Understanding Harmonics in Electrical Systems: Causes, Effects, and NEC Considerations

In today’s modern electrical systems—especially those with extensive use of non-linear loads like variable frequency drives (VFDs), LED lighting, and computers—harmonics have become an important design and maintenance concern. Harmonics can lead to overheating, equipment malfunction, and violations of power quality standards. This article explains what harmonics are, their impact on electrical systems, and where the National Electrical Code (NEC) addresses them either directly or indirectly.

What Are Harmonics?

Harmonics are voltage or current waveforms that are integer multiples of the fundamental frequency (60 Hz in the U.S.). While the ideal AC waveform is a pure sine wave, harmonics distort this waveform due to non-linear electrical loads.

• 3rd harmonic = 180 Hz

• 5th harmonic = 300 Hz

• 7th harmonic = 420 Hz

• … and so on.

These frequencies “stack” on top of the base 60 Hz signal and distort it, leading to total harmonic distortion (THD).

Sources of Harmonics

Harmonics are typically generated by non-linear loads, which draw current in abrupt pulses rather than smooth sine waves. Common sources include:

• Variable Frequency Drives (VFDs)

• UPS systems

• Fluorescent and LED lighting

• Office equipment (computers, printers)

• Battery chargers

• Welders

Effects of Harmonics

Harmonics can have multiple negative effects on an electrical distribution system:

1. Overheating of Conductors and Transformers

   • Harmonics cause increased RMS current, which leads to excess heating.

   • NEC 310.15(B)(5)(c) acknowledges that neutral conductors in 3-phase, 4-wire systems may need to be upsized due to excessive current caused by triplen (odd multiples of 3) harmonics.

2. Voltage Distortion

   • Harmonic currents interact with system impedance, causing voltage waveform distortion.

3. Nuisance Tripping

   • Circuit breakers and GFCIs/AFIs may trip unnecessarily due to distorted currents.
     

4. Reduced Equipment Life

   • Motors can suffer from torsional vibrations.

   • Transformers (especially dry-type) overheat due to eddy currents caused by high-frequency harmonics.
     

5. Increased Losses and Energy Costs

   • Harmonics reduce system efficiency, increasing operating costs.

NEC and Harmonics

The National Electrical Code does not explicitly define or quantify harmonic limits, but it addresses their implications in several sections:

1. Neutral Conductor Sizing – NEC 310.15(B)(5)(c)

“Where the major portion of the load consists of nonlinear loads, the neutral conductor shall be considered a current-carrying conductor.”

In systems with heavy harmonic-generating equipment, the neutral can carry more current than the phase conductors due to triplen harmonics (3rd, 9th, 15th), which add in the neutral.

2. Transformer Loading – NEC 450.3 & 450.9

Transformers supplying non-linear loads (e.g., with VFDs or large LED lighting systems) may need to be derated due to overheating caused by harmonics. Manufacturers typically provide derating guidelines for harmonic-rich environments.

3. Power Quality Considerations – Informative Annex

While the NEC itself doesn’t enforce power quality standards, Annex D and Informational Notes throughout the code reference IEEE standards and utility requirements, which often impose limits on THD (Total Harmonic Distortion).

Relevant IEEE standard:

• IEEE 519 – Recommended Practices and Requirements for Harmonic Control in Electric Power Systems

This is often adopted by utilities or enforced in large-scale installations.

Mitigating Harmonics

Engineers and electricians can mitigate harmonics using several techniques:

• Line Reactors or DC Chokes on VFDs

• Passive Harmonic Filters (tuned to specific frequencies)

• Active Harmonic Filters (dynamic correction)

• K-Rated Transformers (designed for non-linear loads)

• Isolation Transformers (to decouple harmonic-producing loads)

System design and coordination with the utility are crucial in high-harmonic environments.

Conclusion

Harmonics are a byproduct of today’s electronic-rich environments and can cause significant system issues if not properly addressed. While the NEC does not directly limit harmonic distortion, it acknowledges their effects, particularly on conductor and transformer sizing. Understanding and mitigating harmonics is essential for ensuring system reliability, reducing downtime, and maintaining code-compliant installations.

References

• NEC 2023:

  • Article 310 – Conductors for General Wiring (310.15(B)(5)(c))

  • Article 450 – Transformers

  • Informative Annex D and various informational notes

• IEEE Std 519-2014 – Harmonic Control in Electric Power Systems

• NFPA 70B – Recommended Practice for Electrical Equipment Maintenance

• NEMA TP-1 – Guide for Determining Energy Efficiency of Distribution Transformers

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Empower Engineering | July 15, 2025