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Sometimes choosing the correct capacitors for your electrical equipment can be quite challenging. However, this article will help you perform this task easily.

Firstly, in order to correctly choose capacitors for low voltage applications, you must take into consideration the following variables:

  • the ambient temperature;
  • the expected over-current related to voltage disturbances, including the maximum sustained over-voltage;
  • the requested life expectancy;
  • the maximum number of switchings during the year;
  • the level of harmonics in the network.

As you can see, operating conditions can strongly influence the life expectancy of the capacitors. Consequently, all the power factor correction components must be chosen accordingly.

Secondly, you need to calculate the total power of Non-Linear Loads and finally the THDu and THDi levels.

Harmonics and capacitors

Capacitors are very sensitive to non-linear loads and in particular to voltage and current harmonics.

The presence of harmonics in electrical systems implies that current and voltage are distorted. Therefore, they deviate from sinusoidal waveforms.

This phenomenon is very dangerous for capacitors. Their impedance decreases proportionally to the present order of harmonics. In other words, a high order of harmonics leads to the overload of capacitors.

Non-linear loads

Harmonics are caused by non-linear loads. As a result, an indicator of the magnitude of harmonics is the proportion between the total power of non-linear loads and the power supply transformer rating.

In order to calculate the level of Non-Linear Loads, please use the following formula:

equation for determining non-linear loads (total power of non-linear loads/ installed transformer rating)

Example of calculations of Non-linear loads:

Installed transformer rating and reactive power schema


Installed transformer rating: 500 kVA

Total power of non-linear loads: 115 kVA

Non-linear loads = (115 / 500 ) * 100 = 23%

Measurement of Total Harmonic Distortion THDi and THDu

As previously stated, in order to have a more detailed estimation of the magnitude of harmonics, you need to measure the THDi (current harmonic distortion) and the THDu (voltage harmonic distortion) at the transformer secondary, but make sure that when you are performing this step the capacitors are not connected.

Please check the following diagram:

calculation scheme of non-linear loads

Finally, once we obtained the THDu and THDi values, you can choose the suitable capacitors.

Selection of capacitors according to THDu (%) and THDi (%) levels

Below you can check the maximum THDu and THDi values for our Gruppo Energia capacitors.

How to choose the correct capacitor according to the THDu and THDi levels.

ATTENTION: The capacitor type should be selected according to the most restrictive measurement.


If your measurement is giving the following results:

  • THDi = 10%
  • THDu = 4%

….you must select Intact ALLPOWER capacitors.

If your THDu and THDi values are higher than those indicated in the above diagram, the installation of a detuned harmonic reactor is mandatory. Kindly check the example below:

  • THDi = 15%
  • THDu = 5%

As you can see, in this case the THDi measurement is higher than the maximum value handled by capacitors. Therefore, it is necessary to install a harmonic reactor with a combined capacitor.


Non-linear loads are an important parameter to take into account when selecting capacitors. However, this parameter is not sufficient since harmonics in grid may also cause current amplification. Current amplification can only be detected by an in-depth analysis of the grid.

Check out Gruppo Energia power measurement instruments, such as the Multifunctional Network Meter DPA007L2K.

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