NAC Voltage Drop — Method
A notification appliance circuit (NAC) drives sounders and strobes from the panel or a booster power supply. Conductor resistance causes a voltage drop along the run: V_drop = 2 × I × R × L for the two-wire loop, where I is the total circuit current, R is the conductor resistance per unit length and L is the one-way run length. The voltage at the most-remote device equals the supply minus this drop and must remain at or above the devices listed minimum operating voltage — evaluated at the worst-case (lowest) battery voltage, not the nominal 24 V.
Frequently Asked Questions
V_drop = 2 × I × R × L, where I is the total NAC current, R is the conductor resistance per foot (e.g. 14 AWG ≈ 0.00253 Ω/ft) and L is the one-way length. The voltage at the last device = supply − V_drop must stay at or above the device minimum.
Use the worst-case (lowest) regulated voltage the panel provides under battery, often around 20.4 V for a nominal 24 V system, not 24 V. This is the condition the device must still operate in during a mains failure.
It is listed on the appliance — commonly about 16 V for 24 V-nominal devices, but always use the manufacturer's figure. If the end-of-line voltage falls below it, the strobe candela or sounder dB output is not guaranteed.
Increase the conductor size (e.g. 14 → 12 AWG), shorten the run, reduce the load on the circuit, split it into two NACs, or add a NAC booster/remote power supply nearer the appliances.
Current flows out and back on a two-wire circuit, so the total resistance is twice the one-way length. The factor of 2 accounts for both the supply and return conductors.
Fire Alarm NAC Voltage Drop Explained
Notification appliance circuits power the sounders and strobes that warn occupants. Because they run long distances at 24 V DC, conductor resistance can pull the far-end voltage below the level a strobe needs to flash at its rated candela. The code requirement is simple: every appliance must see at least its listed minimum voltage under the worst-case battery condition.
The worst case
Evaluate at the lowest regulated supply voltage (commonly ~20.4 V), with all appliances on the circuit active, and the load lumped at the far end for a conservative result. If the end voltage clears the device minimum there, it clears everywhere.
Fixing failures
If the circuit fails, the usual fixes are heavier copper, a shorter run, fewer devices per circuit, or a NAC booster power supply located closer to the appliances. The same V = 2·I·R·L logic underlies the general voltage drop calculator.
Related: Voltage Drop, Fire Alarm Battery, Cable Sizing.