Understanding Cable Tray Load Capacity

What is Cable Tray Load Capacity? 

Cable tray load capacity is the maximum weight a tray can safely support under stated conditions. Manufacturers commonly express this as a safe working load per metre, using kilograms per metre or newtons per metre.

This figure must always be considered alongside the support spacing. The same cable tray may have a different safe working load when installed across a longer span, so the manufacturer’s data should be checked against the proposed installation.

Why Cable Tray Loading Capacity Matters 

Choosing a cable tray with the correct capacity protects the cables and the wider support system. It also reduces the chance of sagging or movement and minimises the need for remedial work after installation. 

Protecting Cables 

An overloaded tray may sag or twist, placing unwanted pressure on cables around joints and support points. Uneven loading can cause similar problems when heavier cables are concentrated on one side of the tray.

Checking the expected load before installation helps keep cables evenly supported throughout the route. Heavier cables should be positioned and distributed in line with the system design.

Supporting Long-Term Performance 

Cable tray systems often remain in place for many years, during which services may be upgraded or extended. Allowing for planned additions can prevent the tray from reaching its safe limit sooner than expected.

The material and finish must also suit the surroundings. Moisture, chemicals and outdoor exposure can affect unsuitable products over time, even when the initial cable load is within the stated capacity.

Reducing Maintenance Costs 

A properly specified tray is less likely to need extra supports or replacement sections later. Cables also remain easier to inspect, maintain and alter when the containment system is stable and accessible.

Spare capacity can be particularly useful in commercial and industrial buildings where new services are regularly introduced. However, any additions must remain within both the available space and safe working load.

Ensuring Structural Integrity 

Cable tray does not work in isolation. Brackets, channels, threaded rods, anchors and fixings all contribute to the strength of the finished installation.

If one of these parts is underspecified, the capacity of the complete system may be lower than the tray’s published rating. The supporting structure and connection points must therefore be considered as part of the design.

Factors That Determine Cable Tray Load Capacity 

Cable tray capacity depends on several features of the product and its installation:

• Support Spacing: Longer gaps between supports can increase deflection and reduce the weight a tray can safely carry. Check the intended spacing against the manufacturer’s load data.
• Tray Type: Light, medium and heavy-duty cable trays are designed for different applications. Material thickness and sidewall profile can affect their strength, as can perforation patterns.
• Material and Finish: The grade and thickness of the material influence mechanical performance. Its finish should also provide suitable protection for the installation environment.
• Cable Size and Quantity: Heavier or thicker cables may require larger or stronger cable trays. You should not rely solely on the size of a cable tray when determining its strength, however, pay attention to manufacturer specifications and load ratings. 
• Installation Conditions: External trays may be affected by weather conditions. Moisture, vibration and corrosive substances may also influence product selection.
• Accessories: Covers, dividers and other components add to the overall load. Some accessories may also need different support arrangements from straight tray sections.
• Concentrated Loads: Equipment such as junction boxes can apply substantial weight at a single point. These items should only be supported by the tray when the system has been designed accordingly.

How Cable Tray Load Capacity is Determined 

Load calculations should be completed during the design stage using cable manufacturers’ weights and the tray manufacturer’s technical data. 

1. Review the Cable Schedule 

List the cables planned for each part of the route. Record the cable type, number of runs and weight per metre, using the relevant product data sheets.

Avoid estimating weight from the outside diameter. Two cables of a similar size may have different constructions and weights.

2. Calculate the Cable Weight 

For each cable type, multiply its weight per metre by the number of cables being installed:

Cable load per metre = cable weight per metre x number of cables

Add the results together to find the combined load per metre. This calculation assumes that the cables will be distributed evenly across the tray.

3. Account for Accessories and Other Loads 

Include the weight of tray covers, dividers and other components supported by the system. Point loads should be assessed separately rather than spread across the length of the route for calculation purposes.

Cable trays should not be used as walkways or working platforms unless they have been specifically designed and approved for that use.

4. Allow for Future Additions 

Check whether the project specification requires capacity for future cables. The allowance should reflect the likely needs of the installation rather than using a general percentage without justification.

Available space does not necessarily mean that sufficient load capacity remains. Any new cables must stay within the tray’s safe working load.

5. Confirm the Support Spacing 

Measure the proposed distance between supports along straight sections and around changes in direction. Compare this spacing with the spans shown in the manufacturer’s load tables.

Shortening a span may increase the available capacity, but any adjustment should follow the system design and manufacturer’s guidance.

6. Check the Complete System 

Compare the calculated load against the safe working load and deflection data for the chosen tray. Include any margin specified by the manufacturer or project documentation.

The brackets, strut channels, threaded rods, anchors and supporting structure must also be checked. The weakest component determines the capacity of the finished system.

Unusual loads, complex routes and safety-critical installations should be reviewed by a suitably qualified designer or engineer.

Cable Tray Standards and Load Testing 

BS EN 61537:2007 sets out requirements and tests for cable tray and cable ladder systems. This includes testing used to establish mechanical performance and safe working loads.

During controlled testing, tray sections are installed across a specified support arrangement before increasing loads are applied. The tray’s deflection and behaviour under load are then measured, allowing the manufacturer to publish data for different products and spans.

Installers should work from this verified information rather than attempting an improvised load test on site. Placing people or unapproved weights on an installed tray could damage the system and create a serious safety risk. Where proof testing is required by the project, it should follow an engineer-approved method statement.

The completed electrical installation must also meet the applicable requirements of BS 7671, together with the project specification. Before handover, check that:

• The tray and support spacing match the approved design
• Joints, brackets and fixings are correctly installed
• The calculated load remains within the safe working limit
• There is no unexpected sagging or damage 
• Any required earthing or bonding is complete
• Planned additions will remain within the available capacity

Get in Touch With Lenson Select 

Lenson Select supplies light, medium and heavy-duty cable trays, along with the bends, brackets, clips and supports needed to complete an installation. The range includes pre-galvanised and hot-dip galvanised steel trays, with stainless steel products available on request.

We have more than 100 years of collective experience and provide express delivery within 35 miles of our West Bromwich HQ as well as discounts on all orders over £1,000. Whether you’re working from an existing specification or need help finding compatible components, our expert team can help. Contact Lenson Select, call 0121 553 6699 or email sales@lenson-select.co.uk.