The Ferranti effect is a rise in voltage occurring at the receiving end of a long transmission line

Transmission line

In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

, relative to the voltage at the sending end, which occurs when the line is energized but there is a very light load or the load is disconnected.

This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon.

The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.