Higher Rating of CTs

If they are actually metering grade CTs, then they should be most accurate in the lower end of their range. Your current of 30A is close to 25%, so that is good.

The CT manufacturer's specifications and factory test report should give you all you need to be assured of their accuracy. However, I don't know of any good way to make "adjustment" in the field for variation in CT accuracy. The critical item is calibration of your energy meter. It is more often the source of greater error.

Dear friend,

First tell me what is accuracy class of this CT ?

If it is Cl. 0.5 for metering, % error will be +/-0.75% at 20% load instead of +/- 0.5% error at 50 to 100% load. you should choose CT 120% of rated full load current.

Sorry...I have not mentioned Accuracy class...it is 0.2FS5;

Further, I wish to know whether, there is any curve/graph which shows the accuracy at different currents. What is 'Bell Curve' in this context?

Kindly write class according to core of CT and VA rating.

According to Cl 0.2,

at 20 % load of 150=30A

% error will be +/-0.35. and if load is goes to 5% then % error will be +/-0.5 instead of 0.2.

i dont know what you meant by 0.2FS

but if it is 0.2S class CT, then it's magnetic core is made up of mumetal instead of crgo.

therefore, at lower currents also, it will give + or - 0.2% error.

rest assured your CT is working fine and within error limits