Harmonics are multiples of the fundamental frequency of an electrical power system. If, for example, the fundamental frequency is 60 Hz, then the 5th harmonic is five times that frequency, or 300 Hz. Likewise, the 7th harmonic is seven times the fundamental, or 420 Hz, and so on for the higher-order harmonics.

Harmonics can be discussed in terms of current or voltage. A 5th harmonic current is simply a current flowing at 300 Hz on a 60 Hz system. A 5th harmonic voltage occurs from the 5th harmonic current flowing through the impedances present. Sometimes the amount of harmonics is discussed as Total Harmonic Distortion (THD). The formula for calculating the THD for current is as follows:

If, for example...

• I₁= current at 60 Hz = 250 Amps
• I₅= current at 300 Hz = 50 Amps
• I₇= current at 420 Hz = 35 Amps

then...