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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.
Quick Answer
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. Conditions That Can Change the Final Autotransformer Size
The calculated load kVA is the starting point. The final rating can change because of motor starting, simultaneous loads, nonlinear current, phase imbalance, duty cycle and installation conditions.
A fixed percentage margin should not be applied to every application. Different loads produce different current, voltage-drop and thermal requirements.
Motor Loads
Typical applications include imported pumps, fans, blowers, conveyors, mixers, packaging machines and machine tools whose rated voltage differs from the available three-phase supply. A three-phase autotransformer for motor voltage matching can correct the voltage difference when the motor phase, frequency and operating requirements are otherwise compatible. For example, a 30 kVA 480V to 380V three-phase autotransformer may be used to match a 480V three-phase supply to compatible 380V motor-driven equipment, subject to verification of the motor nameplate current, starting method, starting current and allowable voltage drop.
Do not size a motor application from horsepower or kW alone. Use the motor nameplate full-load current whenever possible.
Also confirm the starting method, expected starting current, number of starts per hour, acceleration time and maximum allowable voltage drop.
Direct-on-line, star-delta, soft-starter and VFD starting methods place different demands on the autotransformer.
Multiple Connected Loads
Calculate the maximum load that can operate at the same time rather than automatically adding every connected nameplate rating.
Identify the largest motor, whether several motors can start together and the maximum simultaneous starting combination.
A staggered starting sequence may reduce the required capacity compared with simultaneous starting.
VFDs and Nonlinear Loads
For a VFD application, provide the VFD input current, input-voltage range, rated power and manufacturer requirements.
Confirm whether the autotransformer will be installed on the VFD input side or output side.
VFDs, rectifiers, UPS systems and other nonlinear loads can produce harmonic current and additional heating. Suitability and any required derating should be confirmed.
Unbalanced Three-Phase Loads
The standard three-phase kVA formula assumes a balanced or nearly balanced load.
If the system includes substantial single-phase branches, calculate or measure the current on each phase separately.
Check the maximum phase current, neutral current and loading of each phase to prevent one phase from being overloaded.
Duty Cycle and Installation Environment
Continuous-duty operation can place greater thermal demand on the transformer than intermittent or short-time operation.
Ambient temperature, altitude, humidity, ventilation and enclosure conditions may also affect the final rating or require derating.
Frequency Compatibility
An autotransformer changes voltage but does not change frequency or phase.
For example, converting 480V 60Hz three-phase power to 400V still produces 400V 60Hz output.
If the connected equipment is rated only for 50Hz, a frequency converter or another suitable power-conversion solution may be required.
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.
7. Frequently Asked Questions
Q1: Should I use input voltage or output voltage to calculate kVA?
Use the required output line-to-line voltage and the maximum simultaneous output current when calculating the load kVA of a purpose-built three-phase autotransformer.
Use the input voltage separately to confirm the voltage ratio and estimate the input current.
Q2: Is autotransformer load kVA the same as winding kVA?
No. Load kVA is the total apparent power delivered to the connected equipment, while winding kVA is the portion transferred magnetically through the transformer winding.
When a catalog autotransformer is rated by load or throughput kVA, compare its rating directly with the calculated load kVA.
Q3: How much extra autotransformer capacity should I allow?
Do not apply the same percentage margin to every application. Additional capacity depends on motor starting current, simultaneous loads, harmonics, duty cycle, ambient temperature, altitude, allowable voltage drop and planned expansion.
Q4: How do I size an autotransformer for a motor?
Use the motor nameplate full-load current, rated voltage, frequency and starting method.
Also confirm the starting current, starts per hour, acceleration time and allowable voltage drop. Horsepower or kW alone is not sufficient for final sizing.
Q5: Can an autotransformer change 50Hz to 60Hz?
No. An autotransformer changes voltage but does not change frequency. A 480V 60Hz input converted to 400V will still provide 400V 60Hz output.
Frequency-sensitive equipment may require a frequency converter.
Q6: Can a three-phase autotransformer be used in reverse?
Some fixed-ratio autotransformers can operate in reverse, but only when the manufacturer confirms the connection and rating.
Reversing the voltage direction changes the input and output currents, conductor requirements, protection settings and available voltage ratio.
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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