Checks that a two-wire fire/gas detector loop (initiating device circuit) delivers enough voltage to the farthest device in both its normal standby state and its worst-case alarm state, accounting for the loop's wire resistance, an end-of-line resistor's supervisory current, and each device's own current draw.
Standards followed
Generic Ohm's-law loop-budget model, built the same way as this suite's 4-20mA loop calculator, consistent with the reasoning fire alarm/gas panels use internally (see NFPA 72 IDC design practice) — not tied to one specific panel manufacturer's exact topology
What it can do
- Standby loop current from each device's quiescent draw plus the end-of-line resistor's supervisory current.
- Alarm loop current for a worst-case number of simultaneously-alarming devices, with the rest still at standby draw.
- Voltage available at the farthest device in both states, checked against its minimum operating voltage.
Example problem
24V panel, 10 detectors at 0.5mA standby / 30mA alarm, 1 alarming worst case, 10kΩ EOL, 20Ω round-trip loop resistance, 16V minimum device voltage.
- Standby current = 10×0.5mA + 24V/10kΩ = 5mA + 2.4mA = 7.4mA.
- Standby voltage at device = 24 − 0.0074×20 = 23.85V — passes.
- Alarm current = (1×30mA + 9×0.5mA) + 2.4mA = 34.5mA + 2.4mA = 36.9mA.
- Alarm voltage at device = 24 − 0.0369×20 = 23.26V — passes.
Both standby (23.85V) and alarm (23.26V) states comfortably clear the 16V minimum.
Want the full step-by-step version?
This is a generic loop-budget model, not a substitute for your specific fire/gas panel's own loop-loading tables — some panels bypass the EOL resistor during alarm, or apply different supervision logic; confirm against your panel's installation manual for a real design. Addressable loops (which communicate digitally rather than relying on an analog voltage/current budget) aren't modeled here.