BS 5839-1 Cable Sizing for Fire Alarm Systems: What Engineers Get Wrong

Cable sizing for fire alarms is one of those tasks that looks simple until it catches you out on site. BS 5839-1 isn't prescriptive about conductor cross-sections in the way IEE Wiring Regulations are — it gives you the performance requirements and leaves the calculation to you. That's where the problems start.

This guide covers the core requirements, the calculations that matter, and the three areas where experienced engineers still make mistakes.

What BS 5839-1 Actually Requires

The standard doesn't mandate a specific cable size. What it requires is that the cable:

• Maintains circuit integrity under fire conditions (Clause 26)
• Keeps voltage drop within acceptable limits to ensure all devices operate correctly
• Has adequate current-carrying capacity for the connected load

"Circuit integrity" is the key phrase. Under BS 5839-1, fire alarm cables in a fire-rated installation must maintain their function for the required duration — typically 30 or 60 minutes depending on the application. The standard references BS EN 50200 for the test methodology.

The Circuit Integrity Requirement

Clause 26 of BS 5839-1 requires that cables interconnecting fire alarm components maintain circuit integrity under fire conditions. This means:

Enhanced cables (commonly called "fire-resistant" or "FP") — cables that maintain function when exposed to fire, tested to BS EN 50200 or equivalent. These are required for:

• Cables between the control panel and any component that must function during fire (sounders, beacons, door holders, sprinkler interfaces)
• Cables between repeater panels
• Cables supplying power to the main panel

Standard cables — acceptable for low-risk detection circuits where a cable failure under fire conditions would only affect detection in an already-burning area and would not impair evacuation.

In practice, most responsible designers specify enhanced (FP) cable throughout. Understand what the design specifies before you start running cable, especially on variations or extensions.

Common FP cable types in UK practice:

• FP200 Gold (Draka/Prysmian) — the market standard, available 1.5mm² and 2.5mm²
• MICC (mineral insulated) — for the most demanding environments
• LSF/LSOH alternatives — where low smoke emission is required

Voltage Drop: The Calculation That Gets Skipped

This is where most on-site decisions go wrong. Engineers often pull a cable size from habit (1.5mm² for detection, 2.5mm² for sounders) without calculating whether voltage drop is actually acceptable for the specific run length and load.

The basic calculation:

Voltage drop (V) = I × R

Where:

• I = current draw of all devices on the circuit (A)
• R = total circuit resistance (Ω) — outgoing conductor + return conductor

Cable resistance is given in mΩ/m (milliohms per metre). For 1.5mm² copper: approximately 12.1 mΩ/m per conductor (from manufacturer data at 20°C). For 2.5mm²: approximately 7.41 mΩ/m per conductor.

For a circuit of length L: R = 2 × L × resistance per metre

The factor of 2 accounts for the outgoing and return conductors.

Acceptable voltage drop under BS 5839-1: The standard doesn't give a specific figure, but devices must operate within their rated voltage tolerance. For most 24V DC fire alarm devices, this is ±20%, so you have a 4.8V budget. In practice, most designers work to a maximum drop of 2V (for the device to still operate at 17.6V under battery conditions).

Example: 50 devices, each drawing 25mA quiescent = 1.25A. Run of 300m in 1.5mm² cable.

R = 2 × 300 × 0.0121 = 7.26Ω V drop = 1.25 × 7.26 = 9.08V

That's way outside acceptable limits. You'd need 2.5mm² (R = 4.45Ω, V drop = 5.55V — still over budget) or 4mm² (R = 2.78Ω, V drop = 3.47V — marginal), or you'd need to split the circuit.

Running the calculation before you order cable is a lot cheaper than re-cabling.

Loop Resistance and the 40Ω Rule

On conventional systems with end-of-line (EOL) resistors, total loop resistance affects zone supervision. If the total loop resistance is too high, the panel may interpret the zone as open circuit.

Most conventional fire alarm panels specify a maximum zone loop resistance — commonly 40Ω for the total loop (outgoing + return). Check the panel manufacturer's specification, not the standard.

For 1.5mm² cable: 40Ω gives you approximately 1,650m of total cable run (two-core). For large or multi-building sites, this is a genuine constraint and may require heavier cable or zone splitting.

The Three Mistakes Engineers Make

1. Not accounting for temperature

Cable resistance increases with temperature. At 70°C conductor temperature, resistance is approximately 30% higher than at 20°C. For circuits in high-temperature environments (plant rooms, roof spaces in summer), use temperature-corrected figures in your calculation.

2. Forgetting the CPC

Under BS 7671, the circuit protective conductor (earth) must also be sized correctly. For fire alarm circuits, the CPC is typically integral to the cable (FP200 and similar). But on extensions or modifications where separate CPC runs are added, confirm the CPC cross-section meets the requirements in Table 54.7 of BS 7671.

3. Using quiescent current only

Voltage drop calculations should use alarm current, not quiescent current. Alarm current — all sounders firing, all devices active — can be 5–10× higher than quiescent draw. Running the calc on quiescent current and then discovering voltage drop exceeds limits when the system goes into alarm is a problem you don't want to find during commissioning.

Quick Reference

| Cable size | Resistance (mΩ/m/conductor) | Max run at 2V drop, 1A load | |---|---|---| | 1.0mm² | 18.1 | 55m | | 1.5mm² | 12.1 | 83m | | 2.5mm² | 7.41 | 135m | | 4.0mm² | 4.61 | 217m |

Figures at 20°C, two-core circuit (×2 for total resistance)

Doing the Calculation Faster

IFS Pro includes a BS 5839-1 voltage drop and loop resistance calculator so you can run these numbers on site or during design without working through the formula every time. You put in run length, cable size, and device load — it tells you whether you're inside limits. Available at incognitofiresecurity.com.