Cable Tray Fill Formulas: Area and Cable Count
Cable tray fill formulas: cable area from diameter, tray cross-section area, percent fill and the number of cables for a fill limit, with a worked example.
The formulas
| To find | Formula |
|---|---|
| Area of one round cable | a = π × d² / 4 |
| Tray area used for fill | At = width × loading depth |
| Percent fill | Fill % = n × a / At × 100 |
| Most cables of one size | n = floor(limit × At / a) |
d is the overall cable diameter in millimetres, the tray width and the loading depth are in millimetres, and the limit is a fraction. Tray fill rules differ widely. Some codes limit the cable area to a share of the tray cross-section, others limit cables to one layer, set a spacing for power cables or restrict the fill by cable size. Use the limit from your own code, and check the heating of grouped cables separately.
Worked example: twelve cables on a 300 mm tray
The cables have an overall diameter of 20 mm. The tray is 300 mm wide with a loading depth of 50 mm, and the fill limit is taken as 40%.
- a = π × 20² / 4 = 314.2 mm², so twelve cables take 3,770 mm²
- At = 300 × 50 = 15,000 mm²
- Fill = 3,770 / 15,000 × 100 = 25.13%
- At a 40% limit the tray takes floor(0.4 × 15,000 / 314.2) = 19 cables
Cables of 20 mm on trays with a 50 mm loading depth
| Tray width | Cross-section | Usable at 40% | Cables of 20 mm |
|---|---|---|---|
| 100 mm | 5,000 mm² | 2,000 mm² | 6 |
| 200 mm | 10,000 mm² | 4,000 mm² | 12 |
| 300 mm | 15,000 mm² | 6,000 mm² | 19 |
| 450 mm | 22,500 mm² | 9,000 mm² | 28 |
| 600 mm | 30,000 mm² | 12,000 mm² | 38 |
Frequently asked questions
Is the fill limit the same as for conduit?
No. Trays are open, so the heat and pulling concerns differ. The code or the manufacturer sets the limits for trays.
Does percent fill replace the heating check?
No. Grouped cables need derating, and a tray that meets the fill limit can still overheat the cables in a bunch.
Which depth do I use?
Use the loading depth, which is the usable depth under the side rail and may be less than the overall depth of the tray.
Do large power cables follow the same rule?
Many codes treat large cables differently, for example by requiring a single layer. Check your code for the rule that applies.
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