correct size 35kv wire for 750kva transformer

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Coastalguy1

Coastalguy1

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Virginia Beach, VA
I have a three phase pad mount transformer sized at 750kva 34.5kv primary & 277/480v secondary. My question is what is the calculation to provide the proper 34.5kv size wire? My wire specification is 4-1/c 35kv copper with 133% insulation MV-105 style.

Thanks!
 
Calculate it the same way as any other load. 750/(34.5 x 1.73) = 13 amps. So #12 would do but there is a minimum size for this type of cable. Check out the link below.


5–35kV — Copper Conductors page 17
Conductor

Underground in Ducts – One Cable Per Duct Direct Buried In Air
1 Circuit

Load Factor (%) 50 75 100
6 -------------- 121 115 83 amp rating
4 ---------------158 149 108 amp rating

http://www.kerite.com/pdf/Kerite Sales Catalog.pdf
 
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Anything less than #1 or #2 is going to be hard to find. If you call around with the total length you need, you may be able to get a bigger cable at a much lower price. Some of the plants have reels sitting around that they will make you a deal on just to sell them.
 
HV
That is a good point. It took a while to find a MFG that had the smaller conductors listed in a table.
 
Thank you for the replies! Bob, I thought this was the correct calculation but with the ampacity so small I was questioning myself. The cables terminate on 200amp bayonet fuses so I am going to size for 4/0 after confirming my short circuit study. Again thanks!
 
The cable may terminate on a 200 amp sw but the fuse will not be 200 amps. It should be something that is close to the 13 amps. Maybe 25 amps. The utility will usually use some thing like 2 x FLA to size the fuse.
 
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