Cable Tray Size Calculation for Project Engineers

Cable trays are essential for organizing and supporting electrical and communication cables, as well as assuring safe installations. Choosing the appropriate size and dimensions for a cable tray is critical for performance, maintenance, and potential future improvements.

Importance of Cable Tray Sizes and Dimensions

Selecting the appropriate cable tray dimensions and size is essential for many kinds of reasons:

Proper Cable Performance

The size of the cable tray has to be suitable on account of the kind of cables and the number of cables that it will carry. Overcrowding cables or using a small tray can cause electrical interference, overheating, and poor performance.

Effective Thermal Management

Cables generate heat, and if the tray is too small or packed too heavily, heat dissipation will be insufficient, potentially leading to cable overheating. Proper size facilitates perfect airflow around wires, ensuring safe operating temperatures.

Compliance with Regulations

Many construction regulations and safety standards specify cable tray sizes, particularly in terms of load capacity, heat dissipation area, and overall design. Adherence to these standards is critical for both safety and legal compliance.

Cost and Space Efficiency

Proper sizing ensures that you do not overspend on larger trays than necessary, but it also stops you from purchasing trays that are too small, which could result in costly adjustments or replacements in the future.

Future Cable Additions

Oversizing trays enables for future upgrades or additions, such as more cables, without requiring costly reinstallation. This is especially critical when extending or growing facilities, as new cables may be required later.

What is the size of a cable tray?

Overview of Cable Tray Types and Dimensions

The size of cable trays varies depending on the manufacturer, type, and material. Below are common dimensions for different tray types:

Note: Specific dimensions may vary by manufacturer and application.

How to Calculate Cable Tray Size?

The following elements should be taken into account while calculating the appropriate cable tray size:

Cable Size and Quantity:

• Calculate the cross-sectional area of each cable.
• Add up the total number of cables to be installed.

Cable Type:

• Power cables: Require more space for heat dissipation.
• Data cables: Need careful routing to avoid interference.
• Control cables: Smaller in size but may require special handling for EMI.

Routing Requirements:

• Horizontal routing: Broad base for even cable spread.
• Vertical routing: Higher sidewalls or additional support may be needed.
• Diagonal routing: Custom design for angled placement.

Space Constraints:

• Consider physical limitations in the installation area (e.g., ceiling height, wall space).
• Ensure easy access for future maintenance and modifications.

Load Capacity:

• Cables generate heat, and the tray must allow effective ventilation for heat dissipation.
• The cable  tray loading capacity should account for how well it can manage the heat produced by the cables.
• A thicker plate and stronger materials increase the tray’s loading capacity.
• Material choice and plate thickness are critical for the tray’s strength and ability to handle heavy loads.
• General loading capacity guidelines are provided by the National Electrical Manufacturers Association (NEMA).
• Always consult the manufacturer’s specifications for accurate details on loading capacity.

Example Tray Types and Loading Capacities

Note: These values are indicative and should be confirmed with the manufacturer based on the specific cable tray design and material specifications.

Standards and Guidelines:

• Refer to local codes and industry standards (e.g., NEC, NFPA).
• Follow manufacturer recommendations for spacing, load capacity, and installation.

There are two factors that are used to characterize the size of the cable tray. These parameters are the width and the height.

 Calculating the size of a cable tray primarily involves determining its width.

The following determines a cable tray’s final size:

1. The number of cables on the cable tray
2. The size and overall diameter of each cable
3. The future expansion

The general rule for sizing the cable tray is that all cables must be installed in a single layer, and there must be space between each pair of cables:

1. The diameter of the larger cable is equal to the space between two multi-core cables.
2. In a trefoil configuration, the distance between three single-core cables is double that of other trefoil cables.

After determining the initial cable tray width by measuring the overall width of this arrangement, add the future expansion percentage.

Final cable tray width = Initial cable tray width × (1 + Expansion percentage)

Depending on the manufacturer, the final cable width is usually rounded to the closest standard width, which can be 50, 100, 150, 200, 250, 300, 400, 500, 600, 700, 800, or 900 mm.

Click here for Instrumentation Cables Testing Standards

How to calculate cable tray calculation?

Example Calculations of Cable Tray Size

Find the cable tray size for the following cables, with a future expansion percentage of 20%:

Step 1. Refer the Cable Schedule

2 No’s of 3.5Cx300 Sq.mm XLPE Cable

• Outer Diameter: 59.7 mm
• Weight: 5.9 Kg/Meter
• Purpose: These cables are typically used for power distribution and have a significant diameter and weight.

2 No’s of 3.5Cx400 Sq.mm XLPE Cable

• Outer Diameter: 68.6 mm
• Weight: 6.1 Kg/Meter
• Purpose: Used for higher capacity power distribution, requiring careful consideration for their size and weight.

3 No’s of 3.5Cx25 Sq.mm XLPE Cable

• Outer Diameter: 25 mm
• Weight: 0.5 Kg/Meter
• Purpose: Smaller cables used for control or low power applications.

Click here know more about Instrument Cable schedule

Step 2. Outer Diameter Calculation

Diameter of 300 Sq.mm Cable:

• 2 cables x 59.7 mm = 119.4 mm

Diameter of 400 Sq.mm Cable:

• 2 cables x 68.6 mm = 137.2 mm

Diameter of 25 Sq.mm Cable:

• 3 cables x 25 mm = 75 mm

Total Diameter of All Cables:

• 119.4 mm + 137.2 mm + 75 mm = 331.6 mm

This total diameter represents the combined width of all cables laid side by side.

Click here to know more about Instrument Tray Layout

Step 3. Cable Weight Calculation

Weight of 300 Sq.mm Cable:

• 2 cables x 5.9 Kg/Meter = 11.8 Kg/Meter

Weight of 400 Sq.mm Cable:

• 2 cables x 6.1 Kg/Meter = 12.2 Kg/Meter

Weight of 25 Sq.mm Cable:

• 3 cables x 0.5 Kg/Meter = 1.5 Kg/Meter

Total Weight of All Cables:

• 11.8 Kg/Meter + 12.2 Kg/Meter + 1.5 Kg/Meter = 25.5 Kg/Meter

This total weight must be supported by the cable tray without exceeding its capacity.

Step 4. Total Width Calculation

Total Width of All Cables:

• (Total No of Cables x Distance between Each Cable) + Total Cable Outer Diameter
• (7 cables x 10 mm) + 331.6 mm = 401.6 mm
• Spacing ensures proper ventilation and maintenance access.

Including 20% Spare Capacity:

• 1.2 x 401.6 mm = 481.92 mm (rounded to 482 mm)
• The spare capacity accounts for future expansion or unexpected cable additions.

Step 5. Total Area Calculation

Total Area of Cable:

• Final Width of Cables x Maximum Height of Cable
• 482 mm x 68.6 mm = 33,065.2 Sq.mm

Including 20% Spare Capacity:

• 1.2 x 33,065.2 Sq.mm = 39,678.24 Sq.mm (rounded to 39,678 Sq.mm)
• The total area calculation ensures the tray can accommodate all cables comfortably.

Step 6. Case Studies

CASE I: 500x100mm Cable Tray

Area of Cable Tray:

• 500 mm x 100 mm = 50,000 Sq.mm

Width Check:

• Calculated Width (482 mm) < Tray Width (500 mm) = OK

Depth Check:

• Calculated Depth (68.6 mm) < Tray Depth (100 mm) = OK

How to choose cable tray thickness?

Weight Check:

• Total Cable Weight (25.5 Kg/Meter) < Tray Capacity (120 Kg/Meter) = OK
• This case shows that the tray can handle the weight and depth of the cables but provides limited spare width.

CASE II: 700x100mm Cable Tray

Area of Cable Tray:

• 700 mm x 100 mm = 70,000 Sq.mm

Width Check:

• Calculated Width (482 mm) < Tray Width (700 mm) = OK

Depth Check:

• Calculated Depth (68.6 mm) < Tray Depth (100 mm) = OK

Weight Check:

• Total Cable Weight (25.5 Kg/Meter) < Tray Capacity (120 Kg/Meter) = OK
• This larger tray provides more spare capacity, allowing for future cable additions and better ventilation.

Step 7. Remaining Capacity Calculations

Remaining Cable Tray Width Area

To determine the spare width capacity, we use the following formula:

• Formula: Remaining Width Area = 100% – (Calculated Cable Tray Width / Actual Cable Tray Width) × 100
• Calculation: Remaining Width Area = 100% – (481.92 / 700) × 100 = 31.1%

Remaining Cable Tray Area

Similarly, the remaining area capacity is calculated as follows:

• Formula: Remaining Area = 100% – (Calculated Cable Tray Area / Actual Cable Tray Area) × 100
• Calculation: Remaining Area = 100% – (39,678.24 / 70,000) × 100 = 43.3%

Step 8. Results

• Selected Cable Tray Size: 700x100mm
• Type of Cable Tray: Perforated
• No. of Cable Tray Runs: 1
• No. of Layers of Cables: 1
• Remaining Tray Width Area: 31.1%
• Remaining Tray Area: 43.3%

The selected tray size ensures enough capacity for current and future needs while maintaining safe operational conditions.

Refer the below link for the Instrumentation Cable Tray Installation Checklist and Inspection Procedure

Why Do We Add Space Between Cables?

Adding space between cables is important for many reasons:

Improved Heat Dissipation

• Cables generate heat during operation.
• Without proper spacing, heat cannot dissipate effectively, leading to higher temperatures.
• Overheating can reduce the lifespan of cables and increase the risk of failure or fire.

Derating Factor for Touching Cables

• When cables are in direct contact, their current-carrying capacity is reduced (derated).
• Derating factors for touching cables range from 0.85 to 0.6, depending on the number of cables.
• A reduced derating factor means the cable cannot carry as much current, possibly requiring doubling the cable size, which increases costs.

Derating Factor for Spaced Cables

• Spacing cables allows for proper heat dissipation and better current-carrying capacity.
• Derating factors for spaced cables range from 1.0 to 0.9, which has minimal impact on the cable’s capacity to carry current.

Reduced Risk of Overheating

• Proper spacing reduces the risk of cables overheating by allowing for better airflow around each cable.
• Overheating is one of the leading causes of cable failure and potential hazards such as electrical fires.

Cost-Effective Installation

• When cables are spaced correctly, there is no need to oversize cables to compensate for reduced current-carrying capacity.
• Avoiding unnecessary cable upsizing leads to cost savings in materials and installation.

Compliance with Standards

• Standards such as BS 7671 and IEC 60364-5-52 provide guidelines for cable spacing to ensure safety and optimal performance.
• Adhering to these guidelines helps prevent safety hazards and ensures the system operates within recommended parameters.

Refer the below link for the Instrumentation Cable and Wiring Inspection Procedure: Essential  checklist for Project Engineers

How Do You Calculate the Number of Cable Trays?

To calculate the number of trays you need, use this formula:

Number of trays required=Number of theoretical trays/Tray efficiency

Tray efficiency typically ranges from 50% to 70%. This value depends on factors like the type of tray you’re using and the internal conditions, such as the flow of liquids or gases.

What is the IEC Standard for Cable Trays?

IEC 61537:2023 sets the requirements and tests for cable tray systems and cable ladder systems. These are used to support cables (and sometimes other electrical equipment) in electrical and communication installations.

What Are the 3 Main Types of Cable Trays?

Below are the three most common types of cable trays and their benefits:

1. Ladder Type Cable Tray: This type has side rails connected by crossbars, providing an open structure. It’s great for large or heavy cables and allows for good ventilation.
2. Perforated Cable Tray: Similar to the ladder type, but with perforated sides. It’s suitable for a variety of cables, especially where some protection and ventilation are needed.
3. Solid Bottom Cable Tray: This tray has a solid base, providing full protection for cables. It’s often used for sensitive cables that need to be shielded from external conditions.

Click here for How to do the voltage drop calculation of instrument cable?