What Is a Neutral Conductor (N)?

Neutral conductor (identification: N): conductor electrically connected to the neutral or the mid-point of the AC electrical system and used for the transmission of electric energy (defined in the IEC 60445:2021 [6]).

NOTE. Neutral: common live part of a star-connected polyphase AC system or mid live part of a single-phase AC system.

In the United States, while the term “neutral conductor” is used, this conductor is often also or alternatively identified as “grounded conductor”.

In the book [2] by Kharechko Y.V. the purpose of the neutral conductor and its features are even more detailed:

Neutral conductors together with phase conductors are used in AC circuits of low-voltage electrical installations to provide electrical power to the electrical equipment used in them. The neutral conductor is electrically connected to the common current-carrying part of an AC multi-phase power supply whose windings are star-connected, or to the middle current-carrying part of a single-phase AC power supply. Does not apply to line conductors.

The neutral conductor is referred to as a live part. However, under normal conditions, unlike phase conductors, it is usually under negligible voltage with respect to earth. The neutral conductor is a current-carrying conductor that is counted in the total number of conductors used in an electrical circuit, network or system.

Neutral conductors are primarily used in three-phase, four-wire AC electrical systems that have power supplies with neutrals. Figure 31A1 shows an example of a TN-S system, Figure 20.1 and 31B1 of a TN-C-S system, and Figure 3 of a TT system that has neutral conductors.

TN-S system 3-phase, 4-wire with separate the neutral conductor and the protective conductor throughout the distribution system
Figure 31A1 – TN-S system 3-phase, 4-wire with separate the neutral conductor and the protective conductor throughout the distribution system

In some cases and under certain conditions, the functions of neutral conductor and protective conductor can be combined in one PEN conductor.

Here is an example of a distribution system that has a TN-C-S system earthing type:

General outline of the distribution system
Figure 20.1 – General outline of the distribution system (TN-C-S)

From this TN-C-S system we see that the PEN conductor at the entrance of the installation is re-earthed and separated into a protective conductor PE and a neutral conductor N. This figure shows one example showing where the neutral conductor N in electrical installation of building.

TN-C-S system 3-phase, 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor elsewhere in the electrical installation
Figure 31B1 [1] – TN-C-S system 3-phase, 4-wire where the PEN conductor is separated into the protective conductor and the neutral conductor elsewhere in the electrical installation
TT system 3-phase, 4-wire with the neutral conductor throughout the distribution system
Figure 31F1 [1] – TT system 3-phase, 4-wire with the neutral conductor throughout the distribution system

Neutral conductors can also be used in single-phase AC electrical systems that have power supplies with midpoints (neutrals). Figure 31B3 shows one version of the TN-C-S system.

TN-C-S system single-phase, 2-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation
Figure 31B3 – TN-C-S system single-phase, 2-wire where the PEN conductor is separated into the protective conductor and the neutral conductor at the origin of the electrical installation

Important: In installations where loads are switched between phases, the neutral conductor may not be necessary.

Cross-sectional requirements

IEC 60364-5-52:2009 [3] specifies requirements for neutral conductors. In [3] it is stated that the cross-sectional area of the neutral conductor, if any, shall be at least equal to the cross-sectional area of the line conductors:

  • in single-phase circuits with two conductors, whatever the cross-sectional area of conductors is;
  • in multi-phase circuits where the cross-sectional area of the line conductors is less than or equal to 16 mm2 copper or 25 mm2 aluminium;
  • in three-phase circuits likely to carry third harmonic currents and odd multiples of third harmonic currents and the total harmonic distortion is between 15 % and 33 %.

NOTE. Such harmonic levels are to be met, for instance, in circuits feeding luminaires, including discharge lamps,
such as fluorescent lighting.

For polyphase circuits where the cross-sectional area of line conductors is greater than 16 mm2 copper or 25 mm2 aluminium, the cross-sectional area of the neutral conductor may be lower than the cross-sectional area of the line conductors if the following conditions are fulfilled simultaneously:

  • the neutral conductor is protected against overcurrents according to 431.2 [3];
  • the cross-sectional area of the neutral conductor is not less than 16 mm2 copper or 25 mm2 aluminium.
  • the load carried by the circuit in normal service is balanced between the phases and the third harmonic and odd multiples of third harmonics currents do not exceed 15 % of the line conductor current;

NOTE. Usually, the reduced neutral cross-sectional area is not lower than 50 % of the line conductor crosssectional area.

In [3] (based on Table 52.2) the minimum cross-sections of the neutral conductor are set:

  • in cables and insulated wires of fixed electric wiring – 1.5 mm2 for copper and 10 mm2 for aluminum;
  • in uninsulated wires of fixed electrical wiring – 10 mm2 in copper and 16 mm2 in aluminum;
  • in connecting flexible cables – 0.75 mm2 copper.

Kharechko Y.V. in the book [2] supplements:

« In the current standard [3], the above requirements are supplemented by requirements to increase the area of neutral conductors when third harmonic and multiple harmonic currents are flowing in electrical circuits. Appendix E “Effect of harmonic currents on balanced three-phase systems” contains requirements for the use of correction factors for harmonic currents in four-core and five-core cables with four current-carrying cores, as well as examples of the application of these factors. »

Overcurrent protection requirements

IEC 60364-4-43:2008 (Clause 431.2) [4] contains requirements to protect neutral conductors against overcurrent.

TT or TN systems.

Where the cross-sectional area of the neutral conductor is at least equivalent to that of the line conductors, and the current in the neutral is expected not to exceed the value in the line conductors, it is not necessary to provide overcurrent detection for the neutral conductor or a disconnecting device for that conductor.

Where the cross-sectional area of the neutral conductor is less than that of the line conductors, it is necessary to provide overcurrent detection for the neutral conductor, appropriate to the cross-sectional area of that conductor; this detection shall cause the disconnection of the line conductors, but not necessarily of the neutral conductor.

In both cases the neutral conductor shall be protected against short-circuit current.

NOTE. This protection may be achieved by the overcurrent protective devices in the line conductors. In that case it is not necessary to provide overcurrent protection for the neutral conductor or a disconnecting device for that conductor.

Where the current in the neutral conductor is expected to exceed the value in the line conductors, refer to 431.2.3 [4].

Except for disconnection the requirements for a neutral conductor apply to a PEN conductor.

IT systems.

Where the neutral conductor is distributed, it is necessary to provide overcurrent detection for the neutral conductor of every circuit. The overcurrent detection shall cause the disconnection of all the live conductors of the corresponding circuit, including the neutral conductor. This measure is not necessary if

  • the particular neutral conductor is effectively protected against overcurrent by a protective device placed on the supply side, for example at the origin of the installation, or if
  • the particular circuit is protected by a residual current operated protective device with a rated residual current not exceeding 0,20 times the current-carrying capacity of the corresponding neutral conductor. This device shall disconnect all the live conductors of the corresponding circuit, including the neutral conductor. The device shall have sufficient breaking capacity for all poles.

NOTE. In IT systems, it is strongly recommended that the neutral conductor should not be distributed.

Harmonic currents.

Overload detection shall be provided for the neutral conductor in a multi-phase circuit where the harmonic content of the line currents is such that the current in the neutral conductor is expected to exceed the current-carrying capacity of that conductor. The overload detection shall be compatible with the nature of the current through the neutral and shall cause the disconnection of the line conductors but not necessarily the neutral conductor. Where the neutral is disconnected, the requirements of 431.3 [4] apply.

NOTE. Further requirements regarding protection of neutral conductors are given in IEC 60364-5-52.

The requirements of IEC 60364-4-44:2015 444.4.7 [5] also prescribes that in TN systems, switching from one power supply to another is to be done by a switching device that switches both the line conductors and the neutral conductor, if present in the LV installation. Such switching, shown in Figure 44.R9A, prevents stray currents from generating electromagnetic fields in the main supply system of the LV installation. Since the sum of the currents in one cable must be zero, the switching ensures that neutral currents only flow in the neutral conductor of the circuit that is connected. The third harmonic (150 Hz) currents of the line conductors are added to the neutral conductor current with the same phase angle.

Three-phase alternative power supply with a 4-pole switch
Figure 44.R9A − Three-phase alternative power supply with a 4-pole switch

WARNING! In Figure 44.R9A , the PEN conductors of both power supplies are connected to the neutral conductor. However, IEC 60364-5-54:2011 requires that the PEN conductor be connected to the busbar or clamp that is used to connect the protective conductors.

Color identification and alphanumeric marking

A neutral conductor shall be identified by the colour BLUE. In order to avoid confusion with other colours it is recommended to use an unsaturated colour BLUE, often called “light blue”.

Where a neutral conductor is present, the colour BLUE shall not be used for identifying any other conductor. In the absence of a neutral conductor within the whole wiring system, the colour BLUE may be used for identifying a conductor with any other purpose, except as a protective conductor.

Bare conductors used as neutral conductors shall be either coloured by a BLUE stripe, 15 mm to 100 mm wide in each unit or enclosure and at each accessible position, or coloured BLUE throughout their length [Clause 6.2.2, 6].

NOTE 1. In IEC 60079-11, the colour BLUE is used for the marking by colour of terminals, terminal boxes, plugs and sockets of intrinsically-safe circuits.

NOTE 2. In Canada, the colour identification WHITE or NATURAL GREY for the neutral conductor is used as a replacement for the colour identification BLUE [Annex B, 6].

NOTE 3. In the Russian Federation, the BLUE colour should be used only for identification of the neutral conductors, the mid conductors and the earthed line conductors [Annex B, 6].

NOTE 4. In the United States, the colour identification WHITE or NATURAL GREY for the neutral conductor is used as a replacement for the colour identification BLUE [Annex B, 6].

NOTE 5. In the United States, the use of the colour BLUE is permitted for phase conductors. Neutral conductors are permitted to be WHITE, GREY or with three WHITE stripes on insulation other than GREEN [Annex B, 6].

NOTE 6. In the United States, identification of the terminal for the grounded conductor is by coloration. In the US identification of the terminal for connection of the grounded conductor is by a white colour, or by the word “white” or the letter “W” adjacent to the terminal [Annex B, 6].

The alphanumeric identification of a neutral conductor shall be “N” [Clause 7.3.2, 6].

Color and alphanumeric marking of conductors for three-phase electrical installations of buildings

The Neutral conductor is also referred to as the _____

grounded conductor. [In USA]

A Neutral Conductor Shall Be Placed within the _____

same conduit.

References

  1. IEC 60364-1
  2. Kharechko Y.V. Concise Terminological Dictionary of Low Voltage Electrical Installations. Part 1 // Appendix to the journal “Library of the safety engineer. – 2011. – № 3. – 160 c. Personal English translation by the author of this article.
  3. IEC 60364-5-52:2009
  4. IEC 60364-4-43:2008
  5. IEC 60364-4-44:2015
  6. IEC 60445:2021