A ten-detector initiating device circuit, checked in both its quiet standby state and its worst-case alarm state — both comfortably clear the minimum operating voltage.
| Supply voltage | 24 V |
| Number of devices | 10 |
| Standby draw per device | 0.5 mA |
| Alarm draw per device | 30 mA |
| Devices alarming simultaneously (worst case) | 1 |
| End-of-line resistor | 10,000 Ω |
| Loop wiring resistance (round trip) | 20 Ω |
| Minimum operating voltage at farthest device | 16 V |
One device switches to full alarm draw; the other nine remain at their standby draw.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Standby-state device voltage | ≥ 16 V | 23.85 V | ✓ PASS |
| Alarm-state device voltage | ≥ 16 V | 23.26 V | ✓ PASS |
Key insight: Alarm-state voltage barely differs from standby-state voltage here (23.26 V vs 23.85 V) because only one device alarms at a time in this scenario — the moment more devices can alarm simultaneously (a real possibility in an actual fire event with multiple detectors triggering), alarm current climbs much faster and voltage at the farthest device drops correspondingly, which is exactly why 'worst-case simultaneous alarm count' is a deliberately conservative, adjustable input rather than always assuming just one device trips.
Every input in this example is editable in the live calculator — free, no signup.
Open Fire & Gas Detection Loop Budget calculator →Standby current is what the loop carries essentially all the time (supervising for wiring faults and device health), so it needs its own check for continuous reliable operation — while alarm current is a transient, higher-draw condition that has to be verified separately because it's the condition the whole system exists to detect and respond to correctly. A loop could pass one check and fail the other, so both need independent verification.
The end-of-line resistor provides continuous supervisory current specifically so the panel can distinguish a healthy loop from an open or short circuit — this small but constant EOL current adds to standby loop current and therefore standby voltage drop, which is why the calculator includes it in the standby calculation, though its relative contribution shrinks during alarm when device current dominates.