Tips for Choosing the Right Current Transformer

Unlike a typical transformer (which raises or lowers voltages) a current transformer can monitor and change current in a given circuit. However, it is important to choose the right rating (such as CT class 5p20) for your device. Here is information to help you select the right transformer for your needs.

Current Ratio

CT ratio is measured between primary and secondary current. Primary current is called Ip while secondary current is referred to as Is. For primary current, you can choose between 1 and 7.5 amps. For secondary current, you have two choices, 1 or 5 amp. For relays a distance away, 1 amp is a good choice, because of voltage drop. However, 5 amp is still common, especially in circuits with large relays.

Transformer Accuracy Class

Accuracy class refers to the error potential in the current from primary to secondary. Accuracy depends on factors like load, burden, temperature, frequency, and saturation.


If you have a CT class 5p20 , the “5” refers to the level of accuracy. “P” means it is a protective transformer. “20” is about accuracy limit factor. A protective CT may need to sense very high fault currents, and a “20” means it can handle 20 times the current flow in the secondary.

CT Types

You can choose from three main types of current transformers

  • Wound – The secondary windings are dependent upon the transformer turns ratio.
  • Bar – Primary windings are not coils of wire but a single bus bar which can be the main circuit bus bar also.
  • Toroidal – no primary coil in this doughnut shaped transformer. Split core toroidal CTs can be disconnected and connected without physically removing them from the circuit.

When you need a CT class 5p20 or other type, talk to your CT specialists. They are there to simplify the process of CT selection.

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