Fire Alarm Voltage Drop Calculation for Notification Appliances

Why Voltage Drop Decides Whether a NAC Design Passes

Every fire alarm designer eventually runs into the same field problem: the panel says 24 volts, the strobe is listed for 24 volts, and yet the last device on the run refuses to sync or the sounder output measures low. Nothing is defective. The copper simply took its cut.

“Voltage drop is the reduction in the amount of electrical potential within the electrical path where current is flowing.”

That is the whole idea in one sentence. If a 24-volt source feeds a horn/strobe through a conductor that drops one volt, the appliance sees 23 volts. Drop two volts, it sees 22. NFPA 72 Chapter 23 requires notification appliance circuits to perform as designed under worst-case load, and Chapter 18 sets the audible and visible performance those appliances must deliver — but an appliance that is starved of voltage cannot produce its listed sound pressure level or its listed candela output. The calculation is how you prove, on paper and before installation, that it will.

The Eight-Step Workflow

The tutorial lays out a repeatable sequence. Work it in order and the spreadsheet almost fills itself:

1. Determine how many NACs (notification appliance circuits) are in the design.
2. Determine how many audible and visible devices sit on each NAC.
3. Measure the point-to-point distances, starting from the fire alarm control panel.
4. Determine the alarm current of each audible and visible device from the data sheet.
5. Check the cable resistance per 1,000 meters from the cable specification.
6. Check the operating voltage range of each audible and visible device per the data sheet.
7. Build the table and calculate voltage drop using items 1 through 6.
8. Compare the calculated net voltage at each device against the appliance’s listed operating voltage.

Steps 1 through 6 are pure data gathering. If you shortcut them — estimating cable lengths, using “typical” currents instead of the actual data sheet values — step 7 produces a number that looks authoritative and means nothing.

The Electrical Fundamentals You Are Actually Using

Ohm’s law is the entire engine: V = I × R. Volts for potential, amperes for current, ohms for resistance. Everything else is bookkeeping.

Series circuits. The current is the same at every point in the loop, and the total voltage is the sum of the voltages across each resistance.

Parallel circuits. The voltage is the same across each branch, and the total current is the sum of each branch’s current.

Series-parallel circuits. This is what a real NAC is. As the video puts it:

“The type of the circuit to be considered is series-parallel. Horn/strobes are connected in parallel, while the cable resistance is in series.”

That single sentence is the mental model to hold onto. Each appliance hangs in parallel across the pair, so its alarm current adds to the total the panel must push. But every segment of cable between devices sits in series with everything downstream of it, so each segment takes its own bite out of the available voltage.

In an ideal circuit with zero conductor resistance, the full 24 volts reaches the last horn/strobe. In the real world, the first cable run might drop the circuit to 23 volts, and the second run drops it again to 22 volts at the next device. The voltage keeps stepping down as you walk the circuit, and the last device on the longest run is always the worst case. That is the number your design lives or dies by.

Building the Table: A Worked Example

The tutorial uses a mixed-occupancy building — Group C residential, ground-plus-one floors with a retail unit at ground level — covered by four NACs. Here is how the first two rows of the NAC-1 table come together.

Row 1 — panel to the first device:

• Distance from the FACP (the point of origin) to device 1: 3.6 meters
• Cable resistance from the data sheet: 7.41 ohms per 1,000 meters
• Segment resistance: 3.6 m × 7.41 Ω/1000 m ≈ 0.03 ohms
• Current through this segment: the total current of every appliance downstream — 1.274 amperes
• Voltage drop: V = 1.274 A × 0.03 Ω ≈ 0.038 volts

Row 2 — first device to second device:

• Measure the distance between device 1 and device 2, apply the same cable resistance, and you get a segment resistance of 0.10 ohms in this example.
• Now the current changes. Device 1 has already taken its share, so subtract device 1’s alarm current from the total: 1.274 A − device 1 current = 1.092 amperes flowing through this segment.
• Voltage drop: 1.092 A × 0.10 Ω ≈ 0.109 volts

Repeat that pattern down the circuit — each row uses the remaining downstream current and its own segment resistance — until you reach the final device. The cumulative drop is the sum of every segment above it.

The single most common error in this calculation is using total circuit current for every segment. Only the first run carries the full load. Each subsequent segment carries less, because devices behind you have already peeled off their current. Get that wrong and you will over-report the drop on short circuits and, worse, build a habit of not thinking about where the current actually goes.

Reading the Result

In the worked example, the lowest net voltages came out at 23.66 volts and 22.77 volts at the ends of the circuits.

“By comparing this value with the operating voltage of the fire alarm audible/visible device — if it’s within the operating range, then the design is pass, or in compliance.”

Two practical cautions on that comparison:

• Start from the real supply voltage, not the nameplate. NFPA 72 Chapter 10 requires the system to operate on secondary power, and during battery operation the supply sags well below 24 volts. Running the calculation from a sagging supply voltage is what the worst-case really looks like. A design that only passes from a full 24-volt starting point is not a design that passes.
• Use the listed minimum, not a rule of thumb. Designers often quote “16 volts” for 24 VDC appliances, but the authority is the data sheet. Some strobes have a narrower listed window, and a strobe that is under-volted may still flash while failing to produce its listed candela — a silent, invisible failure that only shows up in a light-intensity test.

Chapter 7 documentation requirements mean this table is not a scratch calculation you throw away. It becomes part of the record of completion, and the AHJ can reasonably ask to see it.

Field Habits That Keep the Math Honest

• Measure the route, not the map. Point-to-point distance means the path the cable actually takes — up the riser, around the beam, through the ceiling void — not the straight line on the floor plan.
• Remember the return conductor. Resistance accrues on both legs of the pair. Confirm whether your cable spec’s ohms-per-1,000-meters figure is per conductor or per loop before you build the table.
• Use alarm current, not standby. Standby current is a fraction of alarm current. Voltage drop is an alarm-condition problem.
• Recalculate when the candela changes. Bumping a strobe from 15 cd to 110 cd to satisfy a spacing issue can multiply its alarm current several times over, and it changes every row upstream of that device.
• Leave margin. A design that clears the listed minimum by 0.1 volts has nothing left for a long splice, a corroded terminal, or the T-tap someone adds during a tenant fit-out.

How Fire Code Mastery Fits Into This

Voltage drop is the kind of topic that separates people who have memorized NFPA 72 from people who can actually use it — and exam questions love it, because it tests Ohm’s law, circuit topology, and code knowledge all at once.

Fire Code Mastery is built for exactly that gap:

• 3,450+ exam questions covering NAC design, notification appliance requirements, and circuit calculations, with explanations that walk through the reasoning instead of just naming the right letter.
• 10+ built-in calculators, including voltage drop and battery calculation tools, so you can run the numbers the same way you would on a real design and check your hand calculations instantly.
• Flash cards for the constants and thresholds you need on recall — operating voltage ranges, conductor resistance figures, and the code sections behind them.
• Case studies modeled on real building designs like the mixed-occupancy example above, where you work a whole NAC layout from device count to final comparison.
• Mock tests under exam conditions, so the first time you work a series-parallel voltage drop problem against the clock is not on test day.

Work through the calculators alongside the question bank and the eight-step workflow stops being a procedure you follow and becomes something you simply know how to do.