Cable tray fill calculator
Compare the area of your round cables with a tray reference rectangle. Add different cable sizes, switch between mm and inches, and share the calculation.
Area fill result
Example loaded. Change the dimensions, then calculate.
- Total cable area
- 1,570.796 mm²
- Reference area
- 30,000 mm²
- Total cables
- 20
This is an area comparison. It does not confirm that the cables can be arranged in the tray.
Round-cable geometry only. No NEC compliance, cable packing, support loading or ampacity check is performed.
Understand fill limits and ampacity ↓How is cable tray area fill calculated?
For a round cable, use its complete outside diameter (OD), including insulation, shielding and the outer jacket. The copper or aluminium conductor size alone does not describe the space the cable occupies. Use the cable manufacturer’s dimensions when available.
Reference area = inside width × reference height
Area fill (%) = 100 × cable area ÷ reference area
Enter each different outside diameter as a separate group. The calculator adds the circular cross-sectional areas and divides the total by your reference rectangle. Cable length does not enter this area calculation. All lengths are converted to the same unit before calculating; area results use square millimetres or square inches.
Which tray dimensions should I enter?
Measure the clear inside width available to the cables, rather than the overall outside width. Enter the height of the reference space you want to evaluate. Side rails, fittings or a project requirement may affect that space, so do not assume the full outside height is usable.
Worked example: 20 cables in a 300 mm tray
Take an inside width of 300 mm, a reference height of 100 mm, and 20 cables, each 10 mm in outside diameter. One cable occupies π × 10² ÷ 4 ≈ 78.540 mm². All 20 occupy approximately 1,570.796 mm².
The reference area is 300 × 100 = 30,000 mm². The geometric area fill is therefore 100 × 1,570.796 ÷ 30,000 ≈ 5.236%. Select “Load example / reset” above to reproduce it. This percentage describes the selected rectangle; it is not an electrical approval.
How does this relate to NEC 392 fill limits?
NEC Article 392 addresses cable tray installations. The fill rules in 392.22 depend on the applicable edition and installation category. Some arrangements are limited by cable area; others require a sum of outside diameters or a particular arrangement. There is no single percentage that applies to every tray and cable combination.
This version calculates geometric area only. It does not select a NEC rule, check cable permissions, apply a code table or verify installation compliance. Before using a result for a project, identify the locally adopted edition and the rules for the tray, cable types, sizes and arrangement involved.
When should ampacity and derating be checked?
A space calculation does not determine current-carrying capacity. NEC 392.80 addresses ampacity; cable construction, ambient temperature, grouping, spacing and covers can affect the applicable evaluation. This calculator does not return an allowable current or a derating factor.
Further reading: Eaton Cable Tray Manual (based on 2014 NEC). This historical guide helps explain the distinction; use the adopted code edition for actual requirements.
Does a low percentage mean the cables will fit?
No. Circles leave gaps between them, and an area sum ignores how they are arranged. A large cable may exceed the tray’s width or your reference height even when the total percentage is low. The calculator flags that simple dimensional issue, but it does not solve packing, cable separation, bend radius, loading or future expansion.
Use the result as a transparent first comparison, then check the physical layout and the applicable installation requirements separately.