Editing Per-unit system (section)

===Three-phase===

Power and voltage are specified in the same way as single-phase systems.  However, due to differences in what these terms usually represent in three-phase systems, the relationships for the derived units are different. Specifically, power is given as total (not per-phase) power, and voltage is line-to-line voltage.
In three-phase systems the equations <math>P = S\cos(\phi)</math> and <math>Q = S\sin(\phi)</math> also hold. The apparent power <math>S</math> now equals <math>S_{\text{base}}= \sqrt{3}V_{\text{base}} I_{\text{base}}</math>

: <math>I_{\text{base}} = \frac{S_{\text{base}}}{V_{\text{base}} \times \sqrt{3}} = 1 \text{ pu}</math>
: <math>Z_{\text{base}} = \frac{V_{\text{base}}}{I_{\text{base}} \times \sqrt{3}} = \frac{{V_{\text{base}}^2}}{S_{\text{base}}} = 1 \text{ pu}</math>
: <math>Y_{\text{base}} = \frac{1}{Z_{\text{base}}} = 1 \text{ pu}</math>