Transformer Turns Ratio and Full-Load Current Calculator

Find the turns ratio, the full-load current on the primary and secondary, and the fault current on the secondary of a transformer.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

The voltage ratio of a transformer equals its turns ratio. The full-load current is the kVA divided by the voltage, and for a three-phase transformer also by root three.

The percentage impedance on the nameplate is the voltage drop across the transformer at full load. The fault current on the secondary is the full-load current divided by that percentage as a fraction.

Formulas used

  • \dfrac{V_1}{V_2} = \dfrac{N_1}{N_2}
  • I_{FL} = \dfrac{S}{\sqrt{3}\,V_L}
  • I_{sc} = \dfrac{I_{FL}}{Z\%/100}

Worked example

With the default numbers in the calculator above:

  1. Turns ratio: 26.51 : 1
  2. Full-load current, primary: 52.49 A
  3. Full-load current, secondary: 1391.21 A
  4. Fault current on the secondary (supply taken as infinite): 27824.11 A
  5. Fault current on the primary: 1049.73 A

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Is this the real fault current?

It is the upper figure, assuming the supply behind the transformer is infinite. The real figure is a little lower and is lower still along a cable.

What is a typical percentage impedance?

Distribution transformers are commonly in the range 4 to 6 percent. Read the nameplate.

Related calculators

Further reading: Standard electrical engineering texts (transformers). Switchgear must be selected by a licensed person.

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