lead

In an AC circuit, lead describes the phase relationship where current reaches its peak before the voltage does. If you graph both as sine waves, the current wave shifts leftward, arriving at zero crossings and peaks ahead of the voltage wave. This happens in capacitive circuits, where the capacitor's charging behavior causes current to surge in anticipation of voltage changes. Lead is measured in degrees; a 90-degree lead represents current peaking a quarter cycle before voltage.

Lead occurs specifically in circuits with capacitance. A capacitor passes current most easily when voltage is changing fastest, which occurs at the zero-crossing points of the voltage sine wave. At the voltage peak, where the rate of change is zero, capacitor current drops to zero. This inverted relationship produces the leading phase angle. In purely resistive circuits, current and voltage are in phase. In inductive circuits, the opposite happens: current lags voltage.

Power factor and practical problems

Excessive lead can reduce power factor, just as lag does. Power factor represents the cosine of the phase angle; a 30-degree lead gives a power factor of 0.866, meaning 13.4 percent of the apparent power does no useful work. Utilities penalise customers whose installations produce large leading or lagging angles because the wasted reactive power requires larger conductors and transformers to carry it.

Lead typically appears in power distribution systems heavily loaded with capacitive equipment: capacitor banks used for power factor correction, AC-DC converter circuits, and fluorescent lighting with certain ballast designs. When correcting lagging power factor by installing capacitors, engineers must ensure they do not overcorrect, introducing lead instead. Protective relays in high-voltage systems monitor for excessive lead as a sign of capacitor failure or mistuning.

The term lead comes from the metaphor of one wave running ahead of another in time, as if the current is leading the voltage forward through the cycle. This distinguishes it from lag, where current trails behind. Both lead and lag are measured as phase angles in degrees or radians, with lead conventionally expressed as negative angle or explicitly stated as leading, depending on the measurement convention used.