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.

 

[kingpb]

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.

Likely a large industrial bus with nothing but three phase loads, typical to drop a small transformer where needed to pick up single phase loads.

 

[kingpb]

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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You can have oversized conductors or maybe the distance is long and some voltage drop needs to be accounted for. What is the load served?

 

[petersonra]

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?


Sent from my iPhone using Tapatalk

if the conductors are 420 amps, that would be 420*480*1.732 = 349 kVA

I don't know where you came up with 200,000/277*1.73.

If you have all 277 V loads, you can get 420 Amps on each line to neutral.

420*277 = 116 kva per line or 349 kVA for all three lines.

note that 480/1.73 = 277.

 

[Isaiah]

Likely a large industrial bus with nothing but three phase loads, typical to drop a small transformer where needed to pick up single phase loads.

The scenario is a building with multiple
loads - some of which are 277V lighting and others


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[Isaiah]

You can have oversized conductors or maybe the distance is long and some voltage drop needs to be accounted for. What is the load served?

The distance really isn’t very far - maybe 150 feet max


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