 300 × 3.071293 ÷ 1,000 ≈ 1.842776 Ω
ΔV = 1.5 × 1.842776 ≈ 2.764164 V
Vlast = 24 − 2.764164 ≈ 21.236 V
That exceeds the entered 16 V minimum, with about 11.517% cable loss. If only 20.4 V is available at the NAC terminals under the battery condition being checked, the same current and cable leave about 17.636 V. Doubling the length to 600 ft doubles the drop to about 5.528 V: the 24 V comparison still clears 16 V, but the 20.4 V comparison leaves about 14.872 V and fails that minimum.
For a positive fixed current and a source at or above the minimum, rearrange the same equation for maximum one-way length:
Lmax = 1,000 × (Vs − Vmin) ÷ (2 × I × r)
For this example, Lmax is about 868.255 ft at 24 V or 477.540 ft at 20.4 V. If the source is already below the minimum, no cable length can fix that. Zero current produces no cable drop and no finite length limit in this mathematical model; enter the actual alarm load before using the estimate.
Point-to-point (segment current) method vs lumped load
Placing the entire fixed current at the far end is the lumped-load method. It is conservative for placement of that same current on the stated radial run because it makes every segment carry all the load.
With devices distributed along a run, each segment carries only the current needed downstream. Apply ΔVsegment = Idownstream × 2 × Lsegment × r ÷ 1,000, then sum the losses to each device. Potter's Advanced Fire Training Manual, section 7 illustrates this segment-current approach.
For N equal-current devices equally spaced at L/N, 2L/N, …, L, summing the segment currents gives our algebraic shortcut:
ΔVdistributed = ΔVlumped × (N + 1) ÷ (2N)
Ten devices at 0.15 A each over the example's 300 ft give a factor of 11/20, or 0.55. The last-device drop becomes 2.764164 × 0.55 ≈ 1.520290 V, leaving about 18.880 V from a 20.4 V source. One device gives the lumped result. Simply halving the lumped drop is an approximation that misses the finite device count.
Unequal spacing or mixed currents need the actual segment lengths and downstream currents. The calculator's device-list mode sums mixed currents for the lumped method; it does not locate those groups along the cable. Branches, return-fed circuits, voltage-dependent current and added module losses require the applicable equipment method.
Fire alarm voltage drop requirements: where the inputs come from
The minimum acceptable device voltage comes from the selected device's listing and instructions. The source voltage comes from the panel or supply documentation for the operating condition being checked, including the panel's end-of-life battery condition. These are separate inputs. Allow for internal panel losses before entering the voltage available at the NAC terminals.
System Sensor's L-Series installation instructions connect the wire-loss calculation to the appliance's rated range and the connected current. Read the instructions for the actual model, settings and supply waveform. For mixed devices, the highest listed minimum provides a conservative common threshold on this simplified radial calculation.
Potter's section 7 uses 20.4 V in its battery-condition example. Confirm the starting voltage for your equipment; neither 24 V nominal, 20.4 V nor the example's 16 V minimum should be assumed for every system. Code navigation reference: NFPA 72 (2022), §10.6.7. Consult your own licensed copy or NFPA's free read-only access. No code text or tables are reproduced here.
Section numbers move between editions, so check the reference against the edition your AHJ has adopted.
A passing voltage estimate checks the entered minimum only. Confirm source capacity, maximum device voltage, circuit ratings, compatibility and the project's required method separately. Use the fire alarm battery calculator for the separate capacity arithmetic, and the NAC vs SLC guide for circuit terminology.
Use the calculation spreadsheet in Excel
On the voltage drop calculator, enter the circuit and select Download Excel workbook. The .xlsx file includes editable inputs and formulas referencing them, along with the tool URL, generation date, draft warning and resistance citation. Change the blue input cells to recalculate. Device groups and included result models reflect the options selected at export; export again to change that structure. The CSV button provides a snapshot of the current calculation.
Return to the Alarm Ladder overview or use the Level II diagnostic to choose a study priority.
Sources
- What is Ohm's Law?, Fluke. Retrieved 17 Oct 2026.
- Copper Wire Tables, NBS Handbook 100, National Bureau of Standards, 1966, §2.2 and appendices 4–5. Retrieved 17 Oct 2026.
- Advanced Fire Training Manual, 8700055 Rev H, Potter Electric Signal Company, May 2017, section 7. Retrieved 17 Oct 2026.
- L-Series wall appliance installation instructions, I56-5845, System Sensor. Retrieved 17 Oct 2026.
- NFPA 72 (2022), NFPA. Public reader checked 17 Oct 2026; section content unavailable. Section reference requires editorial confirmation as noted above.