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 current given the other two variables are known.
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CT Ratio Formula
The following equation is used to calculate the CT ratio.
CT = I_P / I_S
- Where CT is the current transformer ratio (dimensionless)
- IP is the primary current
- IS is the secondary current
To calculate the CT ratio, divide the primary current by the secondary current.
What is a CT Ratio?
Definition:
A CT ratio is the ratio of a current transformer’s primary current to its secondary current (often expressed as a ratio like 600:5, or as a single number such as 120).
A current transformer (CT) is a device that provides an isolated, scaled copy of an AC current using magnetic coupling through a core. Current in the primary conductor produces a changing magnetic flux in the core, which induces a voltage in the secondary winding and causes a proportional secondary current to flow through a connected burden (metering device or protective relay).
CTs are commonly used for metering and protective relaying, helping instrumentation and relays measure large primary currents safely. Conventional CTs require changing current (AC or non-sinusoidal waveforms) and do not measure steady DC current; for DC current measurement, devices such as Hall-effect sensors are typically used instead.
CT cores are often made from laminated silicon steel (for power-frequency applications) or from materials such as ferrite (for higher-frequency sensing). Using thin laminations in steel cores helps reduce eddy-current losses. Accuracy and usable current range depend mainly on the turns ratio, core material and cross-sectional area, the connected burden, and avoiding core saturation.
Good CT designs minimize measurement error by using appropriate low-loss core materials, adequate core sizing to prevent saturation at the intended current levels, and proper winding/burden selection to reduce ratio and phase-angle errors.
