CT Ratio Test Calculator

A CT ratio test verifies that the current transformer produces the expected secondary-current relationship for the selected nameplate tap. The calculator compares the Expected CT ratio derived from the nameplate values with the Measured CT ratio derived from the test-current readings, then reports the percentage deviation.

The result supports commissioning, maintenance, acceptance testing, protective-relay verification, revenue or submetering checks, and troubleshooting of current-transformer circuits. A ratio error can cause a relay, meter, power monitor, or control system to interpret primary current incorrectly even when the conductor, feeder, branch circuit, and connected load are operating normally.

For a 600:5 CT, the expected transformation ratio is 120:1. When 120 A is applied or represented on the primary basis, a correctly ratioed CT should produce 1 A on the secondary basis. The calculator reports both values in multiplier form, such as 120 x, and calculates the difference as a percentage of the expected ratio.

Nameplate and Test Current Basis

The selected CT connection must be identified before comparing ratios. Multi-ratio CTs may have different terminals, taps, or winding arrangements that produce different nameplate ratios. A test performed on one tap cannot be evaluated against the nameplate current values for another tap.

Input	Purpose
CT tap or terminal label	Records the selected nameplate tap or terminal pair used for the test.
Nameplate primary current (A)	Primary current corresponding to the selected CT ratio or tap.
Nameplate secondary current (A)	Rated secondary current corresponding to that same selected ratio or tap.
Measured or injected primary current (A)	Primary-side current used as the test basis.
Measured secondary current (A)	Secondary current measured on the same test basis.
Entered reference deviation (%)	Optional deviation reference from the manufacturer, project specification, or test procedure.
Test basis or instrument note	Documents the test procedure and instrument details separately from the ratio calculation.

The nameplate values establish the expected ratio. The measured or injected current readings establish the field-test ratio. Both current readings must represent the same test condition, whether the test uses primary injection, controlled load current, or another approved ratio-test method.

Do not combine an injected primary-current value from one condition with a secondary-current measurement from another condition. That produces a mathematically valid number but not a valid CT ratio test result.

Ratio Calculation

The calculator uses the selected nameplate currents to determine the expected transformation ratio:

\(\displaystyle \text{Expected CT ratio} = \frac{\text{Nameplate primary current}}{\text{Nameplate secondary current}}\)

It then calculates the field-measured transformation ratio:

\(\displaystyle \text{Measured CT ratio} = \frac{\text{Measured or injected primary current}}{\text{Measured secondary current}}\)

The signed ratio deviation indicates whether the measured ratio is above or below the expected ratio:

\(\displaystyle \text{Signed ratio deviation} = \frac{\text{Measured CT ratio} - \text{Expected CT ratio}} {\text{Expected CT ratio}} \times 100\)

The absolute ratio deviation removes direction and reports the size of the ratio error:

\(\displaystyle \text{Absolute ratio deviation} = \left| \text{Signed ratio deviation} \right|\)

When an Entered reference deviation (%) is supplied, the calculator also reports:

\(\displaystyle \text{Margin vs entered reference} = \text{Entered reference deviation} - \text{Absolute ratio deviation}\)

A positive margin means the calculated absolute deviation is below the entered reference. A negative margin means it exceeds that reference. An entered value of 0% leaves the comparison unassigned; it does not establish that a zero-tolerance acceptance criterion applies.

Calculation Example

For an early CT test record, apply this measured-ratio workflow. Enter the selected tap and the same test basis for both current readings.

Field	Entered value
CT tap or terminal label	Nameplate tap
Nameplate primary current (A)	600
Nameplate secondary current (A)	5
Measured or injected primary current (A)	120
Measured secondary current (A)	1
Entered reference deviation (%)	0
Test basis or instrument note	Record procedure and instrument separately

Expected ratio:

\(\displaystyle \frac{600\text{ A}}{5\text{ A}} = 120\)

Measured ratio:

\(\displaystyle \frac{120\text{ A}}{1\text{ A}} = 120\)

Signed ratio deviation:

\(\displaystyle \frac{120 - 120}{120}\times100 = 0\%\)

The calculated results are:

Result	Value
Expected CT ratio	120 x
Measured CT ratio	120 x
Signed ratio deviation	0%
Absolute ratio deviation	0%
Margin vs entered reference	0%

The test readings reproduce the selected 600:5 nameplate ratio on the stated current basis.

Interpreting Ratio Deviation

A positive Signed ratio deviation means the measured ratio is higher than the expected ratio. For a CT circuit, that condition means the measured secondary current is lower than expected for the stated primary current.

A negative signed deviation means the measured ratio is lower than the expected ratio. That means the measured secondary current is higher than expected for the stated primary current.

The Absolute ratio deviation is generally the comparison value used against a project or manufacturer reference because it shows the magnitude of error without regard to direction. The signed value remains useful for diagnosing the condition and identifying whether the discrepancy is associated with the applied-current basis, the measured secondary current, the selected tap, or test setup.

Field Verification Limits

The calculator evaluates the entered ratio relationship only. Acceptance of a CT installation or test record requires separate verification of the applicable nameplate data, selected terminals, polarity, secondary wiring, test method, instrument accuracy, burden conditions, relay or meter configuration, and the governing project procedure.

CT ratio testing does not by itself confirm correct protection performance. Protective-relay settings must use the installed CT ratio and must be reviewed independently from conductor ampacity, feeder loading, voltage drop, raceway fill, terminal rating, insulation temperature rating, and other installation decisions.

Never leave an energized CT secondary circuit open. Follow the site’s approved isolation, shorting, grounding, testing, and lockout/tagout procedure before working on CT secondary wiring or connected protective and metering equipment.