Enter the primary and secondary voltages of the transformer into the calculator to determine the VT ratio.

VT Ratio Calculator

Enter any 2 values to calculate the missing variable

VT Ratio Formula

The voltage transformer ratio compares the primary-side voltage to the secondary-side voltage. It is a unitless value that shows how much a transformer scales voltage between its two sides.

VTR = PV / SV
Symbol Meaning Notes
VTR Voltage transformer ratio Unitless
PV Primary voltage High-side voltage
SV Secondary voltage Low-side voltage

If you need to solve for a missing voltage instead of the ratio, use the rearranged forms below.

PV = VTR * SV
SV = PV / VTR

Important: both voltages must be interpreted in consistent units. For example, 13.8 kV should be treated as 13,800 V before comparing it with 120 V.

How to Calculate VT Ratio

  1. Enter the primary voltage and secondary voltage, or enter one voltage together with the VT ratio.
  2. Make sure the voltages are on the same basis, such as line-to-line or line-to-neutral.
  3. Calculate the missing value and interpret the result as a numerical ratio such as 60 or 100.

What the Result Means

A larger VT ratio means the secondary voltage is much lower than the primary voltage. For example, a nameplate value of 12,000:120 corresponds to a VT ratio of 100 because the primary voltage is 100 times the secondary voltage.

For an ideal transformer, the voltage ratio matches the turns ratio.

V_P / V_S = N_P / N_S

This relationship is useful in metering, protective relaying, system verification, and converting measured secondary voltage back to the original primary system voltage.

Quick Reference

Primary Voltage Secondary Voltage VT Ratio
480 V 120 V 4
4,160 V 120 V 34.667
7,200 V 120 V 60
12,000 V 120 V 100
13,800 V 120 V 115

Example Calculations

Finding the VT Ratio

If a transformer reduces 7,200 V to 120 V:

VTR = 7200 / 120 = 60

The VT ratio is 60, commonly written as 60:1.

Finding the Primary Voltage

If the VT ratio is 100 and the secondary voltage is 120 V:

PV = 100 * 120 = 12000

The primary voltage is 12,000 V.

Finding the Secondary Voltage

If the primary voltage is 4,160 V and the VT ratio is 34.667:

SV = 4160 / 34.667 \approx 120

The expected secondary voltage is approximately 120 V.

Common Uses of VT Ratio

  • Metering: scales higher system voltage to a safer level for instruments and displays.
  • Protective relays: provides proportional voltage signals for relay logic and fault detection.
  • Commissioning: confirms installed transformer ratios match design and nameplate values.
  • Troubleshooting: helps verify whether measured secondary voltage properly reflects the primary system.

Common Mistakes

  • Mixing units: dividing kilovolts by volts without conversion gives an incorrect result.
  • Reversing the ratio: VT ratio is normally primary voltage divided by secondary voltage.
  • Comparing different voltage bases: line-to-line and line-to-neutral values should not be mixed without adjustment.
  • Confusing ratio with transformer rating: VT ratio does not describe burden, efficiency, impedance, or kVA capacity.
  • Using operating voltage instead of nominal voltage for nameplate checks: field readings may vary slightly from rated values.

VT Ratio FAQ

Is VT ratio the same as PT ratio?

In many electrical applications, yes. VT and PT are often used interchangeably for instrument transformers that reduce voltage for measurement and protection.

Does VT ratio have units?

No. The ratio is dimensionless because it compares one voltage to another.

Can the result be written as a ratio instead of a decimal?

Yes. A calculated value of 100 may be written as 100:1, while an actual transformer may be labeled 12,000:120. Both describe the same voltage scaling relationship.

Why does unit consistency matter?

The ratio is only correct when the primary and secondary voltages are compared on the same unit scale. Converting before dividing prevents errors and keeps the result meaningful.