Oversized Transformer

[vw55]

Assume a 300kVA transformer has been installed and the primary conductors and the transformer primary are protected and the secondary conductors and the transformer secondary are protected correctly for the 300kVA rating. Can the contractor simply reduce the size of the primary and secondary OCPD appropriate for a 150kVA rated transformer and use the 300kVA transformer as a 150kVA transformer as the load the transformer serves has been reduced? The installed primary/secondary conductor sizes will not be changed. Wouldn't the 300kVA transformer inrush current be a problem with the OCPD's?

 

[jim dungar]

On the secondary, yes.
On the primary it may not be possible to reduce the size below 125% and still be able to get the transformer turned on.

 

[electrofelon]

If you are keeping the 300kva transformer, Why change anything? . Am I missing something?

 

[ptonsparky]

If you are keeping the 300kva transformer, Why change anything? . Am I missing something?

We are.

 

[augie47]

I agree with Post #2..inrush current would be your problem.
It might help if you keep your secondary protection at 125% allowing you to increase your primary protection to 250%

 

[charlie b]

You are making things too complicated. Keep the primary OCPD as it is. It will adequately protect the primary conductors and the transformer's primary windings. Select a secondary OCPD for a 150 KVA load.

 

[vw55]

We are.

The contractor used a preliminary set of plans to bid and buy materials (which I always advise against - they should wait for the final plans). Subsequently the load was value engineered by the owner and the panel size reduced from 800A to 400A. He already had the 300kVA transformer installed...the final, city approved plans showed the VE'd 150kVA transformer.

 

[ruxton.stanislaw]

On the secondary, yes.
On the primary it may not be possible to reduce the size below 125% and still be able to get the transformer turned on.

It might require a soft starter at a certain point.

 

[jim dungar]

It might require a soft starter at a certain point.

No.
There is no reason to go towards that initial, and on going, costs when the simple solution is to keep the primary as it has already been installed.

If they are going to spend money, they should replace the oversized transformer and reduce the cost of the ongoing loses.

 

[ruxton.stanislaw]

There is no reason to go towards that initial, and on going, costs when the simple solution is to keep the primary as it has already been installed.

If they are going to spend money, they should replace the oversized transformer and reduce the cost of the ongoing loses.

Right; I am not recommending the option, just advising the potential requirement to operate in the way the poster suggested (assuming it is not feasible to replace the transformer for some reason).