Cable Tray Sizing Calculator — Get the Right Tray for Your Cable Run

Cable tray is an open cable containment system used in commercial, industrial, and large residential installations to support and route cables horizontally and vertically. Unlike conduit and trunking, cable tray is open — cables are laid on top of the tray rather than enclosed within it. This makes cable tray ideal for large cable runs where heat dissipation is important and where cables need to be easily accessible for maintenance, modification, or additional circuits.

Cable tray is commonly found in ceiling voids, service corridors, plant rooms, data centres, factory floors, and external cable routes. It supports power cables, control cables, data cables, and fibre optic cables. The tray provides mechanical support and protection while allowing air circulation around the cables — a significant advantage over enclosed containment for high-current circuits where heat dissipation is critical.

Sizing cable tray correctly ensures that cables fit without overcrowding, that the tray can support the weight of the cables, and that current-carrying capacity is not reduced by excessive grouping. The cable sizing calculator determines the cable sizes, and the tray calculator determines the tray size needed to carry them. For enclosed containment, see the trunking fill calculator and conduit fill calculator.

There are four main types of cable tray used in UK installations, each suited to different applications:

• Ladder tray — consists of two side rails connected by cross rungs (like a ladder). Provides excellent ventilation and is the preferred choice for heavy power cables where heat dissipation is important. Cables are tied to the rungs with cable ties. Most common in industrial and commercial installations.
• Perforated tray — a solid base with regularly spaced holes or slots. Provides moderate ventilation while offering better cable support than ladder tray (smaller cables do not fall between rungs). Common for mixed cable runs with both power and data cables.
• Wire mesh tray — made from welded wire mesh. Lightweight, easy to cut and modify on site, and provides good ventilation. Increasingly popular for data cable installations in office ceiling voids. Lower load capacity than ladder or perforated tray.
• Solid-bottom tray — a continuous metal base with no perforations. Used where additional cable protection is needed or where fire compartmentation requires a solid barrier. Provides the least ventilation and results in the highest cable derating factors.

The choice of tray type affects the cable current-carrying capacity. The installation method reference number — drawn from BS 7671 Appendix 4 Tables 4A1 and 4A2 — determines which current-carrying capacity column applies. Cables installed on open tray (ladder or perforated) use Method E or F (multi-core cables in free air), which typically gives higher ratings than Method B (enclosed in conduit or trunking), because air can circulate freely around the cables. This connects directly with the voltage drop calculations for long cable runs.

Cable tray fill calculation differs from conduit and trunking fill because cable tray is open — cables sit on the tray surface rather than being enclosed. The primary concern is not volumetric fill percentage but rather whether the cables fit within the tray width, considering the recommended installation arrangement.

For power cables, the preferred installation is a single layer with cables laid flat side by side (for flat cables) or touching in a single row (for round cables). Single-layer installation ensures maximum heat dissipation and avoids the derating factors that apply when cables are stacked in multiple layers.

For control cables, data cables, and other low-current cables, multi-layer installation is acceptable because these cables do not generate significant heat. In this case, the calculation is based on the total cross-sectional area of all cables compared to the tray cross-sectional area (width x depth), with a recommended maximum fill of 50% to allow for cable management and future additions.

When power cables and data cables share the same tray, segregation requirements from BS 7671 Regulation 528.1 apply. The tray may need a divider or the cables may need to be separated by a minimum distance. Compartmentalised tray or separate trays are often the simplest solution. For more on containment sizing, see the trunking fill calculator.

To calculate tray fill, you need the overall diameter of each cable. For round cables, this is the single overall diameter. For flat cables, you need the width and height. The following table shows overall diameters for common UK cable types:

These diameters are approximate and vary slightly between manufacturers. Always verify from the manufacturer's data sheet for accurate fill calculations. Elec-Mate includes a comprehensive database of cable dimensions for all common UK cable types, including SWA, T&E, singles, LSOH, and data cables.

BS EN 61537 (IEC 61537) is the standard that specifies the requirements for cable tray and cable ladder systems. It covers mechanical requirements, corrosion protection, fire performance, and testing methods. Key requirements include:

• Load capacity — the tray must support the weight of the cables plus a safety factor. Load capacity is rated per metre and depends on the span between supports. Typical support spacing is 1.5 m to 3 m depending on tray type and load.
• Corrosion protection — steel tray must be protected against corrosion. Options include hot-dip galvanising (HDG), pre-galvanising, powder coating, and stainless steel. HDG is the standard for general indoor use. Stainless steel is required in corrosive environments.
• Electrical continuity — metal cable tray must provide reliable electrical continuity throughout the system to serve as a supplementary CPC. All joints and fixings must maintain continuity.
• Fire performance — tray systems in fire compartments must be fire-stopped at penetrations. Some projects require tray with specific fire ratings.

When selecting cable tray, ensure the manufacturer declares compliance with BS EN 61537 and provides load capacity data for the support spacing planned. The load schedule helps determine how many and what size cables each tray run will carry, feeding into the tray sizing calculation. For overall project documentation, the EIC certificate records the containment system used.