WYE power to Delta power. The Input WYE power is 240V line to ground (415V line to line). The secondary windings are wired as closed Delta configuration. The 3 voltmeters measure the 240V line to line Delta voltages. The 4th voltmeter measures the High leg to Ground. That elusive voltage measures 208V and is derived as follows: The middle transformer is center tapped to provide two 120V sources. The voltage from the High leg to Gnd is not the sum of 240V + 120V = 360V because the phase of the middle transformer lags the phase of the High leg transformer by -120°. Therefore, the actual voltage is 120V x 1.73 (the square root of 2) = 208V. Note: Since EC does not measure rms voltages, peak voltages had to be used. Thats ok as the numbers come out the same. The 3rd transformer secondary voltage lags the High leg transformer voltage by -240°.
