In electrical Systems for laying power cables , cable trays are preferable mode for laying cables over a long distance where digging can’t be feasible or digging cost is very high. Cable trays are also used where space is low. Also cable trays gives aesthetic looks to electrical systems.
There are following recommendations for designing cable trays:-
1. Always take 10-20% as Spare capacity.
2. Distances between cables should be from 5 mm to 10 mm.
Now let’s illustrate the same by taking an example:-
Here we will design cable tray for aluminum cables:-
(i) 10 no’s 2.5 mmsq X 4 Core cables
(ii) 25 mmsq X 4 Core 10 no’s cables
Total Outer Diameter :
Diameter of 2.5 mmSq.mm Cable =No of Cable X Outer Diameter of Each Cable
=10 X 16 = 160 mm
Similarly for 25 mmsq cables = 25X10 =250 mm
Total Diameter of All Cables laying in Tray = (160+250)mm= 410 mm
Now Weight of Cables will be:
Weight of 2.5 Sq.mm Cables =No of Cable X Weight of Each Cable
=10 X 0.56= 5.6 Kg/Meter
Similarly for 25 mmsq cable= 10X1.11= 11.1 Kg/ Meter
Total Weight of All Cables laying in Tray = (5.6+11.1) Kg/Meter= 16.7 Kg/ Meter
Width of all Cables:
Total Width of all Cables = (Total Cables X min. Distance between Cable) + Cable Outer Diameter in total
Total Width of all Cables = (20 X 5) + 410 = 510 mm
Now after taking 10% Spare Capacity of Cable Tray
Final Width of all Cables = 1.1X510 =561 mm
Now total area of cables
Total Area of Cable = width of all Cables X Maximum Height Cable
Total Area of Cable = 561 X 25 =14025 Sq.mm
Taking 10% Spare Capacity of Cable Tray
Final Area of all Cables = 1.1X14025 = 15427.5 mmsq.
Now cable tray selected from above calculations is that either we can install 1 no. 600 mm X 50 mm cable tray having weight carrying capacity of 50 kg/ meter, this cable tray has overall area of 30000 mmsq , Or we can install 300 mm X 50 mmsq 2 no. s cable trays with area = 30000 mmsq.
We can even use single 300 mmsq X 50 mmsq cable tray and placing one cable over another.
Usually perforated cable trays are used in electrical systems.