Enter the primary and secondary currents of the transformer into the calculator to determine the CT ratio. This calculator can also evaluate the primary or secondary currents given the other two variables are known.

CT Ratio Formula

The following equation is used to calculate the CT Ratio.

CT = PC / SC
  • Where CT is the current transformer ratio
  • PC is the primary current or capacitance
  • SC is the secondary current or capacitance

To calculate the CT ratio, simply divide the primary transformer current by the secondary transformer current.

What is a CT Ratio?


A CT ratio measures the ratio between the primary and secondary currents OR capacitances of a current transformer.

A current transformer is a device that measures alternating currents (AC) using a magnetic core. A current flows through the primary winding, producing a magnetic flux that induces a voltage in the secondary winding. A CT is usually placed between the source and load to protect devices in the circuit from large transients due to switching or fault conditions.

These devices are used to measure AC currents of all waveforms (including pulsed currents), but they can also be found in switch mode power supply circuits.* They are used when the CT is not physically located close to the power source, and there is concern about measuring DC currents accurately.*

A core for a current transformer consists of many thin layers of steel lamination stacked together to make a single unit. The number of these laminations determines what range of current will be measured and whether the device will be low resistance or high.

The best CTs have as few laminations as possible, with short wire lengths between them; these reduce both leakage inductance and hysteresis effects, which are significant sources of error in CTs.