ANSI/IEEE C57.12.80 defines an Instrument Transformer as "A transformer which is intended to reproduce in its secondary circuit, in a definite and known proportion, the current or voltage of its primary circuit, with the phase relations and waveform substantially preserved".
In other words, Current and Voltage meassurement transformers are Instrument Transformers.
So, rukmal's question is hard to answer. For low voltage and current transformers I dare to give yo some hints:
For voltage transformers it's useful to begin overdimensioning it with a rated voltage 50 to 100% higher and a rated current 5 or 10 times higher. This first attemp should be prototype tested and according to the objective accuracy some corrections may be implemented (increase of secondary number of turns, for instance).
Current voltage design is much more complicated. Standard EI cores are not advisable at all for such purpose. Special iron is to be used performing very high magnetic properties (permeance) and very low iron losses. If accuracy is not to be very high, same advices as for VT are to be considered. Julian
