Answer / denzel101

Calculate the maximum current you want to measure from your
system. for example calculate the full load current of a
supply transformer. if the full load current is 200 Amps
and your meters have 1amp rating, choose the ratio of 200:1
CT.

Answer / pillay

There is no formula for this you just choose the correct size from what is available.
usually size the ammeter the size of the circuit.example the circuit breaker is 200 amps than the ammeter would be 200amps.The CT would be 200:5 or 200:1 whatever is available so long the ammeter is also of the same rating.

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