Why is impedance important when specifying a transformer?

Impedance is a very important specification. First, it affects the maximum short circuit current a transformer can provide; the higher the impedance, the lower the maximum short circuit current. Secondly, it also affects the transformer's voltage regulation; the higher the impedance the lower the voltage regulation over the transformer's load range. One common misconception; there is no direct correlation between impedance and efficiency. Manufacturers often list a range of impedances their transformers will meet, customers can also specify a specific impedance although this may impact cost.