PRIMARY SIDE OF TRANSFORMER

[binwork91]

Question: if I up size the transformer from 500KVA to 1000KVA, the secondary side of transformer does not change. Do I need to upsize the primary side fuse?
For example, original i have 2000A fuse feed 500KVA step up transformer 208/120 to 480/277, and secondary side feed a 800A fuse.
Now, for some reason, I need to up size the transformer to 1000KVA, secondary side of transformer does not change. Do i need to up size the primary side fuse to 3500A? Can i use 2000A fuse, because the load at the secondary side transformer is not change.

 

[charlie b]

We are allowed to protect a transformer's primary windings at a value no higher than 250% of its primary full load current rating. We are allowed to protect a transformer's secondary windings and the secondary conductors at a value no higher than 125% of its secondary full load current ratings. Those are maximum values. We can protect it at a lower value, if desired. So no, you don't have to upsize the primary protection.

That said, I am curious as to why you need to do this, if the secondary load has not increased.

 

[binwork91]

We are allowed to protect a transformer's primary windings at a value no higher than 250% of its primary full load current rating. We are allowed to protect a transformer's secondary windings and the secondary conductors at a value no higher than 125% of its secondary full load current ratings. Those are maximum values. We can protect it at a lower value, if desired. So no, you don't have to upsize the primary protection.

That said, I am curious as to why you need to do this, if the secondary load has not increased.

Thank you.
I does not know the reason, it is an existing building. The building engineer told me the information about this.

 

[infinity]

I wonder if inrush current will be a problem with the smaller OCPD.

 

[LarryFine]

I agree with Charlie's response, certainly in theory. I'd want to know why the unit is being upsized.

 

[binwork91]

So, the story is there is a 4000A fuse feeds 1000KVA step up transformer. We don;t know what they change on the load side the transformer, they told me the maximum load is 600KVA. Now, they want to rewire the primary side of transformer, and make it feed from 2000A switch with 2000A fuse.

So base on the code, they are allowing to use the 2000A fuse, i think the inrush current will be a problem. They may not able to energize the transformer.