The nominal secondary voltage is defined according to ABNT  as “the voltage appearing at the terminals of a nominal load imposed on the CT at 20 times the rated secondary current, without the ratio error exceeding the specified value.” This means that the CT must withstand a maximum current in its secondary circuit proportional to 20 times its rated current without conducting errors higher than the one specified by its accuracy class.
The rated secondary voltage represents the maximum voltage that the CT must withstand under overcurrent conditions. The value 20 is set by the ABNT as the standardized value for the overcurrent factor. Only the protective duty CTs reach rated secondary voltage. In measuring CTs, the core saturates well before the secondary current reaches this value.
The ratio error depends on the impedance connected to the secondary circuit. The calculation of the total impedance of the circuit for practical purposes of specifying the
TC uses the nominal nominal loads as reference.
However, the CTs for protection service also have a consideration in calculating the impedance of the secondary circuit. ABNT subdivides these CTs into two classes for their impedance.