Why Does Single-Phase Require a Neutral Conductor but not in Three-Phase?

Do you think about why single-phase electricity requires a neutral conductor while three-phase electricity does not?

In a single-phase electrical system, power is transmitted using two conductors: the live conductor (carrying current) and the neutral conductor (completing the circuit). The neutral conductor is necessary to complete the circuit and provide a return path for the current back to the source.

In contrast, three-phase electricity utilizes three conductors, each carrying alternating currents with a phase difference of 120 degrees between them. In a balanced three-phase system, the sum of the currents at any given time is zero, eliminating the need for a neutral conductor. The phases work together in a way that the return paths for the currents are provided by the other phases, negating the necessity of a separate neutral wire for balancing purposes.

However, in some cases, especially in systems where there might be an imbalance in loads or in scenarios where a neutral point is required for specific applications, a neutral conductor might still be present in a three-phase system.

What is the difference between neutral and earth?

A neutral is a piece of conductor that is used to complete the electrical circuit or path. Sometimes the neutral is also connected with the earth or ground conductor, but the purpose of a neutral conductor is to complete the electrical circuit. Earth or Ground is also a path that can complete the electrical path or circuit, but that is not a good path.

An earthed conductor is a piece of conductor that is used to ensure the electrical shock or safe the equipment from short circuits. Earth conductors complete an emergency circuit to drain out the unexpected current to the ground.

To know the details about neutral and earthing and their uses and clarification, you may read another article- the difference between Earth and Neutral.

Why High Voltage Do Not Use Neutral Conductor?

In electrical systems, high voltage transmission lines often don’t utilize a neutral conductor due to a few reasons:

1. System Design: High voltage transmission lines are usually designed as three-phase systems. These systems consist of three conductors carrying alternating current (AC) that are arranged in a way to generate a balanced load. The need for a neutral conductor depends on the type of system and its application. In three-phase systems, the balance of power among phases often obviates the need for a separate neutral conductor.
2. Balanced Loads: In these systems, the loads are often balanced among the three phases, which means the sum of currents at any given time is ideally close to zero. This balanced nature reduces the necessity for a separate neutral conductor, as any residual unbalance can be managed without it.
3. Cost and Efficiency: High-voltage transmission lines are designed with a focus on efficiency and cost-effectiveness. Eliminating the neutral conductor reduces material usage and lowers costs associated with the installation and maintenance of the lines.
4. Safety: High-voltage transmission lines are built with safety in mind. Not having a neutral conductor simplifies the design and reduces potential points of failure. Additionally, high-voltage systems are typically more prone to insulation issues, and having fewer conductors can mitigate these risks.

How Do Calculate Neutral Conductors?

If A, B, and C are the three-phase currents, the formula to find the neutral current is the square root of the following: 

(A² + B² + C² – AB – AC – BC).

The size of the neutral conductor is at least equal to 16 mm2 in copper or 25 mm2 in aluminum.

When harmonics are between 15 % to 33 % the neutral conductor size as per the Standard IEC 60364-5-52  shall be the same size as the active conductor.

The neutral conductor must be sized to carry the maximum unbalanced current in the circuit.

What Color is Used for Phase, Neutral, and Ground Cable?

Cable color is not fixed for all regions or countries. The cable or the wiring color code varies from country to country. We will show the details of the following cable color codes:

    - International Wiring Colours

    - European Wire Colour Standards

    - Old & New UK Wiring Colours

Cable Color Code

If you are interested in knowing more about Standard Colour Code for Electrical Power Cables and Wiring, can visit the article that discussed details about color and standard.