Power Systems calculator
Transformer Calculator
Use this transformer sizing calculator as the project-specific owner for load-to-kVA, voltage, current, loss, and secondary fault-current screening. Enter load kW or kVA, phase, primary and secondary voltage, power factor, planning margin, efficiency, and nameplate impedance to compare sizing, turns ratio, full-load current, operating-point loss, or secondary-terminal fault current. It is intentionally narrower than a full coordination, harmonic, paralleling, or thermal-rise study.
Updated July 16, 2026
Enter load kW or kVA, phase, primary and secondary voltage, efficiency, and nameplate impedance to screen transformer kVA, turns ratio, full-load current, losses, or secondary fault current.
Use sizing mode for load-to-kVA review, parameters mode for voltage/current checks, and fault mode when nameplate impedance is known.
Enter the transformer inputs below to produce the project-specific sizing, current, loss, or fault-current result
Example Calculations
More examples. Open to review 1 additional calculation example.
How to Use
Transformer sizing calculator workflow
- Select the calculation type: sizing, voltage and current, loss screen, or impedance and fault screen.
- Choose single-phase or three-phase and enter primary and secondary voltage.
- Enter a load or transformer rating. Use load kW or kVA for sizing and loss review. Use the transformer nameplate kVA for the fault-current screen.
- If the input is in kW or W, enter the power factor so the page can convert to apparent power.
- For sizing mode, enter the planning margin needed for spare capacity, continuous duty, or future load. For loss screen, enter an estimated operating efficiency. For fault screen, enter the actual transformer nameplate impedance.
What each mode returns
| Mode | Primary outputs | Best use |
|---|---|---|
| Sizing | Calculated kVA, primary and secondary current, and the next common kVA size | Early transformer selection from known load |
| Voltage and current | Turns ratio, voltage relationship, full-load current, and current ratio | Reviewing a known transformer rating |
| Loss screen | Estimated output power, input power, heat loss, and BTU per hour | Simple operating-point loss and cooling discussion |
| Impedance and fault screen | Secondary full-load current, fault-current multiple, and secondary-terminal fault current | Basic available-fault-current screening at the transformer secondary terminals |
What this page does well
- It converts between kW and kVA cleanly with an explicit power factor.
- It applies an optional planning margin before selecting the next common kVA size.
- It uses standard full-load current relationships for single-phase and three-phase transformers.
- It keeps the fault-current screen tied to transformer kVA, voltage, and nameplate impedance instead of pretending to do a full study.
- It makes the loss calculation an honest operating-point estimate based on the efficiency entered by the user.
What this page does not claim to do
- It does not replace short-circuit, protective-device coordination, or arc-flash studies.
- It does not calculate K-factor, harmonics, transformer paralleling, tap-changing behavior, or thermal-rise test performance.
- It does not replace manufacturer catalog review, project-specific spare capacity rules, or nameplate verification.
For a worked review, use the transformer sizing preset above so the result stays tied to the actual load units, power factor, planning margin, primary voltage, secondary voltage, phase, and impedance.
Use the Transformer Sizing Guide as a reference after the calculator has produced a project-specific kVA and current basis. Use the Short Circuit Calculator when feeder impedance matters, the Transformer Impedance Calculator for a dedicated impedance workflow, the Power Calculator for simpler kW or kVA checks, and the Electrical Load Calculator when the real task is service or feeder planning.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
How do you calculate transformer kVA from load kW?
Divide load kW by power factor to get the apparent-power basis. Then use the calculator inputs to keep that kVA basis tied to phase, primary voltage, secondary voltage, current, and the next size review.
How does the calculator find transformer current?
For single-phase transformers, current is kVA × 1000 ÷ volts. For three-phase transformers, line current is kVA × 1000 ÷ (√3 × line-to-line volts).
Does the loss screen replace factory no-load and load-loss test data?
No. It is only an operating-point estimate from the efficiency you enter. It does not split core loss from winding loss or replace manufacturer test reports.
What does the fault-current screen include?
It screens fault current at the transformer secondary terminals from transformer kVA, secondary voltage, and nameplate impedance. Upstream source limits, feeder impedance, and coordination details are outside the page.
Can I use this page for transformer protection sizing or harmonics review?
Not as a final answer. This page intentionally stops at basic transformer math and a simple impedance screen. Protection selection, harmonics, K-factor, and catalog selection still need a deeper review.
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