# Mains power transformer ratings

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.

• It's all down to heat control. Apr 1, 2019 at 12:04
• 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. Apr 1, 2019 at 15:27
• Thanks for the help guys. I have commented under Charles’s answer with a question if you wish to get involved again. Apr 1, 2019 at 21:05