When an installation is to be supplied directly from a MV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking into account the following considerations:

  • The possibility of improving the power factor of the installation (see chapter L)
  • Anticipated extensions to the installation
  • Installation constraints (e.g. temperature)
  • Standard transformer ratings
Apparent power [kVA]In (A)
- 237V -
- 410V -

Figure. 1 : Standard apparent powers for MV/LV transformers and related nominal output currents

3-phase transformer

The nominal full-load current In on the LV side of a 3-phase transformer is given by:

Formula - transformer rating


  • Pa = kVA rating of the transformer
  • U = phase-to-phase voltage at no-load in volts (237 V or 410 V)
  • In is in amperes

Single-phase transformer

For a single-phase transformer:

Formula2 - transformer rating


  • V = voltage between LV terminals at no-load (in volts)

Simplified equation for 400 V (3-phase load)

  • In = kVA x 1.4

The IEC standard for power transformers is IEC 60076.

SOURCE: Schneider Electric


Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.


  1. ravicharan
    Sep 11, 2013


  2. [...] sea level, seismic conditions, and loads with total harmonic distortion above 0.05 per unit.Learn: How to choose transformer rating?Reference: Electric Power Transformer Engieneering – Leo L. Grigsby (Get this book from [...]

  3. [...] and placed upright upon completion.As mentioned previously, the type of winding depends on the transformer rating as well as the core construction. Several of the more common winding types are discussed [...]

  4. [...] that has a step-up transformer increases the voltage with decreasing current, while a step-down transformer decreases the voltage with increasing the current for domestic and commercial distribution. The [...]

  5. shalendra
    Oct 07, 2011

    Hi Edvard

    Thank you for the excellent consise articles you publish so frequently. I am currently employed as an electrical consulting engineer so most of your topics are relevant to the everyday information we deal with. I would really appreciate it if you could forward me any nice literature pertaining to good building services design, power sub stations and residential distribution applications.

    Looking forward to your next article.

    Kind Regards
    Shalendra Singh
    Durban, South Africa

    • Vishnu
      Feb 02, 2013

      Shalendra , good technical guides on design are available for free download at schneider website.

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