On electrical installation or maintenance sites, some electricians can instantly determine the required cable size based on the power, leaving many novice electricians in awe. Today, we’ll delve into the reason behind this.

Let’s examine the power calculation formula for a three-phase asynchronous motor:

P=3×U×I×cos⁡(ϕ)×ηP=3​×U×I×cos(ϕ)×η

In layman’s terms:

Power = Square root of 3 * Rated Voltage * Rated Current * Power Factor * Efficiency

From this, we can derive the formula for current:

I=P3×U×cos⁡(ϕ)×ηI=3​×U×cos(ϕ)×ηP​

Again, in layman’s terms:

Current = Power / (Square root of 3 * Rated Voltage * Power Factor * Efficiency)

Using this formula, we can establish the relationship between current and power. In the International System of Units (SI), power is measured in watts, not kilowatts. Therefore, to calculate the current for a 4 kW motor, the power should be 4000 W. The typical power factor is around 0.8, and the efficiency is usually 90%. After calculation, we find that the current is approximately twice the power.

Under a 380V three-phase power supply condition, the rated current is roughly twice the rated power. For example, a 4 kW motor would have a rated current of about 8A.

It’s important to remember that the current being twice the power only applies to three-phase, 380V AC asynchronous motors.