NAC Voltage Drop: How to Calculate and Verify It

Why voltage drop matters

Notification appliances must receive enough voltage to operate correctly at the end of the circuit under the worst expected conditions. The available voltage is affected by power-supply output, battery condition, circuit current, conductor size, total route length, connection resistance, and the way appliances are distributed along the run.

A circuit can pass continuity testing and still fail during alarm because the voltage collapses under load.

Use the correct starting voltage

Do not automatically calculate from a nominal 24 volts. Use the minimum output voltage identified by the listed power supply for the applicable operating condition. If the circuit must operate on batteries, the design value may be lower than the normal AC-powered output. Manufacturer documentation and the approved submittal should identify the design basis.

Understand the two-conductor path

Current travels out on one conductor and returns on the other, so the resistance calculation includes the complete loop length. For a simple end-loaded circuit, voltage drop equals current multiplied by total loop resistance. For appliances spread along the run, a point-to-point or segmented calculation is more accurate because the current decreases after each appliance.

Using the full circuit current across the full one-way distance is conservative only when the formula already includes the return path. Mixing one-way and loop-length assumptions is a common mistake.

Account for appliance settings

Use the current for the actual candela, horn pattern, temporal setting, speaker wattage, or synchronization mode. A field change from 15 candela to 75 candela can significantly increase load. Use the manufacturer’s listed current table for the specific product and voltage range, not a generic value from a different model.

Include all devices powered by the circuit, including synchronized strobes, horns, sounder bases, relays, and auxiliary modules where applicable.

Worked example

Suppose a circuit carries 1.20 amperes at the source and the farthest appliance is 300 feet away using 14 AWG copper. If the design uses approximately 2.525 ohms per 1,000 feet per conductor, the 600-foot loop resistance is about 1.515 ohms. The simple end-loaded voltage drop is therefore about 1.82 volts.

If the minimum source voltage is 20.4 volts, the estimated far-end voltage is about 18.58 volts. That result must be compared with the listed operating range of every appliance on the circuit. A segmented calculation may show a lower actual drop when devices are distributed along the run.

Verify in the field

Measure voltage at the source and at the farthest or most heavily loaded point while the circuit is active. Compare readings to the design calculation. If actual drop is greater than expected, inspect wire gauge, route length, splices, terminal tightness, corrosion, shared conductors, and appliance settings.

Do not rely on a no-load reading. The circuit must be operating in the condition being evaluated, with monitoring and site procedures coordinated before activation.