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Loads on Phases Change or Rotate in a Transformer

Saturday, May 10th, 2014

Q: I am having a problem with the loads on my panel and transformer schedules. On my 208V panel and the transformer, the loads are shown correctly per phase. On the 480V transformer, the total load from the transformer is correct, but the per phase loads are different. What is going wrong?

A: There is a 30 degree phase shift in delta-wye transformers.

Load Phase A Primary = (Load Phase A Secondary + Load Phase C Secondary) * 0.5

Load Phase B Primary = (Load Phase B Secondary + Load Phase A Secondary) * 0.5

Load Phase C Primary = (Load Phase C Secondary + Load Phase B Secondary) * 0.5

See this answer the Electrical Engineering Stack Exchange site on why this phase shift happens.

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