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Cable Tray Management for Commercial Solar
Tuesday, August 25, 2020
Why cable management is important
With commercial solar projects we’re seeing a shift away from installing both DC and AC cables in their respective conduits and a shift towards using custom solutions for the job at hand. This is due to the number and length of the cables commonly involved. For a 100 kW roof mount system, incorporating, say, 4 x 25 kW, 3 phase inverters, this may result in 15 x DC cable runs down to the inverter so you can imagine how many cables need to be managed for a 500 kW system!
So what’s available to the installer? Effectively the choices can be broken into three categories:
- Cable Ladder
- Cable Tray
- Cable Mesh
1. Cable ladder
Cable ladder is the most robust of the three categories. It is ideal for heavy duty applications, vertical and horizontal runs where the weight and orientation of the cable imparts some severe forces. For example, running 300mm XLPE (Cross Linked Polyethylene) cable up a wall or across a horizontal distance involves some serious weight, hence you will need a suitably rated ladder to accommodate.
2. Cable tray
Cable tray is not as heavy duty as cable ladder but it comes close and is t widely used in commercial solar installations to accommodate both DC and AC cable cable in outdoor and indoor applications.
3. Cable mesh
The third member of the triad is cable mesh and this method of containing cables is for lighter communication cabling. It is typically used indoors for areas such as computer server rooms and similar applications.
Cable tray, standard sizes
Cable tray comes in the following standard sizes:
The standard length is 3000 mm and there are various widths from 150 mm up to 600 mm with different sidewall heights to cater for the different cable diameters.
Cable tray, important points
Look at the load rating when selecting the best options for your cable tray. For example, is there a hot dipped galvanised option for outdoors?
Cable tray, for DC runs
Up on the roof and wondering d how many DC cables can be installed in the tray?
You will have to think about heat transfer when grouping cables together and the subsequent cable derating that must be taken into consideration.
And remember, if the tray is going under something else, the cable tray’s profile is extremely important.
Example: DC cable run
Let’s assume we are using 4 mm DC cable for all our runs on a particular project and have selected a 150 mm tray with a sidewall height of 45 mm.
As the tray is going outside it will be a hot dipped galvanised option as stipulated in standards AS/NZ 4680 and ISO1461.
With this 150 mm tray *can go 14 strings with the main earth vein. Remember the more cable in the tray, the more heat generated and this reduces the CCC (current carrying capacity) of the cable - so do your calculations!
*Decision is up to the designer/installer taking all relevant factors into consideration
Cable tray, design decisions
With larger commercial solar projects there can be a considerable distance between the string that is furthest away from the inverter and the one that is closest. A series of questions have to be asked that include: should the tray transition from smaller cable tray to larger; or should we just use one large tray size as there happened to be a good deal on the bulk purchase of 450mm?
Multiple strings, bends, cost benefit analysis
The question is with multiple inverters and multiple strings do we just run independent trays everywhere or do we combine?
And what about all the bends that are required? Do we take the time to manually cut and fit or do we purchase existing ready made bends? What about how often we fix the cable tray lid down - in other words the distance between fixings?
Also the span between fixings needs to be determined. Do we go to a heavier gauge with less frequent support so less parts less labour but, as a solution, is more expensive?
To arrive at the correct answer requires a cost benefit analysis that includes all the factors under consideration.
Cable tray, for AC runs
So we have taken a cursory look at cable trays for DC but what about the AC side?
Here we are dealing with heavier and thicker cables that weigh more, are harder to handle and exert more force on the tray depending on the diameter of the cable, the number of cables and the orientation of the run.
And when it comes to installation, you may need an indoor scissor lift n which is an extra cost to consider.
Cable tray, Earthing
Concerning earthing of the cable tray, this is referenced in AS3000 under 5.6.3 Bonding Conductors
Under part(a) which states:
“ Conductive piping, cable sheaths and wiring enclosures (i.e. cable tray). The equipotential bonding conductors required in accordance with Clauses 18.104.22.168 to 22.214.171.124 shall have a cross sectional area not less than 4mm2”
So we need to earth the cable tray!
Cable tray management is an integral part of any commercial solar project and making the right selection is crucial to keeping the system safe, minimizing costs and install time and maintaining correct cable ratings. As your projects get bigger, and by default more complex, adopting a prudent selection approach and weighing up the material/labour cost components will help you successfully complete the project.