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i assuming you are asking about the current transformer,
Factors are:
1- load current rating to determine the rated primary current of the CT.
2- the metering or the protection sys. used (1A or5A) to determine the secoundry of the CT.
3-Rated burden of the CT this can be detemined by CT calculation
4-Accurcy class needed
5-aplication (Metering or protection)
6-withstand short circuit current
7-Rated voltage
8- Knee point voltage for saturation