Measure the inductance of the multi-turn winding on the CT, lets assume it is 1H, when in circuit measure the voltages on the multi-turn winding, lets assume a square wave of +/-1V, and the frequency is 50kHz say (10uS each way) di=V.dt/L, So di in this example = 1.10uS/1.0 = 10uA peak to peak, or 5uA peak each way. For a more complex volt waveform you need to integrate the volt seconds applied over a half cycle to get V.dt (easy for a square wave). The magnetising current as seen by the high current winding is calculated by multiplying the above result by the turns ratio, e.g. a 200 turn CT would give 1mA pk each way on the high current winding in this example, if the current being measured is 2 amp peak say, the error is then 0.05%, Regards, Orson Cart.