Fire Alarm Loop Load: engineer's guide

Every SLC loop has a device limit, a current limit, and a resistance limit. Miss any and the panel supervises fault.

By Umar Dar · Fire Alarm/ELV · Published 2026-09-19 · Last updated 2026-09-19

What this calculator does

The calculator sums per-device current and resistance around the loop and reports remaining capacity against the panel-specified limits.

Formulas used

Loop current: I_loop = Σ I_device + I_driver
Loop resistance: R_loop = Σ L_segment × r_{per m}

Variables

I_device = per-device standby current, typically 300 to 500 μA
r = cable resistance per metre (loop = both cores)

Worked example

Scenario: 80 addressable devices at 350 μA, 1.5 mm² twisted pair loop 400 m end-to-end.

I_loop = 80 × 0.00035 + 0.060 driver = 0.088 A — under typical 500 mA limit
Loop resistance = 400 × 2 × 12 mΩ/m = 9.6 Ω — under typical 40 Ω limit
Both within panel limits; loop is compliant

Common mistakes to avoid

Counting per-device current on the driver limit only
One-way length instead of loop-length
Missing the T-tap resistance for spurs
Using shielded cable resistance from cold table (heats up)

Frequently asked questions

Class A or Class B loop?

Class A returns to the panel and survives a single break. Class B is spur. Class A required for critical premises per NFPA 72 Ch. 12.

Maximum devices per loop?

Panel-specified — typically 99 to 250 addressable devices per SLC loop.

Standards and references

NFPA 72, EN 54-13, BS 5839-1

Related tools on this site

Open the calculator that pairs with this guide: Fire Alarm Loop Load.

Verification note

This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.