Current Transformer Ratio

What is this formula?

The current transformer ratio expresses the relationship between the primary current and the secondary current of a current transformer (CT).

When to use it

Use this formula when selecting current transformers, configuring protection relays, calibrating metering systems, and converting measured secondary current to actual primary current.

Example

A current transformer has:

Primary current = 400 A

Secondary current = 5 A

Formula:

CTR = Ip / Is

Substitution:

CTR = 400 / 5

Result:

CTR = 80

The transformer ratio is 80:1, commonly written as 400/5.

Applications

Protective relaying

Power system metering

Substation instrumentation

Switchgear monitoring

Current measurement systems

Note

Most protection and metering CTs use standardized secondary currents such as 5 A or 1 A. Actual performance depends on burden, accuracy class, saturation characteristics, and operating conditions. CT ratios are commonly specified in the form 100/5, 200/5, 400/5, etc.