In this problem we are asked to solve per-phase and in order to do that we have to convert the Voltage to per-phase. That's where the root 3 comes in... |Vp|=|VL|/√3. In 3-phase systems line-line voltage is always assumed given unless specified otherwise. Once the voltages are in per-phase quantities you can treat it as 3 separate systems where the voltage and current magnitudes are identical and the power is 1/3 of the total system. Thus the 3-phase power equation using per-phase quantities is |S|=3*|Vp|*|Ip|Ok, so the answer for this problem requires dividing the transformer power by 3, not root 3. It's 3 phase, wye connected. Why do we divide power by 3 to find output per phase in order to find current?
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