NAC Voltage Drop: There Is No 10% Rule in NFPA 72
Ask ten techs what the allowable voltage drop is on a notification appliance circuit and a good number of them will say 10%. It is one of the most repeated numbers in the trade, and it is not in NFPA 72.
There is no flat 10% voltage drop allowance in the fire alarm code. What the code cares about is whether every notification appliance gets enough voltage to operate as listed. This post covers how a NAC voltage drop calculation actually works, written from the perspective of a NICET Level IV designer who has reviewed a lot of circuits that only passed on paper.
Where the 10% Myth Comes From
The 10% figure probably traces back to general wiring practice, rules of thumb on older panels and calc sheets that got copied from job to job. It feels safe because it is a simple number. The problem is that it measures the wrong thing.
A 10% drop from 24 V suggests the far appliance sees 21.6 V. But the panel is not always running at 24 V. On battery, near the end of its discharge, the starting voltage is lower. And the appliance at the end of the line does not care about a percentage. It either gets its listed minimum operating voltage or it does not.
What Actually Matters: Listed Minimum Operating Voltage
The real requirement is that every notification appliance receives at least its listed minimum operating voltage, under worst-case conditions.
Two pieces drive that:
- The starting voltage. Use the end-of-discharge battery voltage, not 24 V. On a nominal 24 V system that is 20.4 V (85% of nominal). That is the voltage the panel delivers when the system goes into alarm at the end of the required standby period.
- The appliance minimum. Many 24 VDC regulated notification appliances are listed to operate down to 16 V. But that is not a set code number. The minimum comes from each appliance’s datasheet, and it can differ between models and manufacturers.
So the voltage you can afford to lose is the starting voltage minus the appliance minimum. With 20.4 V at the panel and a 16 V appliance minimum, you have 4.4 V to work with across the circuit.
The Calculation Method
Step 1: Get the Wire Resistance
Wire resistance comes from NEC Chapter 9, Table 8, which lists DC resistance per 1,000 ft for each conductor size. Approximate values for uncoated solid copper:
| Wire size | Approx. resistance (ohms per 1,000 ft) |
|---|---|
| 18 AWG | approximately 7.77 |
| 16 AWG | approximately 4.89 |
| 14 AWG | approximately 3.07 |
| 12 AWG | approximately 1.93 |
These are approximate figures for illustration. Use the actual table values for your conductor (solid or stranded, coated or uncoated) and confirm against the edition of the NEC in force.
Step 2: Use the Loop Length
Current goes out on one conductor and comes back on the other. The loop length is 2 x the one-way distance from the panel to the last appliance. A 200 ft run is 400 ft of wire.
Step 3: Calculate Circuit Resistance
R = (loop length in ft / 1,000) x ohms per 1,000 ft
Step 4: Calculate Voltage Drop
V drop = I x R, where I is the circuit current in amps.
Step 5: Check the End-of-Line Voltage
End-of-line voltage = starting voltage minus V drop. Compare it to the appliance’s listed minimum.
Lumped Load vs. Point-to-Point
There are two common ways to model the load.
Worst-Case Lumped Load
Assume the entire circuit current is drawn at the far end of the circuit. It is conservative, fast and easy to review. If a circuit passes the lumped calculation, it will pass a more detailed one.
Point-to-Point (Segment) Method
Break the circuit into segments between appliances. Each segment carries only the current of the appliances downstream of it, so the drop is calculated segment by segment and added up. This is more accurate and often shows that a circuit which fails the lumped method actually works. It takes more effort, and it is what you want when a circuit is long or heavily loaded.
Worked Example
The values below are round illustrative numbers, not from any real product or project. Assume a 16 V listed minimum for the appliances in this example. Your minimum comes from the appliance datasheet.
Example inputs:
- Starting voltage: 20.4 V (end-of-discharge on a nominal 24 V system)
- Circuit current: 1.5 A
- One-way distance to the last appliance: 200 ft
- Loop length: 2 x 200 = 400 ft
- Lumped-load method (all current at the end)
14 AWG (approximately 3.07 ohms per 1,000 ft):
- R = 0.4 x 3.07 = 1.23 ohms
- V drop = 1.5 x 1.23 = 1.84 V
- End-of-line voltage = 20.4 minus 1.84 = 18.56 V. Above 16 V, passes.
16 AWG (approximately 4.89 ohms per 1,000 ft):
- R = 0.4 x 4.89 = 1.96 ohms
- V drop = 1.5 x 1.96 = 2.93 V
- End-of-line voltage = 20.4 minus 2.93 = 17.47 V. Passes.
18 AWG (approximately 7.77 ohms per 1,000 ft):
- R = 0.4 x 7.77 = 3.11 ohms
- V drop = 1.5 x 3.11 = 4.66 V
- End-of-line voltage = 20.4 minus 4.66 = 15.74 V. Below 16 V, fails.
Notice what the 10% rule would have said. A 10% drop from 24 V is 2.4 V. By that rule, the 16 AWG circuit (2.93 V drop) fails and gets upsized for no reason. Meanwhile, the rule tells you nothing about whether the far appliance actually works at end of battery discharge. Calculating against the listed minimum is both more accurate and more honest.
Tips From the Field
- Use the right current values. Appliance current depends on the candela setting, the tone and whether you use the RMS or peak value the manufacturer specifies for calculations. Follow the manufacturer’s instructions.
- Account for every appliance. Spare capacity for future devices is a good habit.
- Check the panel’s NAC rating. A circuit can pass voltage drop and still exceed the panel or power supply’s rated output per circuit.
- Follow the manufacturer’s method. Some manufacturers publish their own voltage drop method or required starting voltage. When they do, it governs.
- Show your work on the drawings. Plan reviewers want to see starting voltage, wire size, resistance, current and end-of-line voltage for each circuit.
For quick checks, use our free NAC voltage drop calculator. Voltage drop pairs with the battery calc, so see fire alarm battery calculations, along with NFPA 72 smoke detector spacing and fire alarm design in AutoCAD.
FAQ
Is there a 10% voltage drop rule for fire alarm NACs?
No. NFPA 72 does not set a flat 10% allowance. The requirement is that every notification appliance receives at least its listed minimum operating voltage under worst-case conditions.
What starting voltage should I use for a NAC voltage drop calculation?
Use the end-of-discharge battery voltage: 20.4 V on a nominal 24 V system (85%), not 24 V. Follow the manufacturer’s method if it specifies a different value.
Where do I find wire resistance for voltage drop?
NEC Chapter 9, Table 8 lists DC resistance per 1,000 ft for each conductor size. Multiply by the loop length (2 x the one-way distance) divided by 1,000.
Voltage Drop Calcs Your AHJ Can Follow
Titan Fire Protection prepares NICET-prepared fire alarm drawings for projects nationwide, with point-to-point voltage drop and battery calculations on every package, designed and prepared by Joseph Montuori, NICET Level IV, SET. Want an existing set checked before submittal? Our flat-fee drawing review covers it. Learn about our design process.
Request a quote or call 860-322-9028.