Fire-Alarm NAC Circuit Voltage Drop (End-of-Line)

The panel's usable output is its regulated minimum (CUSTV), about 85% of nominal per NFPA 72 -- 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 (2.525 ohm/1000 ft) drops 1.01 V to 19.4 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.

available_voltage_v = 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.

NFPA 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.

The Ohm's-law voltage-drop relation is public physics; the 0.85 regulated-minimum (CUSTV) factor is NFPA 72, and the conductor ohms/1000 ft come from NEC Chapter 9 Table 8 (the appliance draws and listed minimums are on the datasheets).

Estimate. AHJ and licensed professional govern.

Field names used by the API: nominal_voltage_v, total_current_a, run_length_ft, resistance_per_1000ft, device_min_v, available_voltage_v, voltage_drop_v, eol_voltage_v

• CUSTV and Class B usable voltage = 0.85 x nominal (NFPA 72 regulated minimum); Class B loop = 2 x length x ohms/1000 ftNFPA 72 / 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