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A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary.

Current transformers are commonly used in métering and protective relaying to facilitate the méasurement of large currents and isolation of high voltage systems which would be difficult to méasure more directly.

Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated conductor (copper or aluminum are typical in electric utility applications) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers (CTs) are used extensively in the electrical power industry for monitoring of the power grid.  The CT is described by its current ratio from primary to secondary.  Common secondaries are 1 or 5 amperes.  The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. Typically, the secondary connection points are labeled as X1, X2 and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications. Often, multiple CTs will be installed as a "stack" for various uses (for example, protection devices and revenue métering may use separate CTs). For a three-stacked CT application, the secondary winding connection points are typically labeled Xn, Yn, Zn.

Specially constructed wideband current transformers are also used (usually with an oscilloscope) to méasure waveforms of high frequency or pulsed currents. One type of specially constructed wideband transformer provides a voltage output that is proportional to the méasured current. Another type (called a Rogowski coil) requires an external integrator in order to provide a voltage output that is proportional to the méasured current.

Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary.