step up transformer from 480v to 12.4KV; NEC 450.3A

[Randall Flagg]

Help! I was wondering what size primary and secondary over current protection would be needed for a 300kva transformer going from 480v to 12.4.. I was panning on feeding it from a SCE 400a service with a 400a disconnect but was told the transformer had to be protected by a 450a disconnect. would my primary side be 1.25% of the FLA since it is 480v? Additionally I have stating that it is being installed in a supervised location so would i have to show secondary protection for the 12.4 KV side?

I also have two 100kva transformers going from 12.4 to 480v and was told by the engineer that i must have primary and secondary protection. I have it noted that It is in a supervised location so secondary protection should not be needed. Am I correct?

Thank you in advance.

 

[lielec11]

I also have two 100kva transformers going from 12.4 to 480v and was told by the engineer that i must have primary and secondary protection. I have it noted that It is in a supervised location so secondary protection should not be needed. Am I correct?

Thank you in advance.

Based on Table 450.3(A), you should be able to get rid of the secondary protection as long as the primary protection is less than 300% (CB) or 250% (fuse).

 

[iceworm]

This has all the earmarks of a looonnnggfeeder to remote equipment - maybe a couple of miles?

Help! I was wondering what size primary and secondary over current protection would be needed for a 300kva transformer going from 480v to 12.4.. I was panning on feeding it from a SCE 400a service with a 400a disconnect but was told the transformer had to be protected by a 450a disconnect. would my primary side be 1.25% of the FLA since it is 480v? ....

300kva, 480V, 3ph ==> 361 FLA X 1.25 = 451A. I would suggest 500A at a minimum - but maybe more. It depends on the transformer inrush. And I would check - cause it will be expensive to change if you get nuisance trips on energization.
Is the transformer designed for step up?
Is it a delta 480V primary?
Can you get any inrush data from the mfg? A 480D/12,470Y step-up is screwie enough the MFG should have the data.

About the worst you could get is if the transformer is a 12.47kvD/480Y and you are operating it reverse fed.

... Additionally I have stating that it is being installed in a supervised location so would i have to show secondary protection for the 12.4 KV side? ....

The NEC does not deal well with step-up transformers. That being said, the section concerning "supervised locations" is 450.3.A, Primary protection over 1000V. That doesn't fit. Rather look at T450.3.B. Yes, there is provision for no secondary protection.

However, consider protection of the secondary conductors, 240.4, 240.21. And if the two 100kva xfm are the loads, the primary OCP for these may well fit for the conductor OCP.

As you know, the 12.47kv conductors will likely be #6 minimum, >60A. The 300kva secondary current is <20A. The 100kva primary current is <10A. You will likely need two 15kv fused disconnects.

... I also have two 100kva transformers going from 12.4 to 480v and was told by the engineer that i must have primary and secondary protection. I have it noted that It is in a supervised location so secondary protection should not be needed. ...

Yes, T450.3.A is the correct section for these, the primary is over 1000V. So no secondary protection is required for the transformer. Again don't forget the secondary conductors require protection, and if terminated in a panelboard, the panelboard will require protection.

Just some thoughts - don't know how well they fit your application

ice

 

[lielec11]

Forgot the secondary conductor protection .. almost always forget the secondary conductors and secondary of the xfmr are considered separate entities since I typically use the same OCPD to protect both.

 

[Randall Flagg]

Thank you.

Thank you.

Thank you everyone!