What cable type is required for fire alarm systems?

BS 5839-1 requires fire-resistant cables for fire alarm circuits to maintain operation during a fire. Standard fire-resistant cable to BS 7629 provides 30 minutes fire resistance. Enhanced fire-resistant cable to BS 8434-2 provides 120 minutes and is the recommended standard for most installations. MICC (mineral-insulated copper-clad) cable provides unlimited fire resistance and is used for critical circuits. The cable must also be coloured red for easy identification.

What are the BS 5839-1 system categories?

L categories are for Life protection: L1 = full coverage of all areas, L2 = coverage of defined areas (escape routes plus rooms opening onto them plus high-risk areas), L3 = escape routes only, L4 = escape routes in HMOs to Approved Document B, L5 = engineered system to specific requirements. P categories are for Property protection: P1 = full coverage, P2 = defined areas. M category is manual call points only, no automatic detection.

How many detectors per zone are allowed?

BS 5839-1 limits conventional (non-addressable) systems to a maximum of 20 detectors per zone, with a zone covering no more than 2,000m² floor area. Each zone should be limited to one fire compartment. Addressable (analogue) systems can have more devices per loop (up to 126 or 250 depending on protocol) but should still be organized into logical zones for the fire brigade. Manual call points should be within 30m of any point on an escape route.

What is the maximum loop resistance for fire alarm cables?

The maximum loop resistance depends on the fire alarm panel specification — typically 40Ω for conventional systems and up to 100Ω for some addressable systems. Loop resistance is calculated as: R = 2 × cable length × resistance per metre. For 1.5mm² cable at 0.0122Ω/m, a 500m loop = 2 × 500 × 0.0122 = 12.2Ω. Excessive loop resistance causes voltage drop, which can prevent devices from operating correctly during a fire.

What spacing is required for fire alarm detectors?

BS 5839-1 specifies maximum coverage per point detector: Smoke detectors on flat ceilings up to 10.5m = 7.5m radius (approximately 100m²). Heat detectors = 5.3m radius (approximately 50m²). Ceiling height affects coverage — higher ceilings require closer spacing or different detector types. Beam detectors can cover up to 1,600m². Detectors must be positioned at least 500mm from walls and not in dead air spaces. Corridors less than 2m wide can use wider detector spacing.

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Fire Alarm Cable Calculator — BS 5839-1

Calculate fire alarm cable quantity, loop resistance, and detector coverage for BS 5839-1 compliant systems. Covers L1-L5 and P1-P2 categories with fire-resistant cable selection.

System Category

BS 5839-1 system category

Cable Type

Fire-resistant cable standard

Cable Size

1.5mm² standard for most systems

Number of Zones

Fire alarm zones or loops

Average Zone Cable Length

Average cable run per zone

Detectors per Zone

Max 20 for conventional systems

How We Calculate This

This calculator estimates fire alarm cable quantities and verifies loop resistance for BS 5839-1 compliant fire detection and alarm systems.

Loop Resistance

R = 2 × Cable length × Resistance per metre (Ω/m)

Cable Resistance Values

1.0mm²: 18.3 mΩ/m
1.5mm²: 12.2 mΩ/m (most common)
2.5mm²: 7.32 mΩ/m (long runs)
Detector coverage: Smoke = ~100m², Heat = ~50m²
Max per zone: 20 detectors (conventional)

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.