Power Systems calculator
Transformer Impedance Calculator
Use this transformer impedance calculator to enter transformer kVA, secondary voltage, percent impedance, source assumption, and optional X/R data before screening full-load current and available secondary short-circuit current. Use nameplate or manufacturer data whenever possible, then carry the calculator result into interrupting-rating, SCCR, arc-flash, and equipment-duty review.
Updated August 4, 2026
Example Calculations
How to Use
How to use the transformer impedance calculator
- Enter transformer kVA, secondary voltage, and phase.
- Enter the transformer percent impedance from the nameplate, submittal, or manufacturer data.
- Choose whether the source is treated as an infinite bus or has known upstream impedance.
- Add X/R ratio data when the study needs asymmetrical fault-current context.
- Use the result panel for full-load current, impedance basis, and available-fault-current screening before device-duty review.
The table below is reference material after you use the calculator. Do not use a typical %Z row as a substitute for the actual transformer nameplate when interrupting ratings or equipment SCCR are being checked.
| kVA | Typical %Z | X/R Ratio |
|---|---|---|
| 75 | 2.5% | 2.7 |
| 150 | 3.2% | 3.5 |
| 300 | 4.5% | 4.5 |
| 500 | 5.0% | 5.0 |
| 1000 | 5.5% | 7.0 |
| 2000 | 5.75% | 9.0 |
Short-circuit current formula
I_SC = I_FLA × (100 ÷ %Z)
Load a preset or enter the project transformer data above, then let the calculator compute the full-load-current basis and available-fault-current screen from your inputs. The formula is shown here so the method is transparent; the project-specific numbers should come from the calculator result.
X/R ratio context
The X/R ratio affects the DC component of fault current:
- Low X/R (1-3): Smaller DC offset, current decays quickly
- High X/R (7+): Large DC offset, longer decay time
- Carry the value into asymmetrical fault-current and device-rating review when the project requires it
For available-fault-current screening, see the Short Circuit Calculator. For arc-flash incident-energy review, use the Arc Flash Calculator.
Common Applications
More applications. Open to review 2 additional use cases.
Frequently Asked Questions
What does transformer %Z mean?
Percent impedance (%Z) is the voltage drop across the transformer under full load, expressed as a percentage of rated voltage. It limits short circuit current - lower %Z means higher fault current. Typical values range from 2-6% depending on size.
Why does higher %Z mean lower short circuit current?
The transformer impedance acts as a current limiter during a fault. I_SC = I_FLA × (100/%Z). So a 5%Z transformer limits fault current to 20× FLA, while a 2.5%Z transformer allows 40× FLA - twice as much. Higher Z is safer but causes more voltage drop.
What is an infinite bus assumption?
Infinite bus assumes the utility source can deliver unlimited fault current (zero source impedance). This is conservative for sizing protective devices but may overestimate fault current. For more accurate results, include the actual available primary short circuit current.
How do I find the transformer %Z and X/R ratio?
Check the transformer nameplate and submittal first. Percent impedance is normally available from the nameplate or manufacturer data, while X/R may require a test report or manufacturer documentation. Use typical values only for early screening.
How does transformer impedance affect interrupting-rating review?
Lower transformer percent impedance allows higher available fault current on the secondary side, so downstream interrupting ratings and equipment SCCR need closer review. Confirm the adopted code edition, utility contribution, transformer nameplate, conductor impedance, and AHJ requirements before final selection.
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