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 November 7, 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 to split impedance into resistance and reactance components; this page does not calculate asymmetrical peak current.
- Use the result panel for full-load current, impedance components, and available-fault-current screening before a separate device-duty review.
If %Z or X/R is left blank, the calculator uses a built-in kVA-based estimate. Treat that output as an early screen only. Enter nameplate or manufacturer values for project review; the built-in estimates have not been validated against a supplied equipment schedule.
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 calculator uses X/R to split the entered impedance into resistance and reactance components. It does not calculate an asymmetrical peak current; use a separate system study for that result and for device-rating review when required.
For available-fault-current screening, see the Short Circuit Calculator. For arc-flash incident-energy review, use the Arc Flash Calculator.
Common Applications
Frequently Asked Questions
What does transformer %Z mean?
Percent impedance (%Z) is a transformer nameplate or manufacturer value used with rated kVA and voltage to screen the secondary current relationship. Lower entered %Z produces a higher calculated current in this simplified model. Use equipment data for a project calculation; the page does not establish typical values for a transformer size.
Why does higher %Z mean lower short circuit current?
With the infinite-bus assumption, the symmetrical secondary current screen is I_SC = I_FLA × (100/%Z). For the same transformer base, lowering entered %Z raises this calculated value. This is not a complete fault study.
What is an infinite bus assumption?
Infinite bus treats the upstream source as having no limiting impedance in this screen. It can produce a higher result than a limited-source input, but whether it is conservative for a specific equipment or protection review depends on the system model. Enter available primary short-circuit data when known.
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.
Related Calculators
Short Circuit Calculator
Screen available fault current from transformer impedance, feeder impedance, or known source impedance.
Transformer Calculator
Screen transformer kVA, turns ratio, full-load current, losses, and a basic impedance-based fault-current check.
Arc Flash Calculator
Screen adjusted incident energy and approximate arc flash boundary from a known base case.
Circuit Breaker Sizing Calculator
NEC-oriented standard breaker review-point screen with simplified conductor protection checks.
Protection Coordination Calculator
One-point screen for coordination time margin, published selective current, and instantaneous pickup checks.
Power Factor Calculator
Evaluate and correct power factor in AC systems