How to Size a Cable Tray: Width, Depth, and Cable Fill

To size a cable tray, list every cable the tray will carry, add up their widths or cross-sectional areas, leave spare capacity for future cables, choose a tray width and side rail depth that hold that total, and then check that the total cable weight stays within the tray's safe working load at your support spacing. Width and depth decide whether the cables fit; the safe working load decides whether the tray can carry them. This guide walks through each step with a simple worked example.

Step 1: List the Cables and Their Sizes

Start with a cable schedule for the route. For every cable, record the outer diameter, the weight per meter, and the cable type (power, control, or data). Use the actual cable manufacturer's data rather than nominal values, since outer diameter and weight vary between cable constructions.

What to record

• Outer diameter of each cable, in mm

• Weight per meter of each cable, in kg/m

• Cable type, since power, control, and data cables are usually separated

• Whether cables will be laid in a single layer or stacked

Step 2: Work Out the Tray Width

For larger cables laid side by side in a single layer, the tray width must be at least the sum of the cable outer diameters, plus any spacing the design requires between cables. For smaller cables bundled in layers, designers work from the total cross-sectional area of the cables and the fill limit set by the project standard. Either way, leave spare capacity so cables can be added later without replacing the tray.

Sizing guidelines

• Large power cables are usually laid in a single layer so heat can escape, with spacing set by the project's derating rules

• Small control and data cables can be bundled in more than one layer, within the fill limit the project standard allows

• Many designs allow roughly 20 to 30 percent spare capacity, but the figure comes from the project specification, not a general rule

• Round the result up to the next standard tray width, rather than down

Step 3: Choose the Tray Depth

The side rail height, or depth, limits how high cables can be stacked. Cables should not rise above the side rails, or they will not be held in place and cannot take a cover. A deeper tray allows more stacked cables in the same width, but also weighs more and costs more, and it changes the load the tray must carry.

Depth considerations

• Single-layer power cable routes usually need only a shallow tray, as deep as the largest cable diameter plus a margin

• Multi-layer control and data routes need a deeper tray, sized to the stacked height of the bundle

• If a cover is required, confirm the cable height leaves room under the cover

Step 4: Check the Load

Fitting the cables is only half the job. Add up the weight of all cables per meter, including future cables you planned spare capacity for, then compare it with the tray's safe working load at your support spacing. The safe working load is normally given in newtons per meter (N/m) for a stated span, and 1 kg/m of cable weighs about 9.81 N/m. Our Cable Tray Load Capacity & Support Span guide explains how span and load interact.

Load check

• Convert total cable weight from kg/m to N/m by multiplying by 9.81

• Compare it with the safe working load for the tray size and span you plan to use

• If the load is too high, shorten the support span, choose a thicker or deeper tray, or split the cables across two trays

A Worked Example

This example is for illustration only. Use your own cable data and project standard for real designs.

Separating Cable Types

Power, control, and data cables are often separated to reduce interference and to meet project or code requirements. The options are separate trays, a divider inside one tray, or different levels of a multi-tier route. Ask for the separation requirements before sizing, since they change the number of trays and the width of each.

Common Sizing Mistakes to Avoid

1. Sizing for today's cables only, with no spare capacity for future additions.

2. Stacking large power cables in layers, which traps heat and may require derating the cables.

3. Checking width but not load, so the tray fits the cables but is overloaded at the planned support spacing.

4. Letting cables rise above the side rails, so they are not held in the tray and cannot be covered.

5. Using nominal cable data instead of the actual cable manufacturer's diameter and weight.

Conclusion

Sizing a cable tray comes down to four checks in order: the cable list, the width with spare capacity, the depth, and the load at the planned support spacing. Using real cable data, leaving room for future cables, and checking load as well as fit are what keep a tray from being undersized or overloaded. Final fill limits and spacing rules come from the project standard, so confirm them before releasing a design.

FAQ

How much spare capacity should a cable tray have?

Many designs leave roughly 20 to 30 percent spare capacity so cables can be added later, but the correct figure comes from the project specification or the applicable standard. A facility that expects frequent changes, such as a data center, may allow more.

Can I stack power cables in a cable tray?

Large power cables are normally laid in a single layer so heat can dissipate, and stacking them may require derating the cables. Smaller control and data cables can usually be bundled in layers within the allowed fill limit. Follow the project standard for the cable types involved.

How do I know if a cable tray is strong enough for my cables?

Add up the weight of all cables per meter, convert it to newtons per meter by multiplying kg/m by 9.81, and compare it with the tray's safe working load at your support spacing. If the load is too high, shorten the span, use a thicker or deeper tray, or divide the cables between two trays.

Need help choosing a size? Send us your cable list and support spacing, browse our cable tray systems and hot-dip galvanized perforated tray, or contact our team for a recommendation.