Three-phase electric power is a method of alternating-current electric power transmission, which is in worldwide use in electric power distribution grids. The system was invented by Nikola Tesla in 1887–1888.
The three conducting wires are commonly colored black, red, and blue. The graphic shows the three correspondingly colored phasors representing voltage and current 120º apart, rotating at 50–60 Hz. Depending on whether the reactance of the load is inductive or capacitive, the voltage leads or lags the current, respectively. (You can refer to the handy mnemonic "ELI the ICEman".) The power factor is defined as the ratio
is the active power (measured in watts), which depends on the resistance, and
, the apparent power (measured in volt-amperes), which depends on the total impedance. The phase angle between the voltage and current phasors is then given by
. The power factor is equal to 1 for a pure resistance, but decreases while the phase angle increases for larger reactance.