Cable Tray Sizing: Cable Layout and Spacing

Cable tray sizing starts with the cable schedule and arrangement, not cable count alone. Different outside diameters, clearances and circuit layouts can require different widths for the same number of cables.

This article complements our tray size guide by focusing on cable layout. Numerical examples illustrate geometry only; they are not an installation design.

1. Collect actual cable data

List cable type, quantity, outside diameter, weight per metre, circuit function, voltage and load. Use the manufacturer's outside diameter: conductor area in square millimetres is not the diameter of the finished cable.

Record bending radius, fixing requirements, ambient conditions, covers and adjacent trays.

2. Establish the circuit arrangement

Identify multicore or single-core cables and the approved arrangement: touching or spaced, single layer, flat or trefoil groups, or any permitted stacking.

The arrangement must match cable calculations, applicable code and product instructions. Single-core phase grouping and parallel-circuit arrangements require electrical review rather than a layout chosen simply to fit the tray.

3. How much space should cables have?

There is no universal millimetre clearance for every installation. Touching, spacing and multiple layers affect heat dissipation and the relevant current-rating and grouping calculations.

Some references show diameter-related spacing for specific arrangements. Do not apply “one diameter” or “20 mm” to all circuits without the conditions of the relevant table.

State whether a drawing dimension is a clear gap or centre-to-centre distance. Two 30 mm diameter cables with a 20 mm clear gap have centres 50 mm apart. This defines geometry; it does not recommend a 20 mm clearance.

4. Calculate the layout width

For circular cables in one row:

Occupied width = sum of outside diameters + sum of clear gaps + specified side clearances.

This geometry check does not replace fill, ampacity, segregation or mechanical checks.

An illustrative example

Assume an engineer has specified six cables of 30 mm diameter, five clear gaps of 20 mm and a 20 mm clearance at each side:

• Cable diameters: 6 × 30 = 180 mm.
• Gaps: 5 × 20 = 100 mm.
• Side clearances: 2 × 20 = 40 mm.
• Required geometric width: 320 mm.

A usable internal width below 320 mm cannot fit this arrangement. Select an available size that accommodates it, including approved barriers, fixings and future provision. The 20 mm values are assumptions, not universal code clearances.

5. Fill percentage is not universal

Do not apply 40% or 50% to every tray. The required method can involve diameters, cable areas or a particular arrangement, depending on cable type, tray type and code edition.

Geometric occupancy, ampacity and future allowance are separate checks. Spare physical space does not automatically make future additions thermally or mechanically acceptable.

6. Segregate power, data and control circuits

Review electrical safety, electromagnetic compatibility and service requirements. Separation distance, a barrier or a separate route depends on circuit types, shielding, specifications and system instructions.

A metal divider does not automatically permit every service combination. Show the approved separation and include barriers and clearances in the usable-width calculation.

7. Review side height, bends and covers

Side height is not permission to stack to the rim. Confirm permitted layers, restraints, access and maintenance. At bends and branches, check cable bending radii and routing space.

Covers and nearby surfaces can affect the installation environment. Review the actual arrangement in the cable calculations. See our cover guide and accessories guide.

8. Check loads and supports separately

Total cable weight per metre and the other required loads must be reviewed with the tray, covers and accessories. Use approved system data for profile, thickness and support span. A sufficiently wide tray is not automatically sufficiently strong.

A 3-metre tray section does not establish a 3-metre support spacing. Section length, cable spacing and support span are different dimensions.

Before ordering

• Actual cable diameters and weights.
• Circuit layout, layers and approved clear gaps.
• Applicable fill and ampacity checks.
• Service segregation and barriers.
• Usable internal width and side height.
• Bend, cover and fixing requirements.
• System loads and support spacing.
• Defined future allowance and approved drawings.

Frequently asked questions

What clearance should power cables have?

Use the clearance established for the actual cable arrangement and current-rating design under the applicable reference. Record it explicitly on the drawing.

Can a higher side wall replace extra width?

Not automatically. A required single-layer or spaced arrangement must still be accommodated.

Can I just divide cable area by tray area?

That alone may not be the applicable fill method. Verify the method for the actual cables and tray, together with thermal and mechanical requirements.

Send the approved schedule to Rovana Trade

Provide the approved layout, sizes, quantities, material, finish and fitting details. Use our BOQ checklist and request a quotation for the project scope.

Technical reading

• Schneider Electric: General cable sizing and grouping factors.
• NEMA: Tray width selection for single-conductor cables up to 2000 V, a reference for that specific scope.
• Eaton: Cable tray catalog, fill and ampacity considerations, subject to its referenced code edition.

Example values are hypothetical. Final design uses the local code, project specification, relevant reference editions and actual cable and system data.