Harmonic Currents in Installations
Harmonic currents from LED drivers, VFDs, and IT equipment — neutral sizing, derating, and mitigation
What Are Harmonics?
Harmonics are multiples of the fundamental 50Hz supply frequency caused by non-linear loads drawing current in pulses rather than smooth sinusoidal waves.
Common harmonic sources in installations
Engineering Recommendation G5/5| Source | Key Harmonics | Typical THD |
|---|---|---|
| LED drivers | 3rd, 5th, 7th | 20-40% |
| Switch-mode PSUs (PCs) | 3rd, 5th | 50-80% |
| Variable frequency drives | 5th, 7th, 11th | 30-50% |
| UPS systems | 5th, 7th | 10-30% |
| Fluorescent lighting (magnetic) | 3rd, 5th | 15-25% |
THD figures are typical for individual equipment — system-level THD depends on load diversity.
Non-linear loads distort the current waveform. Each distortion component oscillates at an integer multiple of 50Hz. The 3rd harmonic is at 150Hz, the 5th at 250Hz, and so on. These higher-frequency currents cause additional heating in conductors, transformers, and switchgear.
Neutral Current in Three-Phase Systems
Triplen harmonics sum in the neutral conductor rather than cancelling, creating a serious overheating risk.
I_neutral = 3 x I_3rd (per phase)- I_neutral
- = Resultant neutral current from triplen harmonics
- I_3rd
- = Third harmonic current per phase
BS 7671 Regulation 523.6.3
Neutral sizing factor based on third harmonic content
IET Guidance Note 1| 3rd Harmonic (% of fundamental) | Neutral Sizing Factor | Action |
|---|---|---|
| 0 - 15% | 1.0 | Standard neutral size |
| 15 - 33% | 1.0 | Size neutral on third harmonic current |
| 33 - 45% | 1.45 | Neutral larger than phase conductor |
| > 45% | Up to 2.0 | Double-sized neutral required |
Where neutral is oversized, cable derating for grouping may also be reduced.
Do Not Rely on Standard Cable Sizing
Cable Derating for Harmonics
BS 7671 Regulation 523.6 requires cables to be derated where harmonic currents increase the total RMS current. The phase conductors carry additional harmonic current that produces extra heat beyond the fundamental component. Where THD exceeds 10%, the cable must be selected based on the total RMS current including harmonics, not just the fundamental design current.
When to Commission a Harmonic Survey
Mitigation Methods
Harmonic mitigation techniques
Engineering Recommendation G5/5| Method | Application | Effectiveness |
|---|---|---|
| Passive filters (tuned) | Single dominant harmonic | Good for 5th/7th |
| Active filters | Multiple harmonics, varying loads | Excellent, up to 97% |
| K-rated transformers | Harmonic-rich supply to equipment | Withstands harmonic heating |
| Detuned PFC reactors | PFC with harmonic loads | Prevents resonance |
| Phase-shifting transformers | Large VFD installations | Cancels 5th and 7th |
G5/5 Harmonic Limits