Fire-Alarm NAC Circuit Voltage Drop (End-of-Line)
Checks that a fire-alarm notification-appliance circuit (NAC) still drives its horns and strobes at the far end.
Example
You enter
- Panel nominal voltage (V) 24
- Total appliance current at EOL (A) 0.8
- Run length (ft, one way) 250
- Conductor resistance (ohm/1000 ft) 3.14
- Device minimum voltage (V) 16
- Panel listed minimum NAC output (V, 0 = 85% of nominal) 0
You get
- Available voltage v 20.4
- Voltage drop (from CUSTV) 1.256
- End-of-line voltage 19.144
Checked against published worked examples from: Fire Alarms Online (Kyle Schuler).
Details, formula, and sources
ScopeThe panel's usable output is its listed minimum NAC output (CUSTV) from its installation manual, or by common convention about 85% of nominal -- 20.4 V on a 24 V panel, not 24. On a Class B circuit the current runs out and back, so the loop resistance is 2 x length x the conductor ohms/1000 ft; lumping the total appliance current at the end is the worst case: V_EOL = CUSTV - I x loop_R. A 0.8 A load on 250 ft of #14 stranded (3.14 ohm/1000 ft at 75 C) drops 1.26 V to 19.1 V -- above a 16 V device minimum, so it PASSES. If it fails, use heavier wire, shorten the run, split the circuit, or add a NAC power extender. A design screen; the panel's regulated voltage, the appliance draws and listed minimums, the wire table, and a signed fire-alarm design and AHJ govern.
Formula or tableavailable_voltage_v = panel_min_output_v when given, else 0.85 x nominal_voltage_v (CUSTV); loop_R = 2 x run_length_ft x (resistance_per_1000ft / 1000); voltage_drop_v = total_current_a x loop_R; eol_voltage_v = available_voltage_v - voltage_drop_v; within_spec when eol >= device_min_v.
Source and editionNFPA 72 (National Fire Alarm and Signaling Code) notification-appliance-circuit power / voltage provisions with the NEC Chapter 9 Table 8 conductor resistance, by name; the listed panel and appliances and the AHJ govern.
Free accessThe Ohm's-law voltage-drop relation is public physics; the starting voltage is the panel's listed minimum NAC output (0.85 x nominal is a common convention when none is listed), and the conductor ohms/1000 ft come from NEC Chapter 9 Table 8 (the appliance draws and listed minimums are on the datasheets).
What governsEstimate. AHJ and licensed professional govern.
API field namesnominal_voltage_v, total_current_a, run_length_ft, resistance_per_1000ft, device_min_v, panel_min_output_v, available_voltage_v, voltage_drop_v, eol_voltage_v
- CUSTV and Class B usable voltage = the panel's listed minimum NAC output, else 0.85 x nominal by convention; Class B loop = 2 x length x ohms/1000 ftpanel installation manual / NEC Ch 9 Table 8
- Worst-case load total appliance current lumped at the end of the line; each device must still see its listed minimumNAC design practice
- Default conductor #14 stranded copper, 3.14 ohm/1000 ft at 75 C (3.07 solid)NEC Chapter 9 Table 8