I know this is an old thread, but I see there is still some confusion as to the turns ratio of a delta-wye transformer (in this case, 480V x 208/120V).
According to mikesh, the turns ratio is 2.3:1 since 480V/208V = 2.3 (approximately).
While that seems like logical thinking, it is incorrect. You must remember that each ONE of the 3 coils of the delta-connected primary induces voltage onto only ONE of the 3 coils in the wye-connected secondary. (Look at a the schematic of a 3-phase transformer for reference.)
In a delta-connected winding, the line voltage is the same as the phase voltage (the voltage across a single coil). So, in this case, the voltage across each coil is 480V.
In a wye-connected winding, the line voltage is 1.73 times larger than the phase voltage. This is a result of two coils having voltages of 120V each, but being 120° out of phase from each other. In other words, 120V × 1.73 = 208V.
Since the voltage across each individual coil in the secondary is 120V (this is the phase voltage), you can clearly see that the turns ratio is 480V ÷ 120V = 4:1
What mikesh has calculated is the line voltage ratio (VL-Pri : VL-Sec). This is a valid calculation for the turns ratio of delta-delta and wye-wye transformers only (not wye-delta or delta-wye).
