Fire Alarm Voltage Drop Calculator

Fire alarm NAC voltage drop uses Vdrop = 2 × I × R × L, where I is the cumulative current of all notification appliances on the circuit, R is wire resistance per foot from NEC Chapter 9 Table 8, and L is the total cable length. Critical detail: in a daisy-chained NAC, the segment closest to the panel carries the current of every device on the circuit, while the last segment only carries the last device's current. The calculator does this point-to-point math automatically — you enter each device's position along the run and it computes per-segment drop with cumulative downstream current.

NFPA 72 doesn't specify a fixed percentage; it specifies a minimum operating voltage at the device. Most 24V notification appliances are listed to operate down to 16V at the device terminals. Per UL 864, the panel must continue operating until batteries deplete to 20.4V. So your worst-case calculation is: 20.4V at panel minus voltage drop must equal at least 16V at the last device. That gives you a 4.4V drop budget — about 21.6% of 20.4V. Most contractors design to a 10% drop on the calculated nominal 24V circuit as a safety margin.

Because the worst-case scenario for any fire alarm system is loss of utility power — the system runs on batteries until they're depleted, and per UL 864 the panel must continue operating until the supervised battery voltage drops to 20.4V. Calculating at the nominal 24V (or worse, 27V float voltage) gives you a passing result that fails in a real outage. Your circuit needs to keep delivering at least 16V to every notification appliance even when the batteries are at end-of-life voltage. This calculator includes a battery backup mode that drops the source voltage to 20.4V automatically.

Each candela rating draws a different current. A 75 cd horn/strobe pulls about 0.180A; a 110 cd pulls 0.235A; a 177 cd pulls 0.305A. On a mixed circuit, you sum the actual current of every device — not an average. The calculator's NAC mode lets you pick each device individually with its candela setting, and it pulls the published UL-max current from the device tables (System Sensor, Wheelock, etc.) so you don't have to manually look up amperage per candela.

Lump Sum (sometimes called End-of-Line) treats the entire circuit as if every device is at the very end of the run. The total current of all devices flows through the full cable length. This is conservative — it always overstates drop. Point-to-Point models the actual cable topology: segment 1 (panel to device 1) carries the full circuit current, segment 2 (device 1 to device 2) carries everything downstream of device 1, and so on. This calculator does true point-to-point math, which means a circuit that fails Lump Sum can still pass if the actual layout has devices distributed along the run.

Yes — and most modern NACs do exactly that. Combo horn/strobes get installed where audible signaling is required (corridors, common areas), strobe-only units are added in restrooms, mechanical spaces, and quiet zones where audible isn't needed. For voltage drop purposes, you're summing per-device currents from the manufacturer cut-sheet, regardless of device type. This calculator supports horn-only, strobe-only, horn/strobe combo, mini-horns, and speaker-strobes on the same circuit, each with its own candela or dB or speaker tap setting.

High-rise stairwells are the brutal case for fire alarm voltage drop. A single circuit running up 20 floors with one notification appliance per floor lands you at 200-400ft of cable carrying significant current. Your options: (1) increase wire gauge to 12 AWG or 10 AWG, (2) split the stairwell across multiple NAC circuits — typically one per 5-8 floors, (3) install NAC extender panels mid-rise. The calculator helps you check option 1 first; if it fails even at 10 AWG, you need option 2 or 3.

Warehouses typically need 110 cd or 177 cd strobes for the open-area coverage requirements per NFPA 72 7.5.4. Higher candela means higher current draw — a 177 cd strobe pulls roughly 0.305A vs 0.180A for a 75 cd unit. Combine that with long warehouse runs (200-400ft is common) and the drop math gets tight fast. Most warehouses end up on 12 AWG NAC even though 14 AWG meets minimum code.

Most AHJs want a per-circuit calculation showing: starting voltage (typically 20.4V battery worst-case), wire gauge and length per segment, cumulative current per segment, voltage at every device, and a clear pass/fail vs the device's minimum operating voltage. Some AHJs require manufacturer-published minimum voltages (like 16V for System Sensor SpectrAlert, or 19V for some older Faraday units). Use the calculator to generate the calculation, then export or screenshot the per-device results table for your submittal package.

14 AWG solid copper FPLR or FPLP is the most common NAC cable in commercial construction — it meets minimum code, it's cheaper, and it terminates fast. 12 AWG cuts wire resistance by about 37% and gives you significant headroom on longer runs or high-candela circuits. Standardizing your shop on 14 AWG and only stepping to 12 AWG when the calc requires it is the most cost-effective approach. For warehouses, high-rise, or long-corridor circuits, default to 12 AWG and save the troubleshooting later.

No — voice evacuation speaker circuits run on 70V or 25V audio distribution, not 24V DC notification. The math is similar (Ohm's law still applies), but the voltage and current scale is completely different. A 70V speaker tapped at 4W draws roughly 0.057A — much less than a 24V horn. Speaker voltage drop is rarely a real problem on typical run lengths because the high voltage / low current geometry minimizes I²R losses. This calculator handles standard NAC (24V horn/strobe) circuits; for 70V speaker tap calculations, use a separate audio distribution calc.