The air coil superconducting fault current limiter consists of two major parts: A primary copper winding and a secondary superconducting winding, made of short-circuited (RE)BCO tapes. Both windings are inductively coupled solenoids. The presented design method enables retrofitting existing, conventional shunt reactors with a secondary superconducting winding. The advantage of the retrofit is a low impedance during normal operation. This lowered impedance depends on different parameters, such as the critical current of the used superconducting tape or the distance between the windings. A calculation example shows that the impedance of a conventional shunt reactor can be lowered from 6% to 2.4% during normal operation and remains roughly the same during fault current limiting.