My question is about load currents from AC transformers.

If for example, we take a 3 phase step down transformer (11kV to 415V) with an apparent power rating of 100 kVA, the following equation, S = 1.73 x V x I, rearranged to give I = S / (1.73 x V) means this transformer is ‘rated’ to supply 139 amps per phase.

My questions are:

  1. At what secondary current do we say the transformer is overloaded? Can a transformer safely supply this full load rated current constantly without causing damage or should the secondary current only be a certain percentage of it’s rated current?

  2. What will happen when the transformer supplies more than 139 amps on the secondary? Will the secondary voltage start to drop below the rated value due to excessive voltage drop over the internal impedance of the windings? Or what does happen?

  3. When sizing a transformer, how much larger should the secondary current be compared to the designed full load current that will be drawn under normal conditions?

Thanks everyone.

  • 1
    \$\begingroup\$ It's all down to heat control. \$\endgroup\$
    – Andy aka
    Apr 1, 2019 at 12:04
  • 1
    \$\begingroup\$ The ambient temperature will make a big difference in deciding whether or not the transformer can sustain short-term overloads. Or even long-term ones. \$\endgroup\$
    – Simon B
    Apr 1, 2019 at 15:27
  • \$\begingroup\$ Thanks for the help guys. I have commented under Charles’s answer with a question if you wish to get involved again. \$\endgroup\$
    – David777
    Apr 1, 2019 at 21:05

1 Answer 1


Rated kVA and current are the values that can be delivered continuously without reducing the expected life of the transformer under rated operating conditions. Rated operating conditions would include the rated primary voltage, a maximum ambient temperature, a maximum altitude / minimum barometric pressure and a humidity specification. There would also be specifications about maintaining adequate cooling and condition of oil for an oil-filled transformer.

Any increase in current above the rated current would shorten the life of the transformer winding insulation according to a curve that is used for all electromagnetic devices.

The output voltage of a transformer is reduced with increased current by the effect of the transformer impedance. The output voltage will be a little above the rated voltage with no output current and reduced inversely proportionally to any current increase. The actual output voltage can be calculated form the transformer and load characteristics.

A transformer should be sized for the actual load current plus any anticipated increase or uncertainty in the load current determination. The size could also be increased to compensate fo any uncertainty about adequate cooling.

  • \$\begingroup\$ Thanks Charles for your response. I have studied transformers but this post always was a question for me. Are these parameters like temperature etc something that could be found on the transformer nameplate? How are these parameters factored into transformer size calculations or is it a matter of individual opinion to an extent? \$\endgroup\$
    – David777
    Apr 1, 2019 at 21:02
  • 1
    \$\begingroup\$ For a 3-phase distribution transformer, I would expect to find the ambient temperature rating marked on the nameplate and probably the altitude also. The transformer impedance is likely to be stated as a percent. Any information that is not on the nameplate is likely to be in documentation shipped with the transformer or available from the manufacturer. \$\endgroup\$
    – user80875
    Apr 1, 2019 at 21:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.