How do I determine cable tray width?

Cable tray width is determined by the number of cables, their outer diameters, and the required spacing between them. For SWA cables laid flat in a single layer: total width = (cable OD × number of cables) + (gaps × (cables - 1)). Allow 5-10mm between cables for ventilation and future additions. The tray width must accommodate this total plus a margin. Single-layer installation is preferred as it avoids derating — stacking cables requires grouping factor correction per BS 7671.

What support bracket spacing is needed for cable tray?

Standard bracket spacing depends on tray width and loading: Light duty (10-15kg/m) = 1.5-2.0m spacing, Medium duty (20-50kg/m) = 1.2-1.5m spacing, Heavy duty (50kg/m+) = 0.9-1.2m spacing. Vertical runs require closer spacing (typically 1.0m). At changes of direction (bends, tees), brackets should be within 300mm of the fitting on each side. Always check the tray manufacturer's data sheet for specific load/span tables.

Should cables be in a single layer on cable tray?

Yes, where possible. BS 7671 current-carrying capacity tables assume cables are laid flat in a single layer with spacing. If cables are touching or stacked, grouping correction factors apply — for touching cables in a single layer, the factor is typically 0.79-0.95 depending on the number of circuits. Two layers of touching cables can reduce capacity to 60-70% of the tabulated value. Single-layer installation also makes it easier to identify and trace individual cables.

What is the weight of SWA cable per metre?

SWA cable is significantly heavier than non-armoured cable due to the steel wire armour. Typical weights: 4-core 2.5mm² SWA = 0.64kg/m, 4-core 16mm² = 2.12kg/m, 4-core 50mm² = 5.50kg/m, 4-core 70mm² = 7.50kg/m. A tray carrying 10 × 4-core 25mm² SWA cables weighs 31kg/m — requiring heavy-duty tray and 0.9-1.2m bracket spacing. Always include the tray self-weight (typically 2-5kg/m) in the total loading calculation.

Do I need to earth cable tray?

Yes — metal cable tray must be earthed as it is an exposed-conductive-part. Per BS 7671, the tray should be bonded to the main earthing terminal using a separate CPC, or verified as providing a reliable low-impedance earth path throughout its length. Tray sections must be electrically continuous — use bonding links or earth straps at joints. The minimum CPC size depends on the circuit protection but is typically 4mm² minimum for the tray bonding conductor.

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Cable Tray Sizing Calculator

Calculate cable tray width and load rating requirements based on cable count, size, and weight. Includes support bracket spacing guidance for SWA and multicore cables.

Cable Type

SWA cable type

Cable Size

Conductor CSA

Number of Cables

Total cables on the tray

Cable Tray Width

Standard tray size

How We Calculate This

This calculator determines the required cable tray width and load capacity based on your cable installation requirements.

Calculation Method

Tray width ≥ (Cable OD × Count) + (Gaps × (Count - 1))

Load rating ≥ Cable weight/m × Cable count

Standard Tray Widths

75-150mm: Small runs, lighting circuits
225-300mm: Standard commercial installations
450-600mm: Main cable routes, risers
750-900mm: Heavy industrial, data centres
Bracket spacing: 0.9-2.0m depending on load

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.