Originally Posted by
Kalbi_Rob
WI | DE
So, easy way to remember how to perform turns ratio is to start with the acronym WIDE where WI stands for Wye where current is equal between phase and line, and DE stands for Delta where E (electromotive force which also defines voltage) or voltage is equal between phase and line.
So, on the Wye side of the transformer you need to take the voltage (assume to be phase voltage if not otherwise noted) and divide by square root of 3 (or ~1.73). So the equation is
(7200V/(sqrt(3))/480V = 4156.9V/480V = 8.66 so the TTR = 8.66:1
Turns ratio is only taken on single phases of a transformer, so all values need to be reduced to individual line values to represent each single phase. This is also why you see a 12kV/480 delta/wye transformer with voltage values of 12kV - 480Y/277V.
If for whatever reason you needed to convert current values from phase to line on a Delta Xfmr, then you take phase current and divide by sqrt(3).