Sizing of Neutral Conductors: A Comparative Analysis of AS/NZS 3000 and IEC 60364-5-52 Standards
Abstract
The neutral conductor, connected to the system neutral point, is integral to electrical circuits for carrying current under normal operating conditions. Both international and national wiring standards mandate its inclusion in primary circuits and specify regulatory requirements for sizing. This paper provides a comparative summary of the neutral conductor sizing stipulations outlined in the Australian/New Zealand Standard AS/NZS 3000 and the international standard IEC 60364-5-52.
1. Neutral Conductor Sizing per AS/NZS 3000
Under AS/NZS 3000, the neutral conductor for general circuits is required to possess a current-carrying capacity equivalent to that of the associated phase conductors, thereby necessitating an identical cross-sectional area.
An exception to this rule applies to multi-phase circuits supplying predominantly balanced multi-phase loads. In such cases, the neutral conductor may be reduced in size, provided it is adequately rated to carry the maximum out-of-balance current, inclusive of any harmonic components.
The presence of substantial harmonic distortion represents a critical consideration. Under harmonic-rich conditions, the neutral conductor may require a larger cross-sectional area than the phase conductors. For multi-core cables, the neutral current may be the determining factor in sizing all conductors.
Table 1: Neutral Conductor Sizing Requirements per AS/NZS 3000
| Circuit Type | Condition | Neutral Conductor Requirement | Relationship |
|---|---|---|---|
| All Consumer Mains, Submains, Final Subcircuits | General Rule | Size not less than the active conductor. | Sneutral = Sactive |
| Multiphase | Substantial 3rd Harmonic Currents | May require a size larger than the active conductors. | Sneutral ≥ Sactive |
| Multiphase | Predominantly Balanced Multi-phase Loads | Must carry the maximum out-of-balance current (including harmonics). | Sneutral ≤ Sactive |
2. Neutral Conductor Sizing per IEC 60364-5-52
IEC 60364-5-52 provides a harmonised framework in which the permissible reduction in the neutral conductor size is explicitly linked to the magnitude of the harmonic current present in the circuit.
For multi-phase circuits, the standard permits a reduced neutral size only when the harmonic current component is less than 15% of the fundamental. When harmonic currents are between 15% and 33%, the neutral conductor must be sized identically to the phase conductors.
For harmonic currents exceeding 33%, more stringent requirements apply. In multi-core cable installations, both the neutral and phase conductors must be sized to carry at least 1.45 times the design load current (IB). For single-core cable installations, the neutral conductor must similarly be rated for at least 1.45 times IB, which may result in its cross-sectional area exceeding that of the phase conductors.
Table 2: Neutral Conductor Sizing Requirements per IEC 60364-5-52
| Circuit Type | Condition | Neutral Conductor Requirement | Relationship |
|---|---|---|---|
| Single-phase 2-wire & Small Conductors | Cu ≤ 16 mm² or Al ≤ 25 mm² | Size equal to the active conductor. | Sneutral = Sactive |
| Multiphase | Harmonic Current < 15% | Must carry the maximum out-of-balance current (including harmonics). | Sneutral ≤ Sactive |
| Multiphase | Harmonic Current 15–33% | Size equal to the active conductor. | Sneutral = Sactive |
| Multiphase (Multi-core Cable) | Harmonic Current > 33% | Neutral & active conductors sized for ≥ 1.45 x IB. | Sneutral = Sactive* |
| Multiphase (Single-core Cable) | Harmonic Current > 33% | Neutral sized for ≥ 1.45 x IB; may exceed active conductor size. | Sneutral ≥ Sactive* |
- *Both conductors are sized for a 1.45 x IB load capacity.
3. Conclusion
A comparative analysis reveals that both AS/NZS 3000 and IEC 60364-5-52 mandate full-sized neutral conductors for general applications. While AS/NZS 3000 allows for a reduced neutral based on load balance, IEC 60364-5-52 introduces a precise, harmonic-dependent methodology. A key distinction arises under high harmonic conditions (>33%), where IEC 60364-5-52 specifies a universal derating factor (1.45 x IB) for conductor sizing and differentiates requirements between multi-core and single-core cable installations. Engineers must apply the relevant standard contextually, with particular attention to harmonic assessments for determining compliant and safe neutral conductor dimensions.
