The National Electrical Code (NEC) uses the word “harmonic” exactly 11 times, all in combination with or explanation of neutral conductors. However, “harmonic” is not defined in the NEC. A definition of the word is located in 3.3.44 and used in 10.1.1.8 in the 2010 edition of NFPA 70B, the Recommended Practice for Electrical Equipment Maintenance: “those voltages or currents whose frequencies are integer multiples of the fundamental frequency.”


An integer is a whole number, either positive or negative in value, such as +1, +2 and +3 and –1, –2 and –3. The fundamental frequency of most applications within the NEC is 60 hertz (Hz) or 60 cycles per second, so an integer would be whole number multiples of 60 Hz. Understanding the fundamental frequency and the harmonics of the fundamental frequency is critical to the knowledge necessary for dealing with proper electrical system installation, troubleshooting and maintenance.


The term “interharmonics,” which is somewhat related to harmonics, is not used in the NEC. The definition of interharmonics is located in 3.3.45 and used in 10.1.1.9 of NFPA 70B and has the following definition: “Not all frequencies that occur on an electrical power system are integer multiples of the fundamental frequency (usually 60 HZ), as are harmonics. Some loads draw currents that result in voltage that are between harmonic frequencies or less than the fundamental frequency. These frequencies are referred to as interharmonics and can be made of discrete frequencies or as a wide-band spectrum. A special category of these interharmonics is called subharmonics, in which the frequencies involved are less than the fundamental power line frequency.”


Both harmonics and interharmonics can be measured and expressed as the total harmonic distortion (THD), which is usually a percentage value of the maximum load capacity of an electrical system. In NFPA 70E, 10.2.1.3.1 provides an example of a circuit with 1 ampere (A) of current on a 30A circuit where the THD is 50 percent of the total-load current. Even though the THD is 50 percent, the harmonic load is only 0.5A on the 30A circuit. If the total load is 25A with a 50 percent harmonic on a 30A circuit, it may be a problem for the circuit components, unless the harmonic loads are taken into account as part of the system design. 


Harmonic and interharmonic loads are often caused by nonlinear loads, such as rectified-input switching-mode power supplies (electronic power supplies and electronic equipment), iron-core and electronic lighting ballasts, some transformers and motors, computers, copy machines, variable frequency drive units, solid-state motor control equipment, and similar electrical/electronic equipment.


As noted in NEC 310.15(B)(5)(c), the neutral current on a 4-wire, three-phase wye circuit, where the major portion of the load consists of nonlinear loads (harmonics and interharmonics), the neutral conductor is considered to be a current-carrying conductor. For a normal 4-wire, three-phase wye system, if the load is balanced and harmonics are not present, the phase currents cancel each other vectorially, and the resulting neutral current is zero. Where harmonics and interharmonics are present, the triplen harmonics, such as the whole number integers of the third, sixth, ninth, etc., harmonics, as well as the fractions of the whole number integers, are additive. This extra current is carried back to the source on the neutral conductor and can be measured using a true rms digital ammeter, a spectrum analyzer, a harmonic analyzer, an oscilloscope or various other types of meters. Care should be taken to increase the size of the neutral for the extra harmonic current that will be added to the normal neutral current carried on the neutral conductor.


Harmonics and interharmonics can create many problems. The main one recognized by the NEC and NFPA 70B 10.2.2.1.1 is excessive neutral current. A host of other problems can be directly attributed to harmonics, such as overheating of transformers, motors, generators, solenoids and ballasts. Circuit-breakers can nuisance-trip, fuses on power-factor-correction capacitors could blow without apparent reason; and unusual noises in switchgear or telephone circuits can occur. One of the main problems can be voltage and current waveform distortion that results in faulty operation, loss of data or failure of computer and other electronic equipment.


While the NEC concentrates on increasing the neutral conductor size, 10.2.5 of NFPA 70B provides a list of solutions to harmonic problems. There are too many recommendations to list here, but check NFPA 70B for solutions.