Check detection and sounder loop voltage drop and size fire-resistant cable to BS 5839-1.
Check loop voltage drop and size fire-resistant cable for detection and sounder circuits to BS 5839-1.
Fire alarm circuits use fire-resistant cable — FP200 Gold, MICC, or SWA with a fire barrier — and must be segregated from power wiring. The key design check is voltage drop along the loop: with the sounders or detectors in alarm, the voltage at the furthest device must stay above its minimum operating voltage (around 19 V for sounders on a 24 V system).
Enter the loop and the calculator returns the alarm current, loop resistance, voltage at the furthest device, and the maximum compliant circuit length for your cable size.
Max Zs values, diversity factors, cable ratings, voltage drop — one printable page. Plus occasional emails with calculator updates and useful tips.