Estimate the VA or kVA rating required for single phase and three phase transformer applications.
Use this transformer sizing calculator to estimate the apparent power requirement for a single phase or three phase transformer.
The tool can calculate an indicative transformer size from either load current or power. It is intended as a practical guide only. Final transformer selection should always be confirmed against the application, load type, duty cycle, installation environment and applicable standards.
This tool provides an indicative estimate only. Final transformer selection should be confirmed against the application, load type, duty cycle, inrush current and installation environment.
Estimate Transformer VA or kVA
Estimate an indicative VA or kVA transformer rating for single phase or three phase applications.
Important: This calculator provides an indicative transformer size only. Final transformer selection should be confirmed by a suitably qualified person and checked against the application, load type, duty cycle, inrush current, installation environment, enclosure requirements and applicable standards.
If you are unsure which transformer size is suitable, contact Boardman Transformers with your voltage, current, phase and application details.
How the Transformer Sizing Calculator Works
The calculator estimates apparent power using common transformer sizing formulas.
For single phase applications where load current is known, the estimate is based on voltage multiplied by current.
For three phase applications where load current is known, the estimate is based on √3 multiplied by line voltage and current.
Where power in kW is known, the calculator estimates kVA using the selected power factor.
A safety margin can then be applied to provide a more practical indicative transformer size.
Transformer Sizing Formulas
Single phase
Three phase
Three phase kVA = √3 × Line Voltage × Current ÷ 1000
kVA = kW ÷ Power Factor
Estimated kVA with margin = Base kVA × (1 + Safety Margin %)
Use the technical calculation sections below to convert between voltage, current, VA, kVA, kW and common three phase values.
Phase-to-Phase / Phase-to-Neutral Voltage
In a balanced three phase system, line-to-line voltage and phase-to-neutral voltage are related by √3.
Line-to-line voltage = Phase-to-neutral voltage × √3
Phase-to-neutral voltage = Line-to-line voltage ÷ √3
For example, a 400V three phase system is approximately 230V phase-to-neutral.
Phase-to-Phase / Phase-to-Neutral Voltage Calculator
Calculate the relationship between line-to-line voltage and phase-to-neutral voltage in a balanced three phase system.
Primary / Secondary Current and Protection Reference
Use this calculator to estimate the primary and secondary full-load current of a transformer from its kVA rating and voltage values.
For three phase transformers, enter the line-to-line primary and secondary voltages. The calculator will also estimate the corresponding phase-to-neutral voltage.
The protection values shown are for reference only. Transformer primary protection should be selected based on the transformer rated primary current, inrush characteristics, protection type and applicable standards. It should not be oversized beyond the transformer VA/kVA rating without proper design confirmation.
Primary and Secondary Current Calculator
Estimate primary and secondary full-load current from transformer rating, primary voltage, secondary voltage and phase. For three phase systems, this tool also estimates phase-to-neutral voltage from line-to-line voltage.
The primary current is the estimated full-load current drawn from the transformer input supply.
The secondary current is the estimated full-load current available from the transformer output side.
For single phase transformers, current is calculated using kVA × 1000 ÷ voltage.
For three phase transformers, current is calculated using kVA × 1000 ÷ √3 × line-to-line voltage.
For three phase voltage systems, phase-to-neutral voltage is estimated using line-to-line voltage ÷ √3.
The protection reference values show the nearest common current ratings at or below and at or above the calculated transformer rated current. These are provided for reference only and should not be treated as final fuse or breaker recommendations.
Primary protection should be based on the transformer rated input current and should not be oversized beyond the transformer VA/kVA rating without proper design confirmation. Transformer magnetising inrush may require the correct protection type, fuse characteristic or breaker curve rather than a higher current rating.
kVA to Amps
This calculation estimates full-load current from transformer rating and voltage.
For single phase transformers:
Current = kVA × 1000 ÷ Voltage
For three phase transformers:
Current = kVA × 1000 ÷ √3 × Line Voltage
This is useful when selecting protection, cable sizing guidance or checking approximate transformer output current.
kVA to Amps Calculator
Estimate full-load current from transformer kVA rating, voltage and phase.
Amps to kVA
Amps to kVA Calculator
Estimate transformer kVA from voltage, current and phase.
kW to kVA
Where power is known in kW, apparent power can be estimated using power factor.
kVA = kW ÷ Power Factor
For example:
10kW ÷ 0.8 = 12.5kVA
Power factor varies depending on the load. Motor loads and inductive loads may require additional consideration.
kW to kVA Calculator
Estimate apparent power in kVA from known power in kW and power factor.
VA to kVA
VA and kVA are both used to describe transformer apparent power.
1kVA = 1000VA
kVA = VA ÷ 1000
VA = kVA × 1000
For example:
2500VA = 2.5kVA
VA / kVA Converter
Convert between VA and kVA transformer ratings.
Transformer Safety Margin Calculator
A safety margin can be applied to a calculated transformer size to allow for practical application requirements.
Required size with margin = calculated size × (1 + margin ÷ 100)
For example:
10kVA with a 20% margin = 12kVA
The correct margin depends on the load type, duty cycle, inrush current and installation conditions.
Transformer Safety Margin Calculator
Apply a safety margin to an estimated transformer rating.
Control Transformer Sizing Guidance
When sizing a control transformer, consider both the sealed/load VA and the inrush VA of connected devices such as contactors, relays, solenoids and control equipment.
The transformer must be selected to support the connected control circuit load while maintaining suitable voltage during operation and start-up conditions.
If you are unsure which control transformer rating is required, Boardman Transformers can help review the application and connected load details.
Suggested Standard Transformer Size
Enter an estimated transformer rating and find the next standard size from a practical rating list.
When to Contact Boardman Transformers
The calculator is useful for creating an initial estimate, but transformer selection often depends on more than voltage and current alone.
Contact Boardman Transformers if your application includes motor loads, high inrush current, continuous duty, unusual duty cycles, non-standard voltages, enclosure requirements, export compliance or specialist installation conditions.
– Motor loads or high inrush current
– Non-standard primary or secondary voltages
– Single phase or three phase custom requirements
– Auto transformer applications
– Control transformer applications
– Enclosure or IP rating requirements
– Continuous duty or unusual duty cycles
– cCSAus or other compliance requirements
– OEM, panel builder or machinery applications
What to Include in Your Transformer Enquiry
Frequently Asked Questions
Below are answers to common questions about this transformer range and how Boardman Transformers can support your application.
What size transformer do I need?
The required transformer size depends on the voltage, current, phase, load type, duty cycle, inrush current and safety margin. The calculator provides an indicative VA or kVA estimate, but final selection should be confirmed against the application.
Primary and Secondary Current Calculator
How do I calculate transformer kVA?
For single phase applications, kVA can be estimated by multiplying voltage by current and dividing by 1000. For three phase applications, kVA can be estimated using √3 × line voltage × current ÷ 1000.
What is the difference between VA and kVA?
VA stands for volt-amperes. kVA stands for kilovolt-amperes. One kVA equals 1,000VA. Smaller transformers are often rated in VA, while larger transformers are usually rated in kVA.
Should I include a safety margin when sizing a transformer?
A safety margin is often useful, but the correct margin depends on the application, load type, duty cycle and inrush current. The calculator includes a default 20% margin as a starting point.
Can Boardman confirm the correct transformer size?
Yes. Boardman Transformers can help review your voltage, current, phase, load and application requirements to confirm a suitable transformer size and specification.
Does this calculator cover custom transformers?
Yes. Boardman Transformers can help review your voltage, current, phase, load and application requirements to confirm a suitable transformer size and specification.
Can this calculator recommend transformer fuse sizes?
No. The calculator provides rated current values and common protection size references only. Final transformer protection must be selected by a suitably qualified person and should consider the transformer rated current, magnetising inrush current, fuse type, breaker curve, cable size, fault level, load type, installation method and applicable standards.
Need Help Sizing a Transformer?
Send your voltage, current, phase and application details to Boardman Transformers and our team will help identify the right transformer option for your requirement.
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