Cable Tray Sizing: Fill Calculation Example
Tray width is a space question first and a loading question second. Here is the fill calculation with a worked example, and the checks that decide whether the answer is actually usable.
The method
- List each cable and its overall outer diameter from the datasheet.
- Cross-section area of a round cable = π ÷ 4 × D².
- Total area = sum of all cable areas.
- Usable tray area = total area ÷ fill ratio.
- Required width = usable area ÷ usable depth; round up to a standard width.
Worked example
Assumptions (illustrative): 20 power cables of 35 mm outer diameter in a single tray, usable depth 100 mm, fill ratio 40% (an assumed design limit; use the project specification).
- Area per cable = π ÷ 4 × 35² = 962 mm²
- Total area = 20 × 962 = 19,242 mm²
- Usable area = 19,242 ÷ 0.40 = 48,106 mm²
- Width = 48,106 ÷ 100 = 481 mm, so choose the next standard width, 500 mm
The other checks
- Weight: total cable weight per metre plus the tray's own weight must be within the tray's rated load at the chosen support spacing.
- Spacing: large power cables are often laid in a single layer with gaps between them so heat can dissipate, which can make the real width larger than the area calculation.
- Derating: grouping cables lowers their current-carrying capacity; check the ampacity of the cables as installed. See cable sizing basics.
- Segregation: power and data cables are normally separated.
- Fire and earthing: tray bonding and fire-stopping where it crosses compartments.
Common mistakes
- Using conductor area instead of overall cable diameter.
- Ignoring spacing for single-core cable trefoil or flat formations.
- Checking fill but not weight and support spacing.
- Leaving no spare capacity.
Related: voltage drop and the conductor sizing calculator.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
How do you calculate cable tray size?
Sum the cross-sectional areas of all cables the tray will carry, divide by the permitted fill ratio to get the required usable area, then divide by the usable tray depth to get the width, and round up to a standard tray width.
What fill ratio should I use?
It depends on the standard, the cable type and whether the tray carries power or control cables. Many designs set a limit that leaves room for installation and future cables; use the value in the project specification or applicable code rather than a general rule.
Is fill the only check for a cable tray?
No. Also check the weight per metre against the tray's load rating and support spacing, the minimum spacing between power cables for heat dissipation, and ampacity derating for grouping.
Should I leave spare space?
Usually yes. Spare capacity for future cables is a common design requirement; state the percentage you used.
Want to learn this properly?
This topic is covered in depth in our Electrical Design – Data Center Specialist program.
Ask us on WhatsAppLearn to do this on a full project in the Electrical Design – Data Center Specialist program, or read the data center design guide.