CT sizing

[Arevaengineer]

What are the standards or criteria for selection of CT primary rating for a motor feeder? If motor full load current is 40A, can I use a 40/1 CT OR the CT has to be rated higher say 50/1 or 60/1?? Let me know the guidelines please. If 40/1 CT cannot or should not be used, what are the disadvantages?

 

[Marmite]

You haven't given enough details to get a detailed response. What protection functions are you providing? Starting current? Make/type of relay etc etc In my view you need to consult the manual for the particular relay you are using for guidance on selecting appropriate CT's. The CT primary rating and the ratio are not one and the same thing, so be careful when you specify CT's. The terms are not interchangeable.
Regards
Marmite

 

[tem1234]

For motor, our standard is that the ct nominal primary current is 3/2 of the motor FLA. Schweitzer have good discussion on their website about this.

Usually, motor are protected by contactor and by fuse, so the relay doesn't have to work under large short-circuit current. If it does, more precaution should be taken for saturation of CT.

If you choose a CT with the same nominal current as the motor, the ct will heat more. You should check the ct datasheet to validate it's ok.