Harmonic Distortion Calculator

Harmonics are multiples of the fundamental frequency (50Hz in the UK). Non-linear loads like LED drivers, VFDs, computers, and UPS systems draw current in pulses rather than smoothly, creating harmonic currents (3rd = 150Hz, 5th = 250Hz, 7th = 350Hz, etc.). These harmonics cause: overheating of transformers and cables, neutral conductor overload, capacitor failures, sensitive equipment malfunction, and increased energy losses. The current UK standard for harmonic limits on DNO connections is ENA Engineering Recommendation G5/5 (issued 2020), which replaced G5/4-1; equipment current emissions up to 16A per phase are limited separately by BS EN 61000-3-2.

In a balanced three-phase system, the neutral should carry zero current because the three phases cancel out. However, triplen harmonics (3rd, 9th, 15th — multiples of 3) do NOT cancel — they add together in the neutral. If each phase carries 10A of 3rd harmonic, the neutral carries 30A of 3rd harmonic. In buildings with high LED or IT loads, the neutral current can exceed the phase current by 1.7× or more. This is why BS 7671 may require oversized neutral conductors. Use our Three-Phase Balancing Calculator to check your phase balance.

ENA Engineering Recommendation G5/5 (2020, which superseded G5/4-1) sets limits for harmonic VOLTAGE distortion on the public supply network. At LV (400V) the total voltage distortion planning level is 5% THDv, with a compatibility level of 8% THDv. The individual harmonic voltage planning levels at LV are 3rd = 4%, 5th = 4%, 7th = 4% and 11th = 3% (these are unchanged from G5/4). A connection is assessed against the 5% planning level. These are network voltage levels measured in volts at the point of common coupling — they are NOT equipment current-distortion thresholds: harmonic CURRENT emissions from equipment up to 16A per phase are limited separately by BS EN 61000-3-2 (in amps). Exceeding the G5/5 voltage limits can require mitigation as a condition of connection.

Methods include: Passive filters (tuned LC filters for specific harmonics — typically 5th and 7th), Active harmonic filters (electronic devices that inject anti-phase harmonics to cancel distortion), Line reactors (3-5% impedance chokes before VFDs — reduces THD from 80% to 30-40%), Multi-pulse rectifiers (12-pulse or 18-pulse drives eliminate lower-order harmonics), Phase shifting transformers (spread harmonics across phases), and Oversized neutral conductors (to handle triplen harmonic current).

If triplen harmonic current is significant, the neutral current can exceed the phase current. BS 7671:2018+A2:2022 Regulation 523.6.3 (with the rating factors in Appendix 4, Section 5.5) addresses this: where the third-harmonic content of the line current exceeds 15% the neutral is treated as a current-carrying conductor, and where it exceeds 33% the cable rating is governed by the neutral current rather than the line current. In practice, for buildings with predominant LED lighting or IT loads, a double-sized neutral (200% of phase) is often specified. Four-core SWA cable inherently has a full-size neutral. Note the neutral carries the sum of ALL triplen harmonics (3rd, 9th, 15th...), so for SMPS/IT-heavy loads it can be higher than a 3rd-harmonic-only estimate.