Cable Tray Fill Calculator
Work out what percentage of a cable tray's cross-section is taken up by the cables running in it. Enter the tray's usable width and depth plus up to four groups of cables (a count and an outer diameter each), and the calculator returns the fill percentage, the total cable area, the tray area and a single-layer width check.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the tray's usable width W and usable depth/height H in millimetres (the inside dimensions, or the maker's stated usable area).
- Enter each cable group as a count and the cable's outer diameter (OD, over the sheath) in millimetres — at least one complete group is required, up to four are allowed.
- Read the fill percentage, the total cable and tray cross-section areas, and whether the cables fit across the width in a single layer.
How it works
Each cable's cross-section is treated as a circle of area (π/4)·d², where d is its outer diameter. The total cable area is the sum of nᵢ·(π/4)·dᵢ² over every group. The tray cross-section is W × H. Fill percentage is 100 × cable area / tray area. Separately, the single-layer width Σ nᵢ·dᵢ is compared with the usable width W to tell you whether the cables can sit side-by-side in one layer or must be stacked (which changes grouping/derating).
Worked example
Worked example. A 150 mm × 75 mm tray has a cross-section of 11,250 mm². Ten cables of 12 mm OD give a cable area of 10 × (π/4) × 12² = 1,130.97 mm², so the fill is 100 × 1,130.97 / 11,250 = 10.05 %. Their combined width is 10 × 12 = 120 mm, which fits within the 150 mm usable width in a single layer.
Common mistakes
- Using the conductor or CSA size instead of the cable's full outer diameter (over the sheath) — always use the OD for tray fill.
- Reading area fill as if it were the allowable limit — typical maximums are around 30–50 %, and heat-dissipation/derating rules can force a lower single-layer fill.
- Entering the tray's overall outside dimensions rather than the usable inside width and depth, which overstates the available area.
Frequently asked questions
What is a safe maximum cable tray fill?
It depends on the wiring rules and the tray manufacturer, but a common guide is around 30–50 % of the tray cross-section. Where cables must be single-layer spaced for heat dissipation the practical fill is lower. Always confirm the allowable fill and any current-carrying derating for your specific installation.
Why does the single-layer width matter?
If the cables fit side-by-side in one layer they dissipate heat better and attract less grouping derating than cables bunched or stacked. The width check (Σ n·d vs W) flags when your cables would have to stack, which affects the current-carrying capacity calculation.
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