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.
This article summarises the requirements of Standards AS/NZS 3000 and IEC 60364-5-52.
Neutral Sizing according to Standard AS/NZS 3000
The neutral conductor of circuits designed to Australian and New Zealand 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
|
Neutral conductor size not less than active conductor size. | Sneutral = Sactive |
Multiphase
|
Neutral conductor may be required to be larger than associated active conductors. Refer to IEC 60364-5-52. | Sneutral ≥ Sactive |
Multiphase
|
Neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. | Sneutral ≤ Sactive |
Neutral Sizing according to 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
|
Neutral conductor size same as the active conductor size. | Sneutral = Sactive |
Multiphase
|
Neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. | Sneutral ≤ Sactive |
Multiphase
|
Neutral conductor size same as the active conductor size. | Sneutral = Sactive |
Multiphase - multicore cable
|
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
|
Neutral sized to carry at least 1.45 times the load current |
Sneutral > Sactive Both sized to carry 1.45 x IB |
