What are harmonics and why do they matter?

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. UK Engineering Recommendation G5/4 sets harmonic limits for DNO connections.

Why does the neutral carry more current than the phases in some installations?

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.

What THD limits apply in the UK?

Engineering Recommendation G5/4-1 sets limits for harmonic voltage distortion on the public supply network: THDv must not exceed 5% at LV (400V). Individual harmonic limits: 3rd = 5%, 5th = 6%, 7th = 5%, 11th = 3.5%. For customer equipment, the contribution should be proportional to their agreed supply capacity. BS EN 61000-3-2 limits harmonic emissions from equipment up to 16A per phase. Exceeding these limits can result in DNO penalties or supply disconnection.

How do I reduce harmonic distortion?

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).

Do I need to oversize the neutral conductor?

If triplen harmonic current is significant (THD > 33%), the neutral current can exceed phase current. BS 7671 Regulation 523.6 addresses this: if the third harmonic content of the line current exceeds 33%, the neutral must be sized for the actual neutral current, not just the phase 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.

TradeCalculator.co.uk

Harmonic Distortion Calculator

Estimate THD and harmonic current content from non-linear loads (LEDs, VFDs, computers). Calculate neutral current from triplen harmonics and determine filter requirements per Engineering Recommendation G5/4.

Load TypeDetermines harmonic spectrum

Fundamental Current (per phase)

50Hz component of load current

Three-Phase Balanced?Affects neutral current calculation

How We Calculate This

This calculator estimates harmonic distortion levels and neutral current based on the type of non-linear load connected.

Triplen Harmonics in Neutral

Neutral current = 3 × I_3rd (balanced three-phase)

Typical THD by Load Type

LED lighting: THD ~35%, high 3rd harmonic
VFDs: THD ~45%, dominant 5th and 7th
Computers/IT: THD ~80%, very high 3rd harmonic
SMPS: THD up to 100%
G5/4 limit: THDv < 5% at point of connection

Frequently Asked Questions

Related Calculators

Cable Sizing Calculator

Calculate the correct cable size based on BS 7671 18th Edition regulations.

Voltage Drop Calculator

Check voltage drop complies with UK wiring regulations.

Maximum Demand Calculator

Calculate maximum electrical demand for domestic installations.

Circuit Breaker Sizing Calculator

Determine the correct MCB rating for your circuit.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.