How to Calculate NAC Voltage Drop: End-of-Line vs Point-to-Point
Why the last strobe on the circuit sets the rule
A notification appliance is listed to operate over a voltage range, and for the common "regulated 24 VDC" horns and strobes that range is 16 V to 33 V. Below 16 V a strobe may not flash at its rated candela and a horn may not reach its rated output, which is a life-safety failure, not a nuisance. The voltage at any appliance is what the panel puts out minus what the wire eats on the way, so the appliance at the far end of the circuit always sees the least. The calculation proves that the farthest appliance still has its minimum voltage under the worst condition the system must survive: on battery, at the end of the standby period, with every appliance in alarm. NFPA 72 (2022) Chapter 7 lists notification appliance circuit voltage drop calculations among the required documentation, and the AHJ will look for them. The adopted edition of NFPA 72, the panel and appliance listings and the AHJ govern the details.
Start from the panel's minimum, not from 24 V
The starting voltage is the lowest voltage the panel is listed to deliver at its NAC terminals, stated in the installation manual. For a 24 V panel running on a discharged lead-acid battery that figure is usually 20.4 V: twelve cells at 1.7 V per cell, the end-of-discharge voltage. Some panels and NAC power supplies publish a different minimum, and some manufacturers require their own worksheet or software; when they do, that method governs. Never start from the nominal 24 V or the 27 V you read with the charger on. The margin you are working with is the panel minimum minus the appliance minimum, and with 20.4 V and 16 V that is only 4.4 V of drop allowed.
The end-of-line (lump-sum) method
The simplest and most conservative calculation assumes the entire alarm current of the circuit flows all the way to the last appliance. The loop resistance is twice the one-way length (out and back) times the wire's resistance per foot, and the drop is current times resistance:
R = 2 × L × (ohms per 1,000 ft) ÷ 1,000, then VD = I × R
| Wire | Ohms per 1,000 ft, solid copper at 20°C (common manufacturer tables) | Ohms per 1,000 ft, NEC Chapter 9, Table 8 at 75°C |
|---|---|---|
| 18 AWG | 6.385 | 7.77 |
| 16 AWG | 4.016 | 4.89 |
| 14 AWG | 2.525 | 3.07 |
| 12 AWG | 1.588 | 1.93 |
Both columns are in use. Most appliance manufacturers' tables and software use the 20°C values; the NEC 75°C values are about 20% higher, give a more conservative answer, and are what the NAC voltage drop calculator uses. Use the table your panel manufacturer or AHJ specifies, and say which one on the sheet.
Worked example: ten horn/strobes at 0.06 A each is 0.6 A, on 300 feet of 14 AWG from a panel with a 20.4 V minimum. R = 2 × 300 × 2.525 ÷ 1,000 = 1.515 ohms. VD = 0.6 × 1.515 = 0.909 V. The last appliance sees 20.4 − 0.909 = 19.49 V, comfortably above 16 V. With the 75°C value the drop is 1.105 V and the end-of-line voltage is 19.3 V; it still passes. Where the lump-sum method bites is a circuit of high-candela strobes: 1.5 A over 400 feet of 14 AWG drops 3.7 V with the NEC values and lands at 16.7 V, with almost nothing to spare.
The point-to-point method
In the real circuit the current is not all at the end. Between the panel and the first appliance the wire carries the full 0.6 A; after each appliance taps off its 0.06 A, the next segment carries less. The point-to-point method figures the drop segment by segment with the current actually flowing in each, and adds them up. Take the same ten appliances spaced 30 feet apart on 14 AWG. Each 30-foot segment has 60 feet of conductor, or 0.1515 ohms. The first segment carries 0.6 A (0.091 V), the second 0.54 A (0.082 V), and so on down to the last at 0.06 A (0.009 V). The total is 0.50 V, and the last appliance sees 19.9 V instead of 19.49 V. For an evenly distributed load the point-to-point drop comes out close to half the lump-sum figure, which is why the calculator offers an "evenly spread" option as a quick estimate.
Point-to-point takes more work and needs the device locations from the drawings, but it rescues a long circuit that fails lump-sum by a few tenths of a volt, and it is what the manufacturers' software does. Many AHJs accept either; some want lump-sum for its margin. Run lump-sum first; if it passes, you are done.
Class A, Class B and the fixes
A Class B NAC ends at the end-of-line resistor, and the length you calculate with is the wire from the panel to the last appliance. A Class A circuit returns to the panel on a separate path and in normal operation is powered from one end, like Class B. Under a single open fault the panel feeds both ends, and if the open is near the panel on the outgoing leg, the farthest appliance is fed through the entire return loop. So the conservative Class A calculation uses the full loop length, out and back to the panel, with the full load; most manufacturers' instructions say exactly that.
When a circuit fails, you have four tools: go up a wire size (12 AWG instead of 14 AWG cuts the drop by about 37%), split the circuit so each run carries less current over a shorter distance, add a NAC power supply near the load so the long run carries only the trigger circuit, or lower the appliance current with lower-candela settings where the Chapter 18 coverage still works out. On the submittal, show the method, the starting voltage and its source, the resistance table, each appliance's current at its installed candela, the lengths, and the end-of-line voltage beside the appliance minimum. Keep the alarm current consistent with the battery calculation; the general voltage drop calculator uses the same resistance math for a second check. Fireforge runs the lump-sum and evenly-spread versions of this calculation offline, and the fire alarm practice exam has a few of these to try.
Step by step
- Get the two voltagesThe panel's minimum NAC voltage on battery from the manual (often 20.4 V) and the appliance minimum from its data sheet (often 16 V). The difference is your budget.
- Total the alarm currentAdd every appliance on the circuit at its installed candela or tap setting.
- Measure the lengthPanel to the last appliance for Class B; the full loop, out and back, for Class A.
- Run the lump-sum calculationR = 2 × L × (ohms per 1,000 ft) ÷ 1,000, VD = I × R, and end-of-line voltage = panel minimum − VD. It must stay above the appliance minimum.
- Go point-to-point if it is closeCompute each segment with the current it actually carries and add the drops; the result is lower and usually accepted.
- Fix and documentUpsize the wire, split the circuit, add a power supply or lower the candela; then put the method, voltages, table, currents and lengths on the submittal.
Frequently asked questions
What starting voltage do I use for a NAC voltage drop calculation?
The panel's listed minimum NAC output on battery, from the installation manual. For most 24 V panels that is 20.4 V (twelve cells at 1.7 V). Never use the 24 V nominal.
What is the minimum voltage for a horn/strobe?
Appliances listed as regulated 24 VDC operate from 16 V to 33 V, so 16 V is the usual minimum. Check the data sheet; some special-application appliances are different.
Is the end-of-line method or the point-to-point method required?
NFPA 72 requires the calculation, not a particular method. Lump-sum is the conservative one most AHJs accept; point-to-point is accepted where the manufacturer's instructions or the AHJ allow it and is what manufacturer software does.
How do I calculate a Class A NAC?
Use the full loop length, out and back to the panel, with the total circuit current, because a single open near the panel can leave the farthest appliance fed through the entire return path.
Written by TapForge Studios, a one-person Android studio run by a tradesman with a background in electrical, HVAC and life-safety work. Reviewed October 7, 2026. This guide is general information, not engineering, legal or tax advice; the adopted code edition, the manufacturer's instructions and the authority having jurisdiction govern.
Free, no ads, made by one person. If this guide helped, you can support the site.
