Calculating 3-Phase current requirements where (2) 1-phase transformers are installed?

Tip DS

Tip DS

I'm here.
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The Great Meme State
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Electrical Engineer
Weird situation: Normal 480V 3-Phase loads total 21.6A
In addition, there are (2) 25kVA 1-Phase transformers connected using different power legs (one of them shared, obviously.)

I get the 25kVA single phase each requires 25,000VA/480V = 52.1A

The issue is the leg that's shared between the two single phase transformers. I wouldn't expect them to double the current on the shared leg to 104.2A, but I also know it won't be the 52.1A alone added to the other 3-Phase loads. I was half inclined to treat the scenario as though there was 25kVA across each par of legs for a total of 75kVA and calculate it as a 3-Phase transformer for 75,000VA/(480*sqrt(3)) = 90.2A and add that to the 21.6A.

What say you?
 
Tip DS said:
The issue is the leg that's shared between the two single phase transformers. I wouldn't expect them to double the current on the shared leg to 104.2A, but I also know it won't be the 52.1A alone added to the other 3-Phase loads.
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If you assume that those two single phase transformers are seeing loads with comparable phase shifts between current and voltage (e.g. all loads resistive, so 0 phase shift), then the two currents you add in the shared leg are 60 degrees out of phase, so the sum has a magnitude of 52.1A * sqrt(3).

Tip DS said:
I was half inclined to treat the scenario as though there was 25kVA across each par of legs for a total of 75kVA and calculate it as a 3-Phase transformer for 75,000VA/(480*sqrt(3)) = 90.2A and add that to the 21.6A.
Click to expand...
Indeed, that will give you the same result as above.

Cheers, Wayne
 
Tip DS said:
The issue is the leg that's shared between the two single phase transformers. I wouldn't expect them to double the current on the shared leg to 104.2A, but I also know it won't be the 52.1A alone added to the other 3-Phase loads. I was half inclined to treat the scenario as though there was 25kVA across each par of legs for a total of 75kVA and calculate it as a 3-Phase transformer for 75,000VA/(480*sqrt(3)) = 90.2A and add that to the 21.6A.

What say you?
Click to expand...

That is the easiest way to get a good approximation.

The true answer will likely be lower since the 25kVA transformers are likely not fully loaded.

As @wwhitney describes, the currents on the shared leg are out of phase, so they don't simply add.
 
It looks like the OP is describing a 3-phase open delta configuration using equally sized transformers. For balanced 3-phase loading this will look like it was a 43.25kVA transfomer (.577x75).
 
jim dungar said:
It looks like the OP is describing a 3-phase open delta configuration using equally sized transformers.
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I believe the OP is talking about sizing a feeder that supplies 21.6A of 3-phase loads and (the primary side of) two single phase transformers.

Cheers, Wayne
 
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