Calculating transformer winding impedance

[THX-1138]

Suppose I want to run a normal low-voltage transformer in reverse -- i.e., running the line voltage through the secondary winding of a filament transformer. The load will generally be negligible, but maybe not. How can I determine whether the transformer can handle this? From the gauge and secondary rating I can make a guess how much current the winding is intended to handle, but I don't know if Ohm's law applies here.

 

Example: For a 120VAC transformer with a secondary winding rated at 28VAC/2.8A, is the winding's impedance 10[math]\Omega[/math]?

 

Thanks!