1000 KVA Transformer wire size

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Joshb

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Can anyone help me with figuring the wire size on the primary feed for a 13,800V primary feed to a transformer. The power company was going to install but that has changed and now we are installing the primary feed and the transformers. The transformers are 1000 KVA. What calculation should I use for this?

Thanks
 
Joshb said:
Can anyone help me with figuring the wire size on the primary feed for a 13,800V primary feed to a transformer. The power company was going to install but that has changed and now we are installing the primary feed and the transformers. The transformers are 1000 KVA. What calculation should I use for this?

Thanks
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I suspect it will end up being the smallest wire you can find suitable for the voltage.
 
Joshb said:
The calculation I used was 1000 x 1000 / 13800 x 1.73 = 42A? Doesnt seem right though...
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That part is right. But we can size a breaker up to 4 times that value, or a fuse up to 3 times that value. The minimum size per the table cited by xformer is #2, and that will have sufficient ampacity for even 4 times 42 amps.
 
Overhead feed or U.G.
Our POCO uses 1/0 for underground up to 2000 kva.
 
augie47 said:
Overhead feed or U.G.
Our POCO uses 1/0 for underground up to 2000 kva.
Click to expand...


It's an U.G. feed. There are 3 transformers (2) 1,000 kva's and (1) 750 kva. I now have added up all the loads for a total
of 2,750 kva. New calculation - 2,750 x 1000 / 13,800 x 1.73 = 115A
 
Take a look at 450.3(A) decide what primary OCP is correct and size accordingly.
 
Joshb said:
It's an U.G. feed. There are 3 transformers (2) 1,000 kva's and (1) 750 kva. I now have added up all the loads for a total
of 2,750 kva. New calculation - 2,750 x 1000 / 13,800 x 1.73 = 115A
Click to expand...

A diagram would be helpful. Make one and then try to calculate.
 
There are 3 steps in order to design a medium voltage cable:
1) short-circuit current withstand
2) ampacity [allowable current-carrying capacity]
3) Voltage drop [mainly if a large induction motor will start D.O.L or Wye- Delta].
1) Usually for 13.8 kV the average rms short-circuit current will be 30 kA and the protection clearance time could be -close 0.1 sec and reserve 0.5 sec. For 30 kA and 0.5 sec fault clearance time, copper conductor, XLPE insulation [250 dgr.C short-time temperature limit] 300 kcmil [150 sqr.mm] will be suitable.
2) Ampacity could be stated from NEC art. 310. Let's say the cable will run in earth at 90 RHO -thermal resistance 20 dgr.C. Transformer rated current will be 1000/13.8/sqrt(3)=41.84 A.
According to Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers for less than 6% transformer rated impedance overcurrent protection required is 600% transformer rated [6*41.84=251 A].
Art.240.4 Protection of Conductors: overcurrent protection shall not be more than feeder ampacity[for less than 800 A could be a little more in certain conditions]
According to NEC Art.310 Table 310.60(C)(83) Conductors Cabled Within an Overall Covering (Three-Conductor Cable), Directly Buried in Earth. For Detail 5[one cable] 5001-35000 V 250 kcmil copper conductor 90oC insulation rated ampacity is 380 A.
 
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