When voltage is read between any two-line conductors that voltage is actually being read between the two phases that feed each line. However, the voltage is not completely additive. Why? Because there is a 120-degree phase difference or time difference between the time that the electrical pressure of each phase is applied. The numeric equivalent of that phase or time difference is expressed by use of the square-root of three or the constant 1.732.
Line voltage divided by the square-root of three or the constant 1.732 = phase voltage.
208-line volts ÷ 1.732 = 120 phase volts.
This is the case because all three-line conductors have equal access to the neutral conductor and therefore will always read 120 volts 1-phase which is the voltage across the coil or phase that feeds each line conductor.
So, the relationship between phase and line voltage in a 3-phase 4-wire wye is governed by the square-root of three or the constant 1.732.
Understanding how to read and work through exam questions very much like this one will help you pass the electrical exam.
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