Autotransformer Calculator (Currents, kVA & Copper Saving)

How to use the autotransformer calculator

Enter the load rating in kVA and the high- and low-voltage sides of the autotransformer (it works the same for step-up and step-down). The calculator returns the current on each side, the current in the common section of the winding, how much of the power is transformed magnetically versus conducted directly, and the copper saved compared with a two-winding transformer of the same rating.

Autotransformer formulas

I_H = S ÷ V_H  ·  I_L = S ÷ V_L  ·  I_common = I_L − I_H

S_transformed = S (1 − K)  ·  S_conducted = S K  ·  K = V_L ÷ V_H

Example

A 10 kVA, 240 V to 200 V step-down autotransformer:

• I_H = 10,000 ÷ 240 = 41.67 A, I_L = 10,000 ÷ 200 = 50.00 A
• Common winding current = 50 − 41.67 = 8.33 A
• Transformed = 10 × (1 − 200/240) = 1.667 kVA; conducted = 8.333 kVA
• Copper saved ≈ 83.3% of a 10 kVA two-winding transformer

Transformed power and copper saving by voltage ratio

Where autotransformers are used

• Motor starting: autotransformer starters apply 50–80% voltage taps, cutting line current to K² of the direct-on-line value.
• Grid interconnection: large 400/220 kV and 220/132 kV interbus transformers, where the ratio is close to 1.
• Variacs: variable autotransformers for test benches.
• Not suitable where isolation is required, or for large ratios such as 11 kV/415 V.

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