Cable Tray Sizing Guide: Width, Depth and Fill Ratio Calculations

Why Tray Sizing Matters Beyond Fill Ratio

A correctly sized cable tray serves three functions: it supports the cable weight without excessive deflection, it keeps cables accessible for maintenance and future addition, and it maintains the physical separation between cable groups that the installation standard requires. Under-sized tray causes deflection, compressed cable sheaths and failed inspections. Over-sized tray wastes material cost and civil support structure. The goal is to get both the width and depth right before the order is placed.

Step 1: Width Sizing from the Cable Schedule

The standard method for tray width selection sums the outside diameters of all cables to be installed in the tray, then applies a fill-ratio allowance. IEC 61537 permits a maximum fill ratio of 75 percent of the usable tray width for single-layer cable placement. NEC 392 uses a slightly different metric: for ladder or ventilated tray carrying cables 4/0 AWG and larger, the sum of cable diameters must not exceed the tray width.

For a practical calculation: list every cable OD, sum them, divide by 0.75 (for IEC) to get the minimum tray width, then select the next standard width upward. Standard IS 12352 tray widths are 100, 150, 200, 300, 450, 600 and 900 mm. Leave a spare capacity of at least 20 percent of tray width for future cables added after commissioning.

• Step 1: List all cable ODs from the cable schedule
• Step 2: Sum the ODs for single-layer placement
• Step 3: Divide sum by 0.75 (IEC 61537) or 1.0 (NEC 392 for large cables) to get minimum width
• Step 4: Add 20 percent spare capacity for future additions
• Step 5: Round up to next standard IS 12352 width (100, 150, 200, 300, 450, 600, 900 mm)
• Step 6: Verify at bends and tees that the bend radius for the largest cable fits within the tray bend radius

Step 2: Depth Sizing by Load Class

Tray depth is selected primarily for structural performance rather than cable count. The deeper the side rail, the greater the moment of inertia and the higher the permitted uniformly distributed load for a given span. IEC 61537 defines four load classes, each with a minimum test load per metre run at the rated span.

• IEC 61537 Class 1 (light): up to 15 kg/m - typical 50 mm depth tray at 1.5 m spans
• IEC 61537 Class 2 (medium): up to 30 kg/m - typical 60-75 mm depth at 2 m spans
• IEC 61537 Class 3 (heavy): up to 50 kg/m - typical 100-110 mm depth at 3 m spans
• IEC 61537 Class 4 (extra heavy): above 50 kg/m - 150 mm or deeper side rail required
• Always verify with the manufacturer's load-deflection table, not the class label alone

Fill Ratio Limits and When They Apply

The 75 percent fill ratio in IEC 61537 applies to single-layer cable placement, meaning all cables lie side by side across the tray floor without stacking. When multi-layer placement is unavoidable, IEC 61537 does not prohibit it but requires that the designer address heat dissipation. In practice, most engineers and project specifications treat 40 to 50 percent fill as the working limit for power cables in multi-layer arrangements, accepting IEC's 75 percent only for instrument and control cables where thermal management is not a concern.

Mixed Voltage and Mixed Cable Type Runs

IEC 61537 clause 5.3 and most project specifications require physical separation between high-voltage power cables and low-voltage control or instrumentation cables. The standard separation methods are: a solid steel barrier fixed to the tray spine (divider plate), a minimum spacing of 200 mm between cable groups within the same tray, or routing on separate trays on different structural brackets. Each project specification will prescribe which method is acceptable; the default for IS projects in India is the divider plate unless the specification explicitly permits the gap method.

Support Span Calculation

Support span is calculated from the tray load table supplied by the manufacturer. The table gives the maximum span in millimetres for each combination of tray width, depth, steel thickness and load class. The structural engineer confirms the actual support spacing from the building frame or cable support structure, and the two are reconciled before finalising the tray specification.

A common project error is specifying a tray class based on the cable weight but installing supports at a wider spacing than the class rating permits. The result is mid-span deflection that exceeds the 1/200 span limit in IEC 61537, which can cause cables at the tray floor to take point loads at support locations over time. Catch this in the design review, not on site.

Sample Sizing Calculation

A run carrying 8 cables: 2 off 50 mm2 XLPE LV power (OD 35 mm each), 3 off 25 mm2 XLPE LV power (OD 28 mm each), 3 off 10-pair instrument cable (OD 22 mm each). Total OD sum: 70 + 84 + 66 = 220 mm. Minimum width at 75 percent fill: 220 / 0.75 = 293 mm. Add 20 percent spare: 293 x 1.2 = 352 mm. Select next standard width: 450 mm. Cable weight per metre: roughly 18 kg/m. Select IEC 61537 Class 2 medium-duty tray. Support span: 2 metres. Tray depth: 60 mm.

Every cable tray sizing exercise starts with the cable schedule, not with a rule of thumb. The schedule tells you the weight, the OD and the voltage class of each cable. Everything else follows from those numbers.