Current transformers, or CTs, are transformers used to step down the current level of high-power transmission systems to measure or monitor the current levels. CTs are required because standard current measuring devices are not configured to measure high-current levels. As a consequence, engineers use CTs to step down the current levels to measure it. Once measured, they use the current transformer ratio, or CT ratio, to calculate the actual current from the measured current.

Find the number of wire turns on the primary side of the transformer. Refer to the circuit diagram or schematic of the CT transformer. As an example, assume you have 15 turns on the primary.

Find the number of wire turns on the secondary side of the transformer. Refer to the circuit diagram of the CT transformer. As an example, assume you have 75 turns on the secondary.

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Determine the voltage ratio. The voltage ratio is proportional to the ratio between the primary and secondary turn. In our example, the voltage ratio is 15:75 or, dividing through by 15, 1:5

Calculate the CT ratio. The CT ratio is the inverse of the voltage ratio. In this example, the voltage ratio is 1:5, so the CT ratio is 5:1. This means the current level is stepped down 5 times where, if the primary current is 200 amps, the CT output is 40 amps.