How to choose the right cable tray size? Filling ratio and weights with a practical example.

Last updated: October 18, 2026

The cable tray size is selected in three steps: First, calculate the total cross-sectional area of the cables using their outer diameters. Then, divide this sum by the permissible fill percentage specified in the project specifications to determine the required net area. Next, select standard widths and heights that will cover this area. Finally, verify that the cable weight per meter is appropriate for the tray thickness and the spacing between supports.

What is the fill percentage in a cable tray and why does it matter?

The fill percentage is the area of the cable cross-sections divided by the net internal area of the tray (width x usable height). A tray is not filled completely for several reasons:

• the heat: Crowded cables exchange heat and do not cool down well, and the designer may have to reduce the current allowed for the cable, i.e., choose larger and more expensive cables.
• Expansion: Projects change, and adding circuits after a year or two is to be expected.
• Easy to withdraw and maintain: A full tray is difficult to pull a cable from or add a new cable without damaging the adjacent cables.
• Natural spaces: The cables are cylindrical, and no matter how you arrange them, there will always be gaps between them, so the "theoretical" fullness of 100% is practically impossible.

The exact ratio isn't a single universal number; it's determined by the project specifications or the consultant's standards. Many specifications set a maximum range of 40 to 50% of the terrarium cross-section, and some require an additional margin for expansion beyond that. Therefore, always refer to your project specifications before calculating.

High power cable condition

Large-section power cables are often arranged in a single layer, sometimes with gaps between them to dissipate heat. In this case, the width is calculated not by area, but by the sum of the cable diameters and the spacing between them, since the height is not utilized.

Steps to calculate cable tray size

1. Compile the cable chart: For each cable segment, write down the number, outer diameter (mm), and weight (kg/m) from the cable manufacturer's catalog. Do not rely on the conductor cross-section (mm²) as it does not represent the outer diameter including insulation and sheath.
2. Calculate the cross-sectional area of each cable: Area = 3.14 × (diameter ÷ 2)². Then multiply by the number of cables of the same type.
3. Add up the areas: To get the total area of the cables.
4. Divide by the percentage of fullness: For example, if the ratio is 40%, then the required area = total area ÷ 0.40.
5. Choose the height: Common side heights include 50, 75, and 100 mm. A higher height reduces the required width but may not fit a ceiling or track.
6. Calculate the width: Width = Required Area ÷ Useful Height, then choose the nearest standard width larger than that.
7. Check the weight: Add up the weights of the cables per meter, add any additional expected loads, and make sure that the thickness of the tray and the span of the supports can withstand them.

Illustrative example of size calculation

The following example is for illustrative purposes only; the diameters and weights are approximate and hypothetical. In the actual project, these are taken from the catalog of the cable actually used. Let's assume a path carrying:

• The filling ratio in the specifications of this hypothetical project 40%, the required area = 8,740 ÷ 0.40 ≈ 21,850 mm².
• If we choose a height of 100 mm, the required width = 21,850 ÷ 100 ≈ 219 mm, so we choose a width of 300 mm.
• If we choose a height of 50 mm, then the width = 21,850 ÷ 50 ≈ 437 mm, so we choose 500 mm.

The first option (300 x 100) is narrower and suitable for busy routes, while the second (500 x 50) is wider and lower, making it suitable for locations with low ceilings. The weight (approximately 18.4 kg/m in the example) is compared to the load-bearing capacity tables based on the thickness and spacing between supports.

Instead of manual calculation, you can enter the cables into Cable Tray Calculator To estimate the size, thickness, and list of materials, then review the result with the design engineer.

Weight, thickness, and distance between supports

The size determines whether the cables can be accommodated, while the thickness and brackets determine whether the trellis can support their weight without sagging or bending. The relationship is simple: the heavier the cables or the greater the distance between the brackets, the thicker or stronger the trellis needs to be, such as a ladder-style trellis.

• A very wide tray loaded with heavy cables may require a greater thickness even if the filling ratio is acceptable.
• Reducing the distance between brackets increases load-bearing capacity, but it also increases the number of brackets and the cost of installation.
• The self-weight of the tray and covers is added to the weight of the cables when calculating the loads of the supports and the roof.

We will explain this relationship in detail in an article. Cable tray sheet thickness and its relationship to load and length. To calculate the weight of the tricycle itself or the iron bars, you can use [the following].Steel Weight Calculator.

Outdoor and covered paths

If the conduit is outdoors, exposed to sunlight, or fully covered, the cables inside will be hotter than in an open indoor conduit. This might lead the designer to choose a larger size to allow for more spacing of cables or to reduce the current they can handle. Therefore, tell the person calculating the dimensions where the conduit will be located and whether it will be covered, not just provide a list of the cables. The covering itself also adds weight that must be factored into the supports, especially in long conduits.

Common mistakes in choosing cable tray size

1. Calculation is based on the conductor cross-section rather than the outer diameter: A 4 x 95 mm² cable has an outer diameter much larger than the number suggests.
2. Neglecting future expansion: The train fills up on the first day, and you have to extend a second track after a year.
3. Uniformity of measurement throughout the entire process: The main line near the electrical room carries more than the distant branches, and the size can be reduced after each branch using reducers.
4. Ignore the cable bending radius: Large cables need elbows with a suitable radius, otherwise they will be damaged when bending.
5. Mixing power and low-voltage cables without a separator: The interval or independent path must be accounted for in the design.
6. Forgetting the dimensions of the actual path: A correctly calculated width may not pass between the ducts and pipes in the ceiling, so check the coordination with the other services.

After determining the size comes the stage of accessories and installation, and you will find instructions for them in Cable Tray Installation Guide. The MITS factory manufactures Cable tray for electrical cable routes In the required sizes and thicknesses.

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