CABLE TRAY
Where Size Does Matter.
Cable Tray design has two main components: Size and Weight.
Both of these are approached in much the same way.
Firstly, ID your cables and work out where each cable will run along the trays. Then break up your Cable Tray into sections that reflect a change in the quantity of cables.
For Example. You may have 6 cable runs:
Cable ID. 1 – Mains: 8 x 300mm2 1C XLPE + E
Cable ID. 2 – Sub-Mains: 4 x 95mm2 1C XLPE + E
Cable ID. 3 – Sub-Circuit P1: 2.5mm2 T+E
Cable ID. 4 – Sub-Circuit P2: 2.5mm2 T+E
Cable ID. 5 – Sub-Circuit P3: 2.5mm2 T+E
Cable ID. 6 – Sub-Circuit L1: 2.5mm2 T+E
And these cables run across 3 sections of Cable Tray:
Tray ID. 1 – Cables: 1, 2, 3, 4, 5, 6
Tray ID. 2 – Cables: 2, 3, 4, 5, 6
Tray ID. 3 – Cables: 3, 4, 5, 6
Now that we have identified each piece of tray, we know what cables will run on each piece. We can now calculate the tray size and weight requirements for each section.
Sizing:
When designing the cable tray and entering your cables, it’s crucial that you think about how each set will be installed on the tray, and how this affects horizontal space.
There is no one way to arrange cables. That being said; when designing a tray size (unless specifically designed otherwise) it’s recommended to always allow a cable space between each of the sets of cables, avoid stacking small cables more than 4 per set (either 4 high x 1 wide or 2 high x 2 wide), and install 3Phase SDI circuits in trefoil/quad. This balances tray efficiency with cable performance.
With the cables from the above example, Cable 1 will be 8 conductors + 1 earth in 2 sets of 4 in Quad. Each set is 2 cables wide by 2 cables high (the earth on top of one set, the earth is of no relevance in this instance), and a space between each set for minimal de-rating. This is expressed in the examples below.
As you can see, with an allowance of 25% spare space for future, Tray 1 we require 324mm of space. This could be satisfied by installing either a 1 x 150mm and 1 x 300mm tray or a single 450mm tray.
Tray 2 requires 99mm of space, which would be satisfied with a 1 x 150mm tray.
*It’s important to note the Tray Size and Quantity Calculator off to the side is used for information only. It is handy for a simple visualization of the potential cable trays required. The most important number is the Total Space Required. Select tray sizes to satisfy the space required and that suit the installation.
Tray 3 requires 31mm of space, which would be again a 1 x 150mm tray to satisfy this.
Loading:
Loading is much the same process as sizing with a slightly different focus, that being cable weight. In some ways this is easier as you simply have to list every cable. (As gravity doesn’t change regardless of how you arrange the cables).
Unlike sizing, where an earth cable can be stacked on top of an SDI set and have no spacing impact. Separate earths will need to be listed separately for loading as they contribute weight.
As you can see from this worked example; Tray 1 requires a loading strength of 31kg/m, Tray 2 5.1kg/m, and Tray 3 about 1Kg/m. As we look up our tray tech data we can see that even with our most demanding section (Tray 1 @ 31kg/m) we are able to install ET3 Steel Tray with Support Spans of up to 3m spacing, which has a load rating of 35kg/m.
Putting It Together
Now we know our tray Size and Type, we can mark these on our drawings. Take off the lengths of each tray section. That will give us our total quantity of tray. Simples!