Neutral Conductor Sizing

Requirements of the Standards

The neutral conductor is connected to the neutral point of a system and is intended to carry current under normal conditions. 

The neutral conductor is required for each main circuit and there are rules in the Standards for its sizing.

Standard AS/NZS 3000

The neutral conductor of circuits designed to Standard AS/NZS 3000 must have the same current-carrying capacity (same size) as the associated active conductors.

When there are substantial harmonics then the neutral conductor size may need to be larger than the active conductors or in the case of a multi-core cable the neutral currents may dictate the size of the active.

An exception is for multi-phase circuits feeding predominantly multi-phase loads (balanced phases).  In this case the neutral conductor size may be smaller than the active conductor size but must be sized to carry at least the out-of-phase current.

Type of Circuit Neutral conductor size requirement
Single phase two-wire
  • One active conductor
  • Multiple active conductors
Multiphase
  • Consumer mains, submains and final subcircuits
Neutral conductor size not less than active conductor size. Sneutral = Sactive
Multiphase
  • 3rd harmonic currents are substantial
Neutral conductor may be required to be larger than associated active conductors. Refer to IEC 60364-5-52. Sneutral ≥ Sactive
Multiphase
  • Feeds predominantly multiphase loads
Neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. Sneutral ≤ Sactive



Standard IEC 60364-5-52

For multiphase circuits:

The neutral conductor size can be smaller than the active size if harmonic current is < 15 %.

When harmonics are between 15 % to 33 % the neutral conductor size shall be the same size as the active conductor.

For multicore cables when harmonics are above 33 % the neutral conductor size and the active conductor size must carry 1.45 times the load current.

For single core cables when harmonics are above 33 % the neutral conductor size must be sufficient to carry 1.45 times the load current and can be larger in size than the active conductors.

Type of Circuit Neutral conductor size requirement
Single phase two-wire
  • One active conductor
Multiphase
  • Copper active size ≤ 16mm2
  • Aluminium active size ≤ 25mm2
Neutral conductor size same as the active conductor size. Sneutral = Sactive
Multiphase
  • Harmonics current < 15 %
Neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. Sneutral ≤ Sactive
Multiphase
  • Harmonics current between 15 % and 33 %
Neutral conductor size same as the active conductor size. Sneutral = Sactive
Multiphase - multicore cable
  • Harmonics current > 33 %
Neutral conductor must be sized to carry at least 1.45 times the active load current Sneutral = Sactive
Both sized to carry 1.45 x IB
Multiphase - single core cable
  • Harmonics current > 33 %
Neutral sized to carry at least 1.45 times the load current Sneutral > Sactive
Both sized to carry 1.45 x IB
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