Transformer impedance equations
Nameplate percent impedance is converted to ohms on the transformer rating base before resolving resistance and reactance.
Ifl = S/(√3V) for three-phaseZbase = V²/S|Z| = Zbase × %Z/100R = |Z|/√(1+(X/R)²)X = R × X/RIsc = Ifl × 100/%Zwhere:
- S
- Transformer apparent-power rating[VA]
- V
- Rated secondary voltage[V]
- %Z
- Nameplate percent impedance
- Isc
- Infinite-bus symmetrical fault current[A]
Add source and conductor impedance for a location-specific result.
How to use the Transformer Impedance Calculator
Calculate transformer full-load current, ohmic impedance, R and X components, X/R ratio, and infinite-bus secondary fault current. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Balanced transformer at rated secondary voltage Infinite upstream bus Nameplate percent impedance on transformer rating base A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Balanced transformer at rated secondary voltage
- Infinite upstream bus
- Nameplate percent impedance on transformer rating base
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. Actual fault current changes with utility, conductor, motor, and generator impedance. X/R affects asymmetrical peak and device duty; use a formal short-circuit method for final design. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
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Assumptions
- Balanced transformer at rated secondary voltage
- Infinite upstream bus
- Nameplate percent impedance on transformer rating base
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Actual fault current changes with utility, conductor, motor, and generator impedance.
- X/R affects asymmetrical peak and device duty; use a formal short-circuit method for final design.