CT Opening Size vs. Conductor Diameter


- How large does a CT window opening need to be to fit a specific conductor diameter (gauge)?
- How large a CT window opening do I need to handle a circuit with a specific current (ampere) rating?

This page covers recommended current transformer (CT) sizes for different conductor wire gauge sizes. This applies to standard round conductors only. If you want to fit multiple conductors into a single CT, see Fitting multiple conductors into a CT for guidance on the appropriate number of conductors.


mm2 - Square millimeter.

The area of a copper conductor and an international standard wire gauge measurement.

AWG - American wire gauge

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American Wire Gauge

The AWG wire gauge standard specifies the diameter of a bare conductor; however, all current-carrying conductors except busbars are insulated, which adds thickness. When sizing a CT, you need to choose an opening size that will accommodate the insulated wire. Insulation thickness can vary, so the following table provides guidance, but it cannot always guarantee that a particular conductor will fit into a particular CT opening. If possible, use a CT with the smallest opening for the conductor you are monitoring to get the best accuracy.

International Wire Gauges


- In general, it is recommended that the CT hole be no larger than twice the wire diameter unless there is no other option.
- Information for compiling this table was taken from American wire gauge table and IEC international standard on insulated conductors and Circular mil.

American wire gauge
IEC 60228
Circular mil