Non-linear loads like computers and LED lights generate harmonic currents—especially 3rd-order (triplen) harmonics—that accumulate in the neutral wire. If not properly sized, the neutral can overheat and affect system reliability. This article explains how to size the neutral conductor correctly in such conditions.
Neutral Sizing for Nonlinear Loads
Single-phase non-linear loads like personal computers, electronic ballasts, and other electronic equipment generate odd harmonics (i.e., 3rd, 5th, 7th, 9th, etc.). The most troublesome harmonics for single-phase loads are the 3rd and odd multiples of the 3rd (3rd, 9th, 15th, etc.). These harmonics are known as triplen harmonics.
What are Triplen Harmonics?
Triplen harmonics are zero-sequence harmonics. In a 3-phase 4-wire system, the phase currents containing these harmonics add algebraically in the neutral conductor rather than canceling out.
For example, if the triplen harmonic current is 5 Ampere per phase, the total harmonic current flowing through the neutral conductor would be:
- 5 A × 3 = 15 A
Neutral Current Under Balanced and Nonlinear Loads
In a perfectly balanced linear system, the sum of all phase currents is zero, and hence, no current flows in the neutral conductor.
However, in systems with balanced non-linear loads, triplen harmonics cause a significant current to flow in the neutral conductor. Hence, proper neutral conductor sizing becomes essential. The size must be capable of handling not just unbalanced current but also harmonic currents.
Impact of Triplen Harmonics on Transformers
In delta-star transformer configurations, triplen harmonics in the neutral conductor cause circulating currents in the delta winding. When these harmonics reach the star side of the transformer, they reflect into the delta side, where they continue to circulate. This leads to increased heat losses and possible overheating of the transformer.
Total Neutral Current in Nonlinear Load Systems
The total current in the neutral conductor is the sum of:
- Current due to system voltage unbalance (
Ib) - Triplen harmonic current (
Ih)
Neutral Current in a Three-Phase Connection
In = Ib + Ih
Where
In = Neutral current
Ib= Current due to voltage unbalance
Ih= sum of triplen harmonics current
Neutral Conductor Sizing Recommendation
Typically, 3 and ½ wire systems are used to handle unbalanced currents. However, in systems with non-linear loads, the neutral conductor size should be equal to or larger than the phase conductor, depending on the level of harmonic distortion.
Neglecting this can result in overheating, equipment failure, and power quality issues.
Practical Example: Neutral Current and Sizing in a Nonlinear Load System
Let’s consider a three-phase 4-wire system supplying power to a commercial office setup with many computers, printers, and LED lighting — all non-linear loads.
Given:
- Each phase supplies 20 A of load current.
- Harmonic distortion due to triplen harmonics is measured and found to be 25% of the fundamental current.
- The system is well balanced (negligible unbalanced load current).
Step 1: Calculate Harmonic Current per Phase
Triplen harmonic current per phase:
Ih (per phase) = 25% of 20 A = 5 A
Since triplen harmonics are zero-sequence components, they add directly in the neutral conductor.
So, total harmonic current in the neutral conductor:
Ih_total = 5 A × 3 = 15 A
Step 2: Determine Total Neutral Current
Since unbalanced current is negligible, neutral current is mainly due to harmonics:
In ≈ Ih_total = 15 A
Step 3: Decide Neutral Conductor Size
In this case, the neutral conductor must safely carry at least 15 A continuously. However, due to heat buildup and safety margins, neutral conductor size should be equal to or slightly larger than phase conductor.
- If phase conductors are sized for 20 A (e.g., 2.5 mm² copper),
- Then the neutral should also be 2.5 mm² or increased to 4 mm² if the harmonic distortion is higher or future expansion is anticipated.
Table: Common Neutral Wire Sizing Mistakes and Their Consequences
| Mistake | Description | Potential Consequences |
|---|---|---|
| Assuming No Neutral Current in Balanced Loads | True only for linear loads; non-linear loads cause harmonics that add in the neutral. | Undersized neutral, overheating, safety risks. |
| Ignoring Harmonic Distortion | Failing to account for triplen harmonics that increase neutral current. | Excessive neutral current, fire hazards, system instability. |
| Using Linear Load Sizing Methods | Designing neutral size the same as for linear systems. | Inaccurate sizing, poor performance, and overheating. |
| Not Planning for Future Loads | No allowance for growth in non-linear devices. | Neutral becomes overloaded as more devices are added. |
| Disregarding Code Requirements | Not following NEC/local code on neutral sizing in harmonic-rich setups. | Code violations, inspection failure, legal issues. |
| Skipping Real-Time Measurement | Relying only on assumptions, not measuring actual neutral current. | Wrong sizing decisions, unexpected overloads. |
| Improper Use of Shared Neutrals | Using one neutral for multiple phases with harmonic loads. | Cumulative harmonic buildup, high neutral current. |
Key takeaways
- Always consider harmonic distortion when sizing the neutral.
- If triplen harmonics are present, neutral may carry more current than phases.
- Size the neutral equal to or larger than the phase conductor for safety.
- Plan for future load increases and follow relevant electrical standards.
Conclusion
Neutral conductor sizing is not just about handling unbalanced loads—in systems with non-linear loads like computers and LED lighting, it’s about managing harmonics, especially triplen harmonics.
These harmonics don’t cancel out like normal currents. Instead, they build up in the neutral wire, which can lead to:
- Overheating
- Equipment failure
- Fire hazards
FAQs on Neutral Conductor Sizing
The neutral is sized based on unbalanced and harmonic currents. In balanced linear loads, it carries minimal current. But with non-linear loads, it should be sized equal to or larger than the phase conductor.
Non-linear loads generate triplen harmonics that accumulate in the neutral. If not sized properly, the neutral can overheat, leading to insulation damage or fire hazards.
It may overheat, cause voltage distortion, reduce power quality, and pose a serious fire risk — especially under harmonic-rich conditions.
Yes. Triplen harmonics (3rd, 9th, 15th, etc.) flow fully in the neutral and can exceed the current in individual phase conductors, requiring upsizing of the neutral.
Not always. In balanced 3-phase loads, the neutral isn’t needed. But for 3-phase 4-wire systems with non-linear or single-phase loads, it becomes essential to handle return currents.
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