Phase angle is the fraction of a cycle, in degrees, that has gone by since a voltage or current has passed through a given value. The given value is normally zero. Referring back to Figure 3, take point 1 as the starting point or zero phase. The phase at Point 2 is 30, Point 3 is 60, Point 4 is 90, and so on, until Point 13 where the phase is 360, or zero. A term more commonly used is phase difference. The phase difference can be used to describe two different voltages that have the same frequency, which pass through zero values in the same direction at different times. In Figure 5, the angles along the axis indicate the phases of voltages e_l and e₂ at any point in time. At 120, e_l passes through the zero value, which is 60 ahead of e₂ (e₂ equals zero at 180). The voltage e_l is said to lead e₂ by 60 electrical degrees, or it can be said that e₂ lags e_l by 60 electrical degrees.

Figure 5 Phase Relationship

Phase difference is also used to compare two different currents or a current and a voltage. If the phase difference between two currents, two voltages, or a voltage and a current is zero degrees, they are said to be "in-phase." If the phase difference is an amount other than zero, they are said to be "out-of-phase."