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How to choose transformer rating?

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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

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on