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Loads on Phases Change or Rotate in a Transformer

Saturday, May 10th, 2014

Q: On my 208V panel and the transformer, the loads are shown correctly on each phase. On the 480V transformer, the total load from the transformer is correct, but the per-phase loads are different. What is happening?

A: In delta-wye transformers, there is a 30-degree phase shift.

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

For an explanation of why this phase shift occurs, see this answer on the Electrical Engineering Stack Exchange.

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