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Transformer Impedance Calculator

Use this transformer impedance calculator to screen percent impedance (%Z), full-load current, X/R ratio, and available secondary short-circuit current. With a 500 kVA, 480 V transformer at 5%Z, the full-load current is about 601 A and the infinite-bus short-circuit screen is 12.0 kA. Use nameplate or manufacturer data before equipment-duty review.

Updated June 21, 2026

Calculator Inputs

Use nameplate value if known, or leave blank for typical

Use nameplate if known, or leave blank for typical

Calculation Results

Enter values above to see calculation results

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Calculation history

Example Calculations

500 kVA at 480V

Calculate short circuit current for typical commercial transformer.

Inputs
  • Transformer Rating: 500
  • Secondary Voltage: 277/480 V three-phase
  • Source Assumption: Infinite bus

How to Use

Transformer impedance quick answer

What is a typical transformer %Z?

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)

Example: 500 kVA at 480V, 5%Z

  • FLA = 500,000 ÷ (√3 × 480) = 601A
  • SC = 601 × (100 ÷ 5) = 12,020A (12.0 kA)

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

Short-circuit studies - calculate available fault current at the transformer secondary

Interrupting-rating review - carry transformer fault-current assumptions into protective-device checks

Arc-flash analysis - provide source current context for an incident-energy study

Protective Device Coordination - carry transformer fault-current assumptions into breaker and fuse review

Generator Paralleling - Match impedances for load sharing

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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