Why is the Neutral Wire Size Smaller than the Phase Wire?

Electrical Technology

Why is the Size of the Neutral Conductor Smaller than the Line Conductor in a Polyphase System?

In a three-phase (poly-phase) system, the Neutral wire may sometimes be smaller than the Line wire under specific conditions. However, the Neutral wire and cable size must match the Hot (Live, Line, or Phase) wire as per NEC and IEC standards.

In three-phase star (wye) systems, the Neutral conductor size might be reduced if:

- The load is symmetrically balanced.
- The total harmonic distortion is low.

This reduction is more common in large-scale distribution systems where engineering calculations confirm its feasibility.

For example, in a three-phase, 120/208V wye system:

- If linear loads are balanced, the Neutral current may be zero.
- If nonlinear loads exist, harmonic currents (especially third harmonics) accumulate in the Neutral, requiring a conductor as large as the phase conductors.

Related Post: Why is the Ground Wire Size Smaller than the Hot Wire?

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Balanced Three-Phase System

The Neutral conductor carries the unbalanced current in a three-phase system. In perfectly balanced systems, the Neutral current is negligible or zero. Because the Neutral typically carries less current, a smaller conductor may suffice in certain situations.

In a balanced three-phase system, the Neutral cable is often smaller than the phase cables because it typically carries only the unbalanced current between phases. This results in significantly less current flow compared to each phase wire, especially when the load is evenly distributed across all phases.

The case is acceptable for systems with mostly linear loads (like incandescent lighting or resistive heating), the Neutral current is minimal due to balanced phase currents.

Unbalanced Three-Phase System

However, this is not always the case. In balanced three-phase systems with nonlinear loads (e.g., computers, LED lights, or motor drives), harmonics can increase the Neutral current, requiring the Neutral conductor to be as large as or even larger than the phase conductors.

For example, consider a three-phase cable supplying three different single-phase circuits. In the case of inductive and electronic devices, the harmonic currents produced will not cancel out in the Neutral. As a result, third harmonic currents can cause the Neutral wire to overheat. Therefore, a larger Neutral conductor is necessary in unbalanced three-phase systems

Single Phase System

The Neutral cable size is sometimes designed to be half the size of the phase cable for specific applications, particularly in older or cost-optimized designs.

In older premises, when there were fewer inductive loads and modern devices such as computers, printers, power supplies, UPS systems, large computer systems, and LED lights, the imbalance current was minimal. Most of the current would flow phase-to-phase rather than through the Neutral conductor.

For instance, in a single-phase 120/240V split-phase system derived from a WYE (star) configuration, if both hot wires carry the same amount of current, no current flows through the Neutral. In such cases, the Neutral wire could be reduced in size.

In another scenario involving a 120/240V system, consider the first hot leg supplying 15 amps and the second hot leg supplying 20 amps to single-phase circuits. The Neutral is intended to return the current difference, meaning only 5 amps will flow back through the Neutral wire to the transformer windings. Since a 240V system does not require a Neutral wire, and the Neutral current in 120V circuits of a 120/240V system is often less, smaller Neutral wires were historically deemed sufficient and used.

In modern times, however, a large amount of harmonic current is produced by modern devices and equipment. This harmonic current is not absorbed by the phase (hot) wires and instead returns to the source via the Neutral wire. In some cases, the returning current in the Neutral can exceed the current in the phase conductors. For this reason, the Neutral cable must now be the same size as the phase cable to safely handle the excessive current.

Neutral Wire Size in Modern Practice

While reducing Neutral cable size was acceptable in some older or specific scenarios, modern installations prioritize safety and compliance with updated electrical standards. It’s now more common to size the Neutral conductor the same as the phase conductor to ensure system reliability, especially with the increasing use of nonlinear and harmonic-rich loads.

Good to know:

Modern electrical codes (like NEC or IEC) often require the Neutral conductor to be the same size as the phase conductor to handle scenarios like unbalanced loads, harmonics, and fault conditions safely.
Designing the Neutral as half the size may not comply with contemporary codes and safety standards, which prioritize robustness and reliability.

NEC & IEC Requirement

NEC

The NEC (National Electrical Code) provides detailed guidelines for Neutral conductor sizing in Article 220, 250, and 310.

The Neutral must be sized to handle the maximum unbalanced load. (NEC Article 220.61).For systems with nonlinear loads (like those generating harmonics), the Neutral conductor must handle the cumulative effect of these harmonics, which could result in a Neutral current higher than the phase current.

When the Neutral conductor carries a significant amount of harmonic current (e.g., third harmonics), it cannot be reduced in size. This ensures safety for systems with computers, fluorescent lighting, or other nonlinear devices. (NEC 310.15(B)(5)(c)).

For grounded systems, the Neutral conductor must be capable of carrying fault current to clear any fault safely, requiring adequate size even if the Neutral normally carries less current. (NEC 250.24(C))

IEC

The International Electrotechnical Commission (IEC) – 60364 standard covers Neutral conductor sizing under various conditions.

In systems with significant harmonics, the Neutral must be at least the same size as the phase conductor. IEC (60364-5-52). For balanced three-phase systems without harmonics, a reduced Neutral size may be acceptable.

The voltage drop along the Neutral conductor must not exceed acceptable limits, even when the Neutral size is reduced. (IEC 60364-4-43).

The standard recommends maintaining the same size for Neutral and phase conductors in critical installations, such as data centers or hospitals, where unbalanced and harmonic currents can fluctuate.

Scenarios When Neutral Size Can Be Reduced

There are specific situations where reducing the Neutral conductor size might be permissible:

Balanced Load Scenarios:

In balanced three-phase systems (e.g., large motors in industrial settings), the Neutral current is minimal or non-existent.

In these cases, the Neutral conductor may be reduced to half the size, as long as voltage drop and fault conditions are verified.

High Voltage Systems:

In high-voltage distribution systems (like 11kV or above), the Neutral conductor often serves as a protective earth (PE) and may not carry load current continuously. This allows for reduced sizing.

Scenarios When Neutral Size Must Match or Exceed Phase Size

The following scenarios mandate full-sized or even oversized Neutrals:

Nonlinear Loads:

Data centers, office buildings, and facilities with high harmonic loads.
Harmonics, especially the third-order harmonics, add constructively in the Neutral, often exceeding phase current.

Single-Phase Loads in Multi-phase Systems:

When a three-phase system supplies single-phase loads (e.g., homes, small offices), the Neutral carries the imbalance and must be sized accordingly.

Fault Tolerance and Safety:

Neutral conductors must handle fault currents safely, ensuring the breaker or protection device trips without overheating the conductor.

The following table shows the code compliance for sizing and selection of a Neutral wire for single phase and three phase systems.

Scenario	Neutral Size (as per NEC/IEC)	Notes
Balanced 3-phase Loads	Can be reduced (half size or less)	Minimal or no current in Neutral.
Unbalanced 1-phase Loads	Must match phase conductor size	Neutral carries the unbalanced current.
Harmonic-rich Loads	Equal to or larger than phase conductor	To handle harmonic currents safely.
Critical Facilities (Hospitals)	Equal to or larger than phase conductor	High reliability required.
Fault Current Conditions	Equal to or larger than phase conductor (per fault analysis)	For safe fault current dissipation.