When electricians are conducting maintenance work, they often use three-phase power sources, with the most common being three-phase four-wire or three-phase five-wire systems.

Whether it's a four-wire or five-wire system, the neutral line is indispensable.

However, there's a question: when connecting loads, how do you calculate the current on the neutral line? Many electricians are unclear about this, mistakenly believing that there is no current flowing through the neutral line. In fact, many electricians have caused electric shock accidents as a result. So, how exactly should you calculate the current on the neutral line?

This article will use the simplest and most intuitive method to help novice electricians easily learn this technique!

The calculation methods for neutral line or midpoint current are similar, and maintenance electricians mainly deal with two situations: balanced and unbalanced three-phase loads.

I. Balanced Three-Phase Load Situation

Whether it's a three-phase four-wire or three-phase five-wire system, the three-phase power supply is balanced, with angles between each phase being 120°.

For three-phase loads, if the load is balanced, such as common three-phase motors, three-phase heating elements, three-phase air pumps, etc., these loads themselves are balanced.

In this case, the current on the neutral line is 0 amperes. It should be noted to novice electricians that current is a vector with magnitude and direction, so the vector sum of the current on the neutral line is 0 amperes. This situation is relatively simple and applicable to balanced three-phase loads.

In addition to the quick calculation method, we also have a standard calculation method: √(a² + b² + c² - ab - bc - ac).

Now, let's illustrate with a specific example so that novice electricians can easily grasp it.

For instance, if the currents in phases A, B, and C are all 10 amperes. According to the calculation formula, we can determine that the current on the neutral line is 0 amperes. This is the standard calculation method. Experienced electricians usually have an intuitive sense that the current on the neutral line is 0 amperes in the case of balanced three-phase loads, which is very practical!

II. Unbalanced Three-Phase Load Situation

Next, let's discuss the situation of unbalanced three-phase loads. For unbalanced three-phase loads, the current on the neutral line is not 0 amperes. So how should it be calculated? We have a simple and quick calculation method: √(a² + b² + c² - ab - bc - ac).

Now, let's illustrate with a specific example and explain how to use and calculate.

In the diagram, we can see that the three-phase load is unbalanced, with different currents. For example, the current in phase A is 10 amperes, in phase B is 20 amperes, and in phase C is 30 amperes. In this case, the current on the neutral line is not 0 amperes.

Let's look at the specific calculation method: current on the neutral line = √(a² + b² + c² - ab - bc - ac); substituting 10 amperes, 20 amperes, and 30 amperes into the formula for calculation, the result is approximately 17.3 amperes.

This is the method for calculating the current on the neutral line under the situation of unbalanced three-phase loads.

Novice electricians should not always assume that the current on the neutral line is 0 amperes, which is very important!

What are the causes of three-phase imbalance?

Three-phase imbalance refers to the situation where the amplitude or phase difference of three-phase voltages or currents in a three-phase power system are not equal. The causes of three-phase imbalance can be multifaceted, including:

1. Load imbalance: When the loads in the system are unevenly distributed among the phases, it will lead to current imbalance, thus causing three-phase imbalance. For example, if a larger load is connected to one phase while smaller loads are connected to the other phases, it will cause current imbalance.

2. Power supply imbalance: If the system power supply is unbalanced, for example, due to faults in the power supply transformer or uneven configuration of the power supply system, it will lead to three-phase voltage imbalance.

3. Line imbalance: Imbalance in the lines themselves can also cause three-phase imbalance. This may be due to reasons such as mismatched cable impedances, poor contacts, or line damage.

4. Equipment faults: Some equipment faults or damages may cause three-phase imbalance, such as a fault in one phase of the transformer or internal faults in electric motors.

5. Changes in operating conditions: During the operation of the system, changes in load, system adjustments, or other external factors may cause three-phase imbalance.