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A three-phase autotransformer should be sized according to the maximum apparent power delivered to the connected load. The basic calculation uses the required output line-to-line voltage and the maximum simultaneous output line current.
Calculate the required three-phase load capacity using:
kVA = √3 × Output Voltage × Maximum Output Current ÷ 1000
Select a purpose-built autotransformer with a catalog load rating equal to or greater than the calculated result. Then verify motor starting, load imbalance, harmonics, duty cycle, ambient temperature, altitude, wiring configuration and voltage-drop requirements.
PowerHome offers three-phase autotransformers for fixed industrial voltage matching. Standard and custom configurations can be used to match common three-phase supply and equipment voltages, including 200V, 220V, 230V, 240V, 380V, 400V, 415V and 480V systems.
1. What Information Is Needed Before Sizing?
Before calculating the required kVA, collect the electrical and operating data for both the available power supply and the connected equipment. Use the measured site voltage and equipment nameplate values whenever possible.
These details determine the required voltage ratio, load capacity, wiring configuration and operating conditions.
| Category | Parameter | Information Required |
|---|---|---|
| Electrical Data | Site supply voltage | Measured input line-to-line voltage |
| Required load voltage | Equipment-rated line-to-line voltage | |
| Maximum load current | Maximum simultaneous output line current | |
| Phase | Confirm a three-phase supply and three-phase load | |
| Frequency | Confirm whether the system operates at 50 Hz or 60 Hz | |
| Load and Wiring | Load type | Motor, heater, VFD, rectifier, welder or mixed load |
| Starting method | Direct-on-line, star-delta, soft starter or VFD | |
| Wiring system | Three-wire or four-wire; delta or wye | |
| Neutral requirement | Whether the connected equipment requires a neutral conductor | |
| Duty cycle | Continuous, intermittent or short-time operation | |
| Installation | Installation environment | Ambient temperature, altitude, humidity, ventilation and enclosure requirements |
For motor-driven equipment, also provide the motor rated power, full-load current, power factor, efficiency, starting current and starting method.
When several loads share one autotransformer, identify which devices will operate or start at the same time. The transformer should be sized for the maximum simultaneous load rather than automatically adding every nameplate rating.
A clear equipment nameplate photo and the measured site voltage are usually the most useful starting points.
2. How to Calculate Three-Phase Load kVA
For a balanced or nearly balanced three-phase load, calculate the required apparent power using the equipment's required output line-to-line voltage and maximum simultaneous output line current.
Where:
- Sload is the required load capacity in kVA.
- Voutput is the required output line-to-line voltage in volts.
- Ioutput is the maximum simultaneous output line current in amperes.
- √3 is approximately 1.732.
| Use for the Calculation | Do Not Substitute |
|---|---|
| Required output line-to-line voltage | Input or site supply voltage |
| Maximum simultaneous output current | Current of only one device when several loads operate together |
| Three-phase kVA formula | Single-phase VA or kVA formula |
| Motor nameplate full-load current when available | Motor horsepower or kW alone |
Use the voltage required by the connected equipment, not the supply voltage, when calculating output load kVA. The current must represent the maximum load operating at the same time.
The formula assumes a balanced or nearly balanced three-phase load. If the system includes substantial single-phase branches, check the current on each phase separately.
When a motor nameplate provides only kW or horsepower, motor efficiency and power factor must also be considered. Whenever possible, use the motor's nameplate full-load current as the primary sizing value.
3. Worked Example: Sizing a 480V to 400V Autotransformer
Assume the following operating requirements:
First, calculate the required three-phase load capacity:
For a purpose-built autotransformer rated by load or throughput kVA, select a catalog rating equal to or greater than 69.3 kVA.
Selection Result
- Calculated load: 69.3 kVA
- Minimum standard catalog size: 75 kVA
- Voltage ratio: 480V input to 400V output
- Phase: Three-phase
- Final checks: Load type, starting current, duty cycle, harmonics, ambient conditions and allowable voltage drop
In this example, the calculated load is 69.3 kVA, so the next suitable standard capacity is 75 kVA. A 480V to 400V three-phase autotransformer should therefore have a catalog load rating of at least 75 kVA, subject to confirmation of motor starting current, harmonics, duty cycle, ambient conditions and allowable voltage drop.
The ideal input current can also be estimated:
This input-current value is an ideal calculation and does not include transformer losses or operating tolerances.
Final conductor sizing and overcurrent protection should be based on the transformer nameplate currents, manufacturer instructions and applicable electrical requirements.
4. Load kVA vs. Winding kVA
Load kVA
Load kVA is the total apparent power delivered to the connected equipment. In the 480V-to-400V example:
For a purpose-built autotransformer rated by load or throughput kVA, 69.3 kVA is the value used to select the finished-product capacity.
Winding kVA
An autotransformer transfers part of its power conductively through the shared winding connection. Only the voltage difference is transformed magnetically.
The approximate winding kVA can be calculated as:
Where VH is the higher voltage and VL is the lower voltage.
For a 480V-to-400V autotransformer supplying a 69.3 kVA load:
The 11.55 kVA value represents the approximate portion of power transformed magnetically. It does not mean that an 11.55 kVA catalog autotransformer can supply a 69.3 kVA load.
| Rating Term | Meaning | How It Is Used for Selection |
|---|---|---|
| Load kVA | Total apparent power required by the connected load | Used to determine the required finished-product capacity |
| Winding kVA | Portion of the power transformed magnetically | Used for transformer design or as a capacity reasonableness check |
| Buck-boost nameplate kVA | Rating of an individual two-winding transformer | Use the manufacturer's dedicated buck-boost selection table |
| Catalog load kVA | Maximum load capacity assigned to a purpose-built autotransformer | Compare directly with the calculated load kVA |
5. What Affects the Required Autotransformer kVA?
Calculated load kVA is only the starting point. Motor starting, simultaneous loads, harmonics, phase imbalance, duty cycle and installation conditions may increase the required rating. A fixed safety margin should not be applied to every application.
- Motor Loads: For motor-driven equipment, size the autotransformer using the motor nameplate full-load current rather than horsepower or kW alone. Also confirm the starting method, starting current, acceleration time, starts per hour and allowable voltage drop. For example, a 30 kVA 480V to 380V three-phase autotransformer may match a 480V supply to compatible 380V equipment, provided the motor current, frequency and starting requirements are verified.
- Multiple Connected Loads: Base the calculation on the maximum load operating at the same time. Consider the largest motor, simultaneous motor starts and the maximum starting combination. Staggered starting may reduce the required capacity.
- VFDs and Nonlinear Loads: For VFD applications, provide the input current, voltage range, rated power and installation position. VFDs, rectifiers and UPS systems can create harmonic current and additional heating, so suitability and derating should be confirmed.
- Unbalanced Three-Phase Loads: The standard three-phase kVA formula assumes a balanced load. For systems with substantial single-phase loads, check each phase current, neutral current and maximum phase loading separately.
- Duty Cycle and Environment: Continuous operation, high ambient temperature, altitude, poor ventilation, humidity and enclosure conditions may require a larger rating or derating.
- Frequency Compatibility: An autotransformer changes voltage but not frequency or phase. For example, 480V 60Hz converted to 400V remains 400V 60Hz. Equipment requiring 50Hz may need a frequency converter.
6. Three-Phase Autotransformer Selection Process
Follow these steps to confirm the electrical requirements, calculate the load and select an appropriate catalog rating.
- Confirm the measured site supply voltage.
- Confirm the equipment's required operating voltage.
- Confirm the phase, frequency and wiring configuration.
- Calculate the maximum simultaneous three-phase load kVA.
- Check motor starting, VFDs, rectifiers and harmonic-producing loads.
- Check phase balance, neutral requirements and maximum phase current.
- Select the next verified catalog load rating equal to or greater than the calculated requirement.
- Confirm the enclosure, cooling, terminals, conductors and protection requirements.
To confirm the correct three-phase autotransformer size, provide the input and output voltage, maximum output current, frequency, load type, starting method, wiring and neutral requirements, duty cycle, ambient conditions, and preferred enclosure and cooling method.
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