PT Ratio Calculator

A PT ratio calculator determines the measured potential transformer ratio from an observed primary-voltage reading and secondary-voltage reading:

\(\displaystyle \text{Measured PT Ratio}=\frac{\text{Primary Voltage}}{\text{Secondary Voltage}}\)

The result expresses how many primary volts correspond to one secondary volt. This ratio is used when reviewing potential transformer (PT), voltage transformer (VT), metering, and protective-relay voltage inputs. It helps confirm whether measured transformer behavior aligns with the entered nameplate or test-reference ratio before voltage values are used in a metering check, relay review, or commissioning record.

The calculator also compares the measured ratio against the entered nominal ratio and reports the percentage ratio error.

PT Ratio From Voltage Readings

A potential transformer reduces a system voltage to a lower proportional voltage suitable for instruments, meters, relays, and test equipment. For example, a PT intended to supply a nominal 120 V secondary from a 12,000 V primary has a nominal transformation ratio of 100:1, commonly displayed as 100 x.

The calculator uses these fields:

Input or result	Electrical meaning
Primary voltage	Measured voltage on the primary side of the PT, entered in V
Secondary voltage	Measured voltage on the secondary side of the PT, entered in V
Nominal ratio	The PT nameplate ratio or approved test-reference ratio, entered as x
Measured ratio	Primary voltage divided by secondary voltage
Nominal ratio used	The entered nominal ratio used for comparison
Ratio error	Percent difference between measured ratio and nominal ratio
Primary voltage used	The entered primary-voltage value used in the calculation
Secondary voltage used	The entered secondary-voltage value used in the calculation

A ratio result near the nominal value indicates that the two entered voltage readings are proportionally consistent with the selected nominal ratio. It does not independently establish PT condition, calibration status, or suitability for service.

Ratio Error Calculation

The calculator first calculates the measured ratio:

\(\displaystyle R_m=\frac{V_P}{V_S}\)

Where:

• R_m = measured PT ratio
• V_P = primary voltage
• V_S = secondary voltage

It then compares that value with the Nominal ratio:

\(\displaystyle \text{Ratio Error}=\frac{R_m-R_N}{R_N}\times100\%\)

Where:

• R_N = nominal ratio entered in the calculator

A positive result means the measured ratio is higher than the nominal ratio. A negative result means the measured ratio is lower. A result of 0% means the arithmetic ratio from the entered voltage readings equals the entered nominal ratio exactly.

Calculation Example

For the following readings:

Field	Value
Primary voltage	12,000 V
Secondary voltage	120 V
Nominal ratio	100 x

The measured ratio is:

\(\displaystyle \frac{12{,}000\text{ V}}{120\text{ V}}=100\)

\(\displaystyle \text{Measured ratio}=100\text{ x}\)

The ratio error is:

\(\displaystyle \frac{100-100}{100}\times100\%=0\%\)

The calculator result is:

Result	Value
Measured ratio	100 x
Nominal ratio used	100 x
Ratio error	0%
Primary voltage used	12,000 V
Secondary voltage used	120 V

The primary and secondary readings therefore produce a 100:1 measured PT ratio, matching the entered 100 x nominal ratio.

Electrical Use of the Result

PT ratio verification supports several downstream electrical calculations and reviews:

• Metering review, where a PT multiplier affects the relationship between instrument voltage and actual system voltage.
• Protective-relay review, where relay voltage inputs must correspond to the assumed transformer ratio used in settings and coordination work.
• Commissioning documentation, where recorded primary and secondary values can be checked for proportional agreement with the PT reference ratio.
• Voltage indication troubleshooting, where an unexpected meter or relay voltage may result from an incorrect assumed ratio, incorrect wiring, an unsuitable measurement reference, or an abnormal transformer condition.
• Load and voltage review, where primary-system voltage is reconstructed from a known secondary measurement using the approved PT ratio.

For a known nominal ratio, the inverse relationship is often used during field review:

\(\displaystyle V_P=V_S\times R_N\)

A 120 V secondary reading with a 100 x PT ratio corresponds to 12,000 V on the primary side. That conversion is useful only when the PT ratio, measurement locations, and circuit configuration are confirmed.

Field Verification Limits

This calculation performs ratio arithmetic only. It does not determine PT accuracy class, rated burden, phase-angle error, polarity, insulation condition, winding condition, fuse status, relay-setting correctness, calibration compliance, or acceptance-test status.

A ratio that matches a nominal value does not prove that a PT installation is correct. Field decisions should separately verify the PT nameplate data, applicable test method, meter and relay scaling, secondary-circuit wiring, phase relationship, grounding arrangement, instrument burden, test-equipment accuracy, and the governing project requirements or AHJ requirements.

Use measured values taken with equipment and procedures appropriate for the voltage level and available fault energy. Primary-voltage measurements on medium-voltage or higher systems require qualified personnel, an approved safe-work process, and properly rated test equipment.