Substation Yard Sizing

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timm333

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Minneapolis, MN
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Electrical Design Engineer
How should we determine the size (length x width) of the medium voltage substation yard at the incoming supply point of the utility? NEC does not give much information about this. The substation has 25kV pad mounted transformer and 25kV pad mounted switchgear. We can get the footprints of the equipment, and add to it the electrical fence safety clearances as mentioned in Table 110-1 of NESC. This will give us the perimeter of the fence. Is this the right way to determine the dimensions of the substation yard? Thanks.
 
If all of your equipment is completely enclosed, like padmount gear, then you do not need any yard or fencing per the NEC. Anything beyond that is a design choice.
 
Sorry only the switchgear is pad-mount type. The transformer is substation type which requires fence. The transformer has air terminal channels (ATC) for primary and secondary terminations.
 
timm333 said:
Sorry only the switchgear is pad-mount type. The transformer is substation type which requires fence. The transformer has air terminal channels (ATC) for primary and secondary terminations.
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Only open air terminations require fencing. I mentioned padmount only as an example.
 
Actually transformer manufacturer told that fence is required because some part of the cables is exposed before the cables enter into the termination-chamber. I will check more into it.

In case fence is required, will it be Ok to find the size of substation (length x width) by using the method which I described above?
 
I supplied a good link for a substation design guide.
it would e a good resource. If you aren’t sure how to size the yard you will surely have other questions which this guide will answer. It also has clearances you need.

Are you wanting to size it to minimum? What about maintenance? What about changing the XF and needing to get equipment inside? You also need clearances for equipment.
 
Figure 4-21 of "Design Guide for Rural Substation" gives the clearances of equipment from the fence. In this way we can determine the perimeter of the fence which will be the boundary of the yard.

Yes we want to keep space for maintenance and future expansion. If the transformer is moved inside, the switchgear would still be outside because the switchgear has to be at the utility interface point. The switchgear is pad-mounted so fence will not be required, but the ground grid study would still be required.

If we move the 25kV transformer inside, let’s say on the second floor of the electrical building, then shall we have to perform the ground grid study for the first floor (even if the transformer is on the second floor)?
 
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