Loop resistance (out and back)
R = 2 × L × r / 1000 + Σ R(isolators)
- L = the loop’s cable length (m), as drawn, with the riser
- r = the core’s resistance (Ω/km, IEC 60228): 1.0 mm² 18.1 · 1.5 mm² 12.1 · 2.5 mm² 7.41
STANDARDS & FORMULAS
FA Wiring
FA Wiring works with Fire Alarm: they share these references. Fire alarm loops: wiring, numbering, the loop’s DC voltage drop check and the riser diagram.
Standards and regulations
- NFPA 72National Fire Alarm and Signaling Code
Detector spacing, manual call points, notification appliances and installation notes.
- BS 5839-1Fire detection and fire alarm systems for buildings (non-domestic premises)
The BS option of the installation details and notes.
- IEC 60228Conductors of insulated cables
The copper core resistance (Ω/km at 20 °C) for the loop and 100 V line checks.
Formulas
Voltage at each device
V(device) = V(panel) − Σ I(alarm) × R(cable up to it)
- I(alarm) = each device’s current in alarm (detector quiescent, sounder / beacon alarm current)
- Checked against the lowest voltage the device works on (17 V as standard)
- Class A: worked out fed from one end (a break by the panel) and from both ends
Design rules it applies
- Numbering in the loop’s order: FACP-1/SD/01, FACP-1/HD/01 …, written into FA Address
- Class A loops drawn back to the panel
- Riser diagram per panel, drawn from the loops: every floor, every device with its number, isolators and the legend
For reference. MO.tool works these out to help your design; the engineer remains responsible for it. Always check the edition of each standard your project uses and the requirements of the local authority (DEWA, ADDC, Civil Defence…).