Current Transformer

Watts the matter with that?

CTs and VTs and power transformers are all rated in Volts x Amps because :-

1. The more amps you need (without most of the power disappearing in the resistance of the transformer windings) the thicker the wire you need [at least in proportion] and the more weight of conductor.
2. The more volts you need (for the same number of turns) the more magnetic core area you need (because, for the same core material, the maximum flux density, flux per unit core cross section, is limited by saturation - so the only way to get more flux through the winding is to increase the area). That means a bigger core with more material, in proportion to the volt increase. N.B. Instantaneous voltage depends on rate of change of flux - if you have defined a certain frequency and waveform, the only way to increase voltage is to increase maximum flux.
   
3. Since the weight of winding and core materials are proportionally (or worse) increased, size and material cost go in proportion to VA rating.
   

Current transformer is a device that transfer the power from one circuit to another circuit and deals in apparent power. VA, or volt-amperes, is a measure of the output of the transformer. Active and reactive power depends on the power factor and may change with change in the power factor. But Apparent power always independent of power factor. The VA remains constant irregardless of the load characteristics, and so is a much better indicator of transformer performance. This is the reason why, a current transformer has to be rated apparent power (VA).
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