Calculating secondary transformer current on center tapped delta

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mull982

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We have an application where we are using a 75kVA Delta center tapped 240/120V transformer to supply 3-Phase 240V motors, Single phase 208V loads, and 120V single phase lighting loads.

When calculating the full load on the secondary of the transformer do you use the 240V L-L 3-phase voltage like you would for conventional calculations. Thus secondary current is 75kVA/.240kV/1.732 = 180A? Or do you need to somehow account for the 120V and 208V legs in the calculation?

When you are putting together the load calcs for the loads on the secondary of the transformer do you add the loads like you typically would on each phase or is there something special needed to account for the various voltages?
 

GoldDigger

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I would take the total load, both 220 and 240, on the center tapped winding to get a power value. I would then use that figure along with the 240V loads on the other two windings. I would not expect any line to neutral load on the high leg.
Given the three power (or VA) numbers, I would treat it like any unbalanced delta calculation.
 
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