Transformers and Power Supplies
Autotransformer Rating Calculator
The purpose of Autotransformer Rating is to turn high-side voltage, low-side voltage, and low-side load current into autotransformer winding rating. Keep the source condition with the saved result so later comparisons remain meaningful.
Calculate Autotransformer winding rating
Enter ratings or measurements that describe one scenario.
Understanding the equation
This page evaluates winding VA = (Vhigh − Vlow) × Iload. The entered quantities are High-side voltage, Low-side voltage, and Low-side load current.
The defaults produce 1,800.0 VA. Replace them with measurements from one operating state.
Estimate autotransformer winding rating and load apparent power. A separate apparent power result is available from Transformer VA Rating Calculator.
Values this page needs
Record waveform, frequency, load level, and winding or device temperature. Nameplate and meter values are not interchangeable unless they describe the same condition.
Check every prefix before entry; micro, milli, kilo, and mega are not interchangeable.
- High-side voltage
- Default example: 240 V. Enter high-side voltage in V.
- Low-side voltage
- Default example: 120 V. Enter low-side voltage in V.
- Low-side load current
- Default example: 15 A. Enter low-side load current in A.
Checks that catch bad inputs
Mixing nominal and measured data is a common source of error.
Recheck decimal placement and unit conversion when the answer looks unusual.
Practical limits
This ideal calculation does not include losses, temperature rise, or isolation.
The equation does not include isolation, creepage, waveform shape, and manufacturer operating limits. Do not hide omitted behavior inside an unexplained correction factor.
Record waveform, frequency, load level, and winding or device temperature.
Sensitivity check
Change High-side voltage from 240 V to 288 V without altering the remaining fields. The calculated value shifts from 1,800.0 VA to 2,520.0 VA.
This isolates one variable; a real system may change several at once.
Common questions before using the result
What else is required before final selection?
No. Compare the value with voltage stress, current stress, ripple, loss, regulation, and temperature and apply relevant equipment and installation requirements.
Should intermediate values be rounded?
Retain the raw result for comparison and create a separately rounded reporting value if needed.
Can an input legitimately be zero?
Check the field definition: some quantities can be zero, while others represent positive ratings or dimensions.
How should I prepare the Autotransformer Rating inputs?
Choose entries that describe the same case. Use voltage and current values from the same side of the transformer or converter.