1000 KVA Transformer wire size

[Joshb]

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

 

[petersonra]

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

I suspect it will end up being the smallest wire you can find suitable for the voltage.

 

[Joshb]

The calculation I used was 1000 x 1000 / 13800 x 1.73 = 42A? Doesnt seem right though...

 

[david luchini]

The calculation I used was 1000 x 1000 / 13800 x 1.73 = 42A? Doesnt seem right though...

Seems right to me.

 

[xformer]

look at 310.106A

 

[charlie b]

The calculation I used was 1000 x 1000 / 13800 x 1.73 = 42A? Doesnt seem right though...

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.

 

[augie47]

Overhead feed or U.G.
Our POCO uses 1/0 for underground up to 2000 kva.

 

[Joshb]

Overhead feed or U.G.
Our POCO uses 1/0 for underground up to 2000 kva.

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

 

[augie47]

Take a look at 450.3(A) decide what primary OCP is correct and size accordingly.

 

[topgone]

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

A diagram would be helpful. Make one and then try to calculate.

 

[Julius Right]

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.