Sizing a dry type XFMR

[Isaiah]

I’m trying to size a 3ph 480V-480/277V transformer. Construction has already purchased a 4/W600kCMIL feeder (for the secondary to panelboard)
- using 310.16 the amps are 420 at 75 degC
So if I size the XFMR secondary to the conductors I’d get a 200kVA with 480V-480-277V
200000/277 x 1.732 = 417A
Is this correct?


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[ron]

Usually you determine the load first, then pick the feeder and transformer, but I'll play along.

If the non-continuous load is 420A or less, then the transformer would be 420x480xsqrt(3)=350kVA.
However, most use standard sizes such as a 300kVA transformer with a 400A secondary breaker and a 450A or larger primary breaker to overcome inrush.

 

[petersonra]

I am guessing someone decided to go with a 400 Amp panelboard so they chose feeder wire for it that is appropriate for a loaded 400 A panelboard.

There is no real requirement to select a transformer that can feed 400 A though, but it probably would make the most sense since they already bought the panelboard and feeder.

I think I would be inclined to use a 300 kVA transformer as suggested by the previous poster, since the next standard size is 500 kVA. 300 kVA is only 360 Amps at 480 V but I suspect they are a lot easier to get than a 500 kVA transformer, and probably substantially less cost.

 

[Isaiah]

Usually you determine the load first, then pick the feeder and transformer, but I'll play along.

If the non-continuous load is 420A or less, then the transformer would be 420x480xsqrt(3)=350kVA.
However, most use standard sizes such as a 300kVA transformer with a 400A secondary breaker and a 450A or larger primary breaker to overcome inrush.

Thanks Ron


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[tortuga]

Interesting this is a 480 delta primary to 480/277 wye?
What size is the upstream delta transformer?
In this situation, I always first try to replace the 480 delta transformer upstream with a wye.
When comparing numbers I include the transformer losses over the next 10 years.