CT Ratio Selection Guide for Power Measurement

How to think about current transformer ratio, accuracy and measurement range.

The CT ratio links primary current to the smaller secondary current used by a meter or protection device. Selecting only by the largest expected load can reduce accuracy at normal operating current.

Start from normal current

List normal, peak and fault-related conditions separately. The meter should measure the normal operating range with useful resolution while the CT and circuit remain within their ratings.

Match the meter setting

The CT primary and secondary values must agree with the meter configuration. A mismatch creates a consistent scaling error that can affect energy reports and alarm thresholds.

Consider burden and wiring

Long secondary wiring, terminals and connected devices add burden. Confirm the CT burden and accuracy class for the intended circuit, and never leave a current-transformer secondary open while the primary is energized.

Plan maintenance access

Label the ratio and circuit clearly. Provide a safe test or shorting arrangement where required by the equipment design and local practice.