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How to calculate the self-supporting capability of a large conductor?

How to calculate the self-supporting capability of a large conductor?

How to calculate the self-supporting capability of a large conductor?

(OP)
We are trying to install some 2 - 3/C #500kcmil 15kV cables in a cable tray to go to some horizontally mounted open switches. The switches are located outside 30' up on a rack type structure.  

The design shows these cables routed directly to the switches and no support for the last 15'. The cables will break out of the multi-conductor at the end of the cable tray. The individual phase conductors weigh about 2.3 pounds per foot, so worst case this will be about 35lbs of weight per cable unsupported.

Is there any kind of calculation that can be done that can verify this design is correct? See the attached sketch.

 

RE: How to calculate the self-supporting capability of a large conductor?

What forces will wind and ice loading add?  
Can the cable hold itself up and maintain reasonable vertical alignment with the switch terminals in heavy winds?
What is the weight rating of the terminal and how much side force can it take?

Take a 10' piece of of 500 kcmil and hold it horizontally to get an idea of its strength.  9 feet is a long ways to self support.

If you ask the cable manufacturer they will probably refer to the codes:  NEC requires maximum 6 foot spacing, Canadian Code is 5 feet.

I woudl add a support.

 

RE: How to calculate the self-supporting capability of a large conductor?

In addition to the wind and ice loading, I'd be concerned with seismic loads. Because of the unsupported bend in the cables, you have what amounts to a structure that is supported on one end.They are free to swing back and forth about an axis which is formed by a line through the end off the cable tray and the logs at the top of each switch phase. The centers of gravity of each cable lie somewhere above this line. Knowing the seismic design criteria for your locality, one could calculate the forces developed by a maximum expected event and compare that to the switch manufacturer's specs.

If you could arrange to have the cables supported near the bends down to the switch locations, this would reduce this swinging effect significantly.

RE: How to calculate the self-supporting capability of a large conductor?

I don't the answer, but you also have to be concerned about magnetic forces during a fault.  If you can possibly comply with the applicable codes, I'd do that.   

RE: How to calculate the self-supporting capability of a large conductor?

I think it is not the actually situation sketch. The cable termination-at least for an outdoor termination-has to be erected upward so the cable
has to be lowered more. In the same time you have to keep the minimum permissible bending radius in some places and to support the cable
in vertical position in order to avoid the cable weight to be supported by terminals .
The termination has to be supported firmly in place.
The minimum bending radius= 12*overall dia.
According to IEEE-525 "IEEE Guide for the Design and Installation of Cable Systems in Substations" ch.10.2.2:
"The weight of a vertical cable should not be supported by the terminals to which it is connected"
 

RE: How to calculate the self-supporting capability of a large conductor?

(OP)
7anoter4,

Thanks for giving it a shot...but the switches can't be fed the way you are showing. The attached picture shows a similar installation.

Based on these answers, we will be supporting the cables all the way to the switches.

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