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Home / Technical Articles / How to choose transformer rating?

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 –
100244141
160390225
250609352
315767444
400974563
5001218704
6301535887
80019491127
100024361408
125030451760
160038982253
200048722816
250060903520
315076734436

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

where:

  • 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

where

  • 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

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

10 Comments


  1. Ahmed Almayyahi
    Jan 13, 2019

    Hi,

    For three-phase Formula – transformer rating, is the voltage a line or a phase voltage ? , my understanding it should be line voltage if you multiply by root square of 3
    where:

    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


  2. Duncan Lance
    Oct 18, 2018

    Wow, I had no idea that there was so much math involved when it comes to choosing a transformer for your electrical needs. Although, I suppose that it does make sense since there is so much power involved. After all, you wouldn’t want to accidentally overload your transformer or anything like that.


  3. sirajuddin Hamdard
    May 28, 2018

    Hi Sir , hope you fine and healthy:
    Please kindly write the formula of (Average of actual load in(Amp))
    Rated Capacity of Tr (Amp)
    Percentage of Tr loading
    Over loading(%).
    i saw in an electrical artical the average of actual load(amp) is =(R+S+T/3)
    and Rated capacity (amp) is =(Total KVA/0.692) what is (0.692)
    percentage of TR loading is =(Rated capacity(amp))/average of actual load
    over loading(%) is= (percentage of loading-100)
    I confused with such formulas , please clear how much is correct that and where we use these Formula.
    I am waiting to your response or reply ,help me please.


  4. krishan
    Oct 12, 2016

    Dear admin,
    Thank you for the excellent consise articles you publish so frequently. I am currently employed as an project 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.

    Best regards,


  5. krishan
    Oct 12, 2016

    how to estimate electrical load in new constriction building plz help me


  6. Khaldoun
    Aug 24, 2015

    Dear Sir
    I have an air compressor powered by 120 v and I am going to use the compressor in my country where main power used in my country is 220 v , Would you please advise me what transformer rate can be used.
    for more technical specification of compressor I will supply you ASAP.

    Regards


  7. Ebukanepa
    Jan 26, 2015

    Dear admin,

    Thank you for the excellent consise articles you publish so frequently. I am currently employed as an project 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.

    Best regards,
    Ebukanepa
    Lagos, Nigeria


  8. ravicharan
    Sep 11, 2013

    NICE


  9. 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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