Oversized Transformer?

[jds3838]

I have a 208/120v 3 phase buck and boost transformer that is rated at 416 amps, that is providing 380/220v. It was used for a larger load in the past but I now have a smaller load. Could I use this transformer with a smaller 208/120v feed? It was fed with a 400 amp breaker before but my question is can I feed it with a 225 amp breaker since my load is reduced to 30kVA. If not, I would need to purchase a smaller transformer or a larger OCPD to feed the transformer. Time is critical on this project and I don't think I can get a smaller transformer of this type here in time to meet the deadline. Any feedback would be greatly appreciated.

 

[gar]

171221-0943 EST

I don't think I should answer, but the mods will have to indicate whether it is OK. Your job function would seem to indicate that tou could answer your own question.

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[jds3838]

171221-0943 EST

I don't think I should answer, but the mods will have to indicate whether it is OK. Your job function would seem to indicate that tou could answer your own question.

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Understand gar.

I only design distribution from our panels for our manufacturing lines. I had an EE that is no longer with us that did all of our electrical calculations and managed everything from the street feed to the electrical panels. I understand if you cannot answer the question.

 

[augie47]

I appreciate gar's concern but feel the question falls within Forum rules since the answer is somewhat generic.
You can feed a transformer with a circuit less than allowed if there is a reduced load. The only thing that may be a problem is tripping due to inrush current. The calculations needed to assure that will not happen are within the realm of engineering.

 

[gar]

171221-1537 EST

Several comments on transformers.

An ideal transformer has no inrush current when voltage is applied, and no steady-state magnetizing current, nor power loss. Therefore, for an ideal transformer the only thing you see at the input is a reflection of the load on the output back to the input. The coupling from the primary to the secondary will determine the reflection ratio.

In this ideal situation the size of the transformer is of no importance other than it needs to big enough.

In the real world you have to consider some of these parasitic effects.

Auto-transformer is simply a transformer where the primary and secondary share some portion of the same coil. Thus, no electrical isolation between primary and secondary.

A typical application of an auto-transformer is to provide a means to increase or decrease the output voltage of a source by adding or subtracting some voltage from the source. Usually not over about 10%. Thus, assuming the 10% value, to supply a particular load the VA sizing of the transformer might be about 10% of what would be required of a transformer that had to carry the full load.

An excessively sized auto-transformer might produce an inrush current problem. You would have to have an extremely oversized unit to probably worry about steady-state core losses.

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[winnie]

As others have said, the only significant problem is with 'inrush current'.

When you first turn on a transformer, depending on the design of the transformer and the particular place in the AC cycle that things are switched, you _may_ see a huge excess of magnetizing current flow for a fraction of a second.

This 'inrush current' is caused by the transformer iron core saturating, temporarily reducing the inductance of the transformer, so that more current flows. Because the transformer coils have resistance, this inrush quickly damps out to the steady state operating condition.

This can be a problem because the transient current can trip the primary side breaker as you turn the transformer on.

My best guess: try it out. It sounds like you have a 150 KVA transformer that you want to use with a breaker suitable for about a 50KVA transformer. It will probably work. If it doesn't there are techniques to 'precharge' a transformer to avoid the issue of inrush.

-Jon