In daily life, we all know that the commonly used power supply voltage is 220V, while the voltage of many large machines in the factory workshop is 380V. Generally, two wires are 380V, and one wire is 220V. Why is this?

Electricity is a vector quantity. It has direction and magnitude. The three – phase coils in the alternator stator are symmetrically arranged at an electrical Angle of 120 degrees. Our daily electricity supply is through the power distribution transformer step-down power supply, the secondary side of the transformer is star connection, it has three live lines (respectively A, B and C three phase lines) and work at the neutral point to zero, a line is drawn out is the zero line.

We call the voltage of two different phases (live line) the line voltage, the voltage of the phase (live line) and the neutral line the phase voltage; From the vector diagram of line voltage, we can see that one live wire is 220V, and two live wires are 380V.

This should start from China’s national conditions, because China has stipulated that the voltage level of residential electricity should be 220/380V. Then why are two firewires 380V and one 220V? Let’s look at the following:

First, the source of electric energy

Electrical energy is usually generated by generators (of course, nowadays new energy sources such as: Solar power generation, wind power generation, etc.), and then through the power plant’s step-up transformer to the grid voltage level, after line transmission to the load center, and then through the step-down transformer will reduce the grid voltage to suitable for users to use the voltage level, that is, we often say 380V (the actual transformer secondary side voltage is 0.4kV, Because the line loss pressure drop should be considered), so 380V is produced. The whole production, voltage change and transportation process is shown in the following figure:

Two, the formation of 380V voltage

From the figure above, we can see that the transformer T3 is the user terminal transformer. This transformer reduces the primary voltage to the residential voltage level, and leads to the PEN line on the secondary side of the transformer, which is also known as the neutral line.

With this PEN line, three-phase and four-wire power supply can be formed, which is what we often see on the village pole (three phase lines and one PEN line). Then, why the four lines become three lines after each household, and why it is 220V? See the picture below:

The formation of 220V voltage we often say 380V refers to the line voltage between the three phase lines ABC (L1, L2, L3) on the figure, and the voltage (UA0, UB0, UC0) of each of them is 220V.

Since the power distribution of each household is designed according to single-phase power distribution (380V power distribution for rich people and large households), our voltage is all single-phase voltage 220V, which is the voltage of each phase (L1, L2 or L3 to PEN line) to PEN line in the figure above. (If the PEN is repeatedly grounded before entering the house, and N wire and PE wire are separated, then the electricity consumption in the house will become a TN-C-S system, so we will see that the original two wires become three wires after entering the house).