Use of over-sized Transformer

[Brian Jackson]

Im adding 15KVA of 208/120V, 3 phase load to my building. I have 480/277V 3 Phase power available.
I have an existing 75KVA transformer Id like to use for this new 15KVA load.
Can I repurpose this 75KVA transformer for my 15KVA loads? If so: what is the minimum feeder amps and wire size I'd need to feed this?

 

[jim dungar]

The inrush characteristics of a transformer are independent of the load on its secondary.

You may get lucky but I would be surprised if you can get the primary side protective device to be much smaller than 125% of full load.

 

[electrofelon]

It was very common in the past to supply a 75 KVA transformer with 100 amp fuses without issue. New more efficient units probably have higher inrush and are more likely to be an issue, but I do not have any direct experience with 100 amp supplies for these. What infrastructure do you have available on the 480 side to feed the transformer? Another consideration is losses. The no load losses are really going to add up for that size transformer over what you actually need, particularly if this is an older transformer

 

[winnie]

As others have alluded to, you can certainly use a 75kVA transformer to feed 15kVA of load.

The problems come when you energize the transformer. The transformer will draw significant inrush current, and this means that you need to size your supply circuit to be sufficient for this inrush. If you only needed to supply the load, you could get away with a 15kVA feeder, but you need to tolerate starting the transformer.

The inrush current to the transformer depends on things like the impedance of the supply to the transformer.

-Jon

 

[Jraef]

Transformers also have a fixed amount of internal losses that are based on the size of the transformer. So the fixed losses of a 75kVA will be roughly 5x the fixed losses of a 15kVA and you will see those losses all of the time it is energized.

 

[strap89]

I would recommend you size the primary and secondary feeders considering full rating of the transformer. See NEC Table 450.3(B). If you go significantly smaller you will have to look at the primary breaker trip curve to see if roughly 5X to 10X or so of the transformers full load rating for a a few milliseconds. Interestingly, the inrush varies depending on the phase position when the transformer is energized so it can sometimes start without a hitch, but other times open the primary OCPD. Still, you should design for a condition where inrush would never be an issue.