A catalog width is not a guarantee
Cable trays support and organise wiring in industrial buildings, equipment rooms and distribution routes. A tray width in a catalogue does not by itself guarantee that a particular set of power and control cables is suitable. The calculator answers a narrower, geometric question: will these cables fit side by side in one row, with the gaps and reserves you choose?
1. What “fill” can mean
Two geometric ideas must not be confused. Single-layer width is the horizontal space the cables occupy side by side. Cross-sectional occupancy is the sum of the circular cable areas divided by the inside rectangle of the tray. Occupancy alone cannot prove that a stack of cables can be arranged, or that it is thermally acceptable: it is only an area comparison.
2. Why outside diameter matters
Cables are specified by conductor size (for example 4 × 35 mm²), but tray fit depends on the overall diameter including insulation, sheath and armour. Two cables with the same conductor size can have different diameters, so always use the cable datasheet.
3. The single-row equation
For N cables of diameters d1 to dN, a uniform edge-to-edge gap g, and reserves cL and cR at the edges:
Compare the result with the usable inside width. With several cable groups, add the diameter of every cable and count the gaps between all neighbouring cables only once, across the whole row. The calculator never adds a hidden spacing percentage: you choose the gap, and 0 mm means touching cables, not a recommendation.
4. Worked example
Twelve cables of 22 mm outside diameter, a 5 mm gap and a tray with 300 mm usable inside width:
319 mm are needed, so the twelve cables do not fit in one such row of a 300 mm tray (margin −19 mm). A 400 mm usable width leaves 81 mm of spare geometric width, which is still not an installation approval. In a divided tray, check each compartment separately.
5. Cross-sectional occupancy is different
The twelve 22 mm cables occupy about 4,562 mm². In a usable 300 × 50 mm rectangle (15,000 mm²) that is about 30.4 %, yet the single-row width is still 319 mm. Area occupancy must not be confused with conduit or trunking fill rules, manufacturer limits or grouping derating.
6. What IEC 61537 addresses
IEC 61537:2023 covers cable tray and ladder systems: construction, testing, loading and supports. It is not a rule that a tray filled to 40 % or 50 % is compliant. Spacing, grouping corrections, current-carrying capacity and mechanical load must be checked against the applicable installation standards, the cable arrangement and the tray manufacturer's support-span data.
A well-planned tray also considers access and future replacement, bending radius, connectors, support spacing, load per metre, electromagnetic separation, heat dissipation, protective bonding, corrosion and the environment.
7. Common mistakes
- Using the conductor size instead of the cable outside diameter.
- Using the nominal tray width without checking the real inside width or dividers.
- Treating area occupancy as proof that a single-row layout fits.
- Treating a geometric fit as a current-carrying-capacity or support-load approval.
- Quoting a universal IEC fill percentage without naming the applicable installation rule.
8. Using the calculator
Enter the usable inside width, the cable counts and diameters and the gap. KiloVolt returns the required width, the margin and, if you enter candidate widths, the smallest one to investigate. It is a preliminary layout check, not a final installation approval: confirm the thermal installation and the cable and tray manufacturers' data. Related: the LV Cable Sizing guide for current-carrying capacity.