There are different standards, which define medium voltage levels:
- IEC 60038 (International Electrotechnical Commission)
- IEEE (Institute of Electrical and Electronics Engineers)
- NEMA (National Electrical Manufacturers Association)
Standardization of system voltages has not been achieved worldwide. Different countries and regions have different voltage levels, but IEC voltage standard is approaching universal acceptance, so it is worth to focus on this standard.
IEC 60038 – voltage classification
IEC 60038 standard voltages are within 1 kV – 35 kV range. There are two series defined in the IEC standard. The series number one is within 3.3 kV – 35 kV range and second is from 4.16 kV to 34.5 kV. Only one series is recommended to be used. The below table shows AC standard voltages based on IEC 60038 ed. 7.0 2009.
|Series I||Series II|
|Highest voltage for equipment (kV)||Nominal system voltage (kV)||Highest voltage for equipment (kV)||Nominal system voltage (kV)|
[a] Unless otherwise indicated, these are generally three-wire systems. The values are phase-to-phase voltages. The values specified in parentheses shall be treated as non-preferred values. Recommendation: these values should not be used for newly built systems in future.
[b] These values shall not be used for new public distribution systems.
[c] These are four-wire systems and the values are phase-to-phase voltages. The phase-to-neutral voltage is equal to the indicated value divided by 1.73. [d] The unification of these values is under consideration.
[e] The values of 22.9 kV for nominal voltage and 24.2 kV or 25.8 kV for highest voltage for equipment are also used in some countries.
Note 1: It is recommended that in any one country the ratio between two adjacent nominal voltages should be not less than two.
Note 2: In the Series no. 1, the highest voltage and the lowest voltage do not differ by more than approximately ±10 % from the nominal voltage of the system. In a normal system of Series no. 2, the highest voltage does not differ by more then +5 % and the lowest voltage by more than -10 % from the nominal voltage of the system.