Calculate transformer rating, primary and secondary voltage, full-load current, or terminal voltage ratio for single-phase and three-phase systems.

RMS magnitudes; balanced three-phase formulas use line-to-line voltage and line current. Ideal ratio calculations omit losses. These results do not select conductors, protection or a safe installation.

Transformer Formula

The transformer calculator uses total kVA, RMS terminal volts, line amps, and primary-to-secondary terminal voltage ratio. Use line-to-line voltage for balanced three phase. The terminal ratio equals winding turns ratio only for single phase or matching winding connections. It converts MVA to kVA, kV to V, and mA to A before calculating.

S & = (F × Vₚ × Iₚ) / (1000) \ Vₚ & = (S × 1000) / (F × Iₚ) \ Iₚ & = (S × 1000) / (F × Vₚ) \ TR & = (Vₚ) / (Vₛ) \ Vₛ & = (Vₚ) / (TR) \ Iₛ & = Iₚ × TR
  • S = transformer rating in kVA
  • F = phase factor, 1 for single phase or √3 for balanced three phase
  • Vp = primary transformer voltage in volts
  • Vs = secondary transformer voltage in volts
  • Ip = primary full load current in amps
  • Is = secondary full load current in amps
  • TR = primary-to-secondary terminal voltage ratio; mixed delta/wye winding turns ratios require a connection correction

Select the value to solve for and enter only the relevant inputs. Rating, primary voltage and primary current use the kVA relationship; secondary voltage, secondary current and terminal ratio use ideal terminal-ratio relationships.

Phase Factors and Unit Conversions

Phase selection Factor used Rating formula
Single phase 1 kVA = V × A / 1000
Three phase √3 ≈ 1.732050808 kVA = √3 × V × A / 1000

Input unit Base unit used Conversion
MVA kVA 1 MVA = 1000 kVA
kV V 1 kV = 1000 V
mA A 1 mA = 0.001 A

Example Calculations

Example 1: Find three-phase transformer rating

You have a three-phase transformer with a 480 V primary voltage and 120 A primary full load current.

Use the rating formula:

S = √3 × 480 × 120 / 1000 ≈ 99.766127 kVA

The transformer rating is about 99.77 kVA.

Example 2: Find secondary voltage from terminal ratio

You have a primary RMS terminal voltage of 2400 V and a primary-to-secondary terminal voltage ratio of 10.

Use the secondary voltage formula:

Vs = 2400 / 10 = 240 V

The secondary transformer voltage is 240 V.

FAQ

What does transformer full load current mean?

Full load current is the current a transformer carries when it is delivering its rated kVA at the specified voltage. Primary full load current is on the input side. Secondary full load current is on the output side.

Why is √3 used for three-phase transformers?

The calculator uses √3 at full numeric precision for balanced three-phase apparent power with RMS line-to-line voltage and line current: kVA = √3 × V × A / 1000. The old factor 1.73 was a rounded approximation.

How many fields should be entered?

Choose a named solve mode, then fill its relevant fields. Select the system for kVA/current/voltage calculations. Ratios use matching terminal-voltage bases; losses, winding connection conversions, inrush and protection selection are outside this calculator.