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Fire Alarm Cable Size Calculator

Size fire alarm cables and check voltage drop per BS 5839-1

Reference Info & Formulas
System Categories

L1: Full coverage (all areas)

L2: High risk areas + escape routes

L3: Escape routes only

L4: Circulation spaces only

L5: Custom (fire risk assessment)

M: Manual call points only

BS 5839-1 Clause 8

Cable Types

FP200 Gold: Standard, 2hr fire resistance. Used for most installations.

MICC: Unlimited fire resistance. Used in high-rise and critical applications. More expensive.

SWA: With fire barrier for industrial use.

BS 5839-1 Clause 26

System & Circuit
Select the system category and circuit type
Devices & Current
Enter device count and alarm current draw
mA

Alarm-mode current for worst-case calculation

Cable Details
Select cable type, size, and enter total loop length
m

Total cable run from panel to furthest device

Data: BS 5839-1:2017 -- Fire detection and alarm systems

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Fire alarm cable sizing is driven by voltage drop, not current-carrying capacity. The cable must deliver adequate voltage to the furthest device on the circuit.

Unlike power cables where thermal capacity is the primary concern, fire alarm cables carry very low currents. The critical design factor is ensuring that the voltage at the furthest device on the loop remains above the manufacturer's minimum operating voltage. The calculator determines the total circuit current in alarm conditions, calculates the loop resistance for the chosen cable, and checks the resulting voltage drop.

V(furthest) = V(panel) - (Ia x R x L x 2)
V(panel)
= Panel output voltage (24V or 48V DC)
Ia
= Total alarm current in amps
R
= Cable resistance per metre (Ω/m)
L
= Cable length to furthest device (m)
2
= Factor for go-and-return conductors

BS 5839-1:2017 Clause 26.2

If the calculated voltage at the furthest device falls below the minimum, the calculator recommends upsizing the cable or reducing the circuit length. It also calculates the maximum circuit length achievable with the selected cable size.

Quick Reference -- Device Currents

Typical fire alarm device currents

Manufacturer data (typical values)
Device TypeQuiescent (mA)Alarm (mA)Notes
Smoke detector0.335Optical or ionisation
Heat detector0.230Fixed temp or rate-of-rise
Sounder—25–60Varies by model
Sounder/beacon—80–150Visual alarm device
Call point—NegligibleSwitch device only

Always use the specific manufacturer data for the devices installed. These are typical values for estimation purposes only.

Practical Notes

Fire-Resistant Cable Is Mandatory

All fire alarm circuits must use enhanced fire-resistant cable complying with BS 8434-2 (such as FP200 Gold) or mineral insulated cable (MICC). Standard PVC, LSF, or SWA cable without a fire barrier is not acceptable for fire alarm wiring per BS 5839-1 Clause 26.2.

Addressable vs Conventional

Addressable systems typically use 2-wire loops where all devices share a single pair. Conventional systems use separate zone cables for detection and separate cables for sounders. Addressable systems usually have lower cable requirements because a single loop can serve many more devices and the panel can identify individual device addresses.

Cable Segregation

Fire alarm cables must be physically separated from all other cables. Where they run in parallel, maintain at least 300mm separation or use a metallic barrier. In shared containment, a segregated compartment within the trunking is required. Crossing at right angles is acceptable.

Frequently Asked Questions

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