Acquire a power generator with a known power output from an electronics shop. Make sure the generator has an output of 240 or 120 volts.
Divide the power rating in watts, and the quotient is the current load of the generator.
Connect a cable from the live terminal of the generator to the primary current input terminal of the transformer.
Connect another cable from the secondary current output of the transformer to the positive terminal of the ammeter.
Connect another cable from the ammeter's negative terminal to the neutral terminal of the generator to complete the circuit.
Determine the current transformer (CT) ratio, the magnitude of the current input in the transformer as compared to its output. This is usually provided in the manual handed to you during purchase of the transformer, such as 300:5. In this example, for every 300 amps of current input to the transformer, there is a 5 amp output.
Power on the transformer, and check the reading on the ammeter to see if the ratio of the current input on the transformer to the current output is equal to the CT ratio. This happens only if the transformer is ideal.
Calculate the difference between the two ratios, then divide it by the CT ratio from your manual.
Multiply this quotient by a hundred to expressed the difference as a percentage. The percentage you obtain is the transformer accuracy.