Answer / guest

I dont know of any formula to answer your question, but I
follow a trial method which worked out well for ME.First
know your full load current and the expected running
amperes which would be below your full load amperes.Now
choose your ampere meter which will show your running /full
load amperes at about 75% of the chosen amp meter scale.Now
you can select your CT .
In this case a 200 KW motor full load current is about 370
amps and for 75% scale reading would be 500 amps.So your CT
rating would be 500/5 and the amp meter also of the same
rating.

Answer / k.prakashchandra

Another method.Divide KW by 0.75 (lowest pf expected) to get
KVA. Calculate current which is about 380 A at 400 V line
voltage.You can choose a CT 0f either 400 or 500 A as
primary current.The secondary current of CT is decided by
meter and relays.This is generally either 1 or 5 A .

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