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Transformer Calculator

Full-load current, turns ratio and transformer type for single phase and three phase.

Number of phases

Results
Primary full-load current
Secondary full-load current
Turns ratio (Np : Ns)

Show the working
Results are theoretical rated values. Always verify against the manufacturer's nameplate, the applicable code and your own engineering judgment. Engineering World

How to Use This ?

Enter a transformer's kVA rating and its primary and secondary voltages to get the full-load current on each side, the voltage ratio, and whether it is a step-up, step-down or isolation transformer. It works for single and three phase, and the working is shown so you can check it.

Formulas Used in the Transformer Calculator

Single phase: Ip = S ÷ Vp and Is = S ÷ Vs
Three phase: Ip = S ÷ (√3 × Vp) and Is = S ÷ (√3 × Vs)
Voltage ratio: Vp ÷ Vs

S is the rating in volt-amperes (multiply kVA by 1,000). Vp and Vs are the primary and secondary voltages in volts. For three-phase units, use line-to-line voltages. The current shown is the line current.

For a single-phase ideal transformer, the voltage ratio equals the turns ratio (Np : Ns). For three-phase units, it equals the turns ratio only when both windings use the same connection, such as delta-delta or wye-wye.

Wye (Y) connection

Wye connection diagram Three windings joined at a common neutral point N. The other end of each winding is a line terminal A, B or C. The line-to-line voltage is the square root of three times the phase voltage, and the line current equals the phase current. N A B C Vph VL (line-to-line)
  • VL = √3 × Vph
  • IL = Iph

Delta (Δ) connection

Delta connection diagram Three windings connected end to end in a triangle. The corners of the triangle are the line terminals A, B and C. The line-to-line voltage equals the phase voltage, and the line current is the square root of three times the phase current. A B C Vph = VL
  • VL = Vph
  • IL = √3 × Iph

L means line (measured between line terminals) and ph means phase (measured across one winding). This is why connection type matters for turns ratio. For example, a 13.8 kV delta primary with a 480 V wye secondary has a line-to-line voltage ratio of 28.75, but a per-phase turns ratio of about 49.8.

Worked Examples

Worked Example 1: Single Phase

A 50 kVA, 4,000 V to 400 V transformer: S = 50,000 VA. Ip = 50,000 ÷ 4,000 = 12.5 A. Is = 50,000 ÷ 400 = 125 A. Voltage ratio = 4,000 ÷ 400 = 10 : 1, so it is a step-down transformer.

Worked Example 2: Three Phase

A 500 kVA, 13.8 kV to 480 V transformer: S = 500,000 VA. Ip = 500,000 ÷ (√3 × 13,800) = 20.918 A. Is = 500,000 ÷ (√3 × 480) = 601.407 A. Line-to-line voltage ratio = 13,800 ÷ 480 = 28.75 : 1 (115 : 4).

Frequently Asked Questions

How do you calculate transformer full-load current?
Divide the rating in VA by the voltage for a single-phase unit (I = S ÷ V). For a three-phase unit, divide by √3 times the line-to-line voltage (I = S ÷ (√3 × V)).

Why is the three-phase formula divided by √3?
Three-phase apparent power equals √3 times the line-to-line voltage times the line current. Rearranging for current gives the √3 in the denominator.

What is the difference between a step-up and a step-down transformer?
A step-down transformer has a higher primary voltage than secondary voltage. A step-up transformer is the opposite. If both voltages are equal, it is an isolation transformer.

Is the turns ratio the same as the voltage ratio?
For a single-phase ideal transformer, yes. For a three-phase transformer, only when both windings have the same connection type. Delta-wye and wye-delta units differ by a factor of √3.

Can I use full-load current to size a breaker or cable?
It is the starting point, not the final answer. Breaker and conductor sizing involve code-based multipliers and conditions, so check the electrical code that applies to your installation.

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