Small Transformers

The primary current, with open secondary, is the magnetizing current and should be small but mostly reactive (inductive). This current is design specific and depends on the input voltage, frequency, transformer core material, size, maximum flux density, primary number of turns, and winding geometry.

The resistive (lossy) component in the winding and the core material is what makes most idling transformers warm to the touch.

I would expect the primary magnetizing current (real+reactive) to be approximately 10%-15% of rated load for small transformers. Much lower on larger and commercial transformers.

Thank you. The purpose of my question was to determine, if I leave a small transformer permanently connected to the mains without a secondary load, if it will shorten the transformer life and increase the amount of electricity used, to any appreciable degree. A bell transformer circuit is a perfect example.