Calculate transformer impedance in ohms from kVA, voltage and nameplate %Z, find percent impedance from a short-circuit test, and estimate fault current.

Uses the nameplate %Z with an ideal stiff upstream source; fault current is transformer-only.

Use total three-phase rating when three-phase is selected.

Three-phase: line-to-line voltage. Single-phase: voltage across the selected winding terminals.

Optional settings

Transformer Impedance Formula

The following formula is used to calculate transformer impedance percentage:

%Z = (VSC) / (Vrated) × 100
  • %Z is the transformer impedance percentage
  • VSC is the short-circuit test voltage (volts applied to primary to drive rated current through a bolted secondary short)
  • Vrated is the rated primary voltage

Divide documented short-circuit test voltage by rated voltage on the same winding and voltage basis, then multiply by 100. The test must be at rated current and frequency under the applicable procedure. Use actual nameplate or test data; qualified personnel must handle electrical testing.

What Transformer Impedance Percentage Represents

Transformer percent impedance is a per-unit magnitude expressed as a percentage. Equivalent impedance contains resistance R and leakage reactance X, with |Z| = √(R² + X²). Resistance causes real winding loss; reactance represents reactive voltage drop. The magnitude alone does not identify R, X or X/R.

A specialist short-circuit test determines the voltage required for rated current under specified conditions. Reference temperature, tap, frequency and winding/voltage basis matter. This calculator uses a supplied test result; it does not provide an electrical test procedure or a universal expected %Z range.

Use Actual Nameplate and Manufacturer Data

Rating alone does not determine impedance or X/R. Use actual manufacturer nameplate, test-report and project-specification data. The previous unsourced rating-based ranges are omitted because they were not verified as IEEE or IEC limits.

On the selected side, use total three-phase VA with line-to-line volts, or single-phase VA with full-winding terminal volts. Manufacturer data and model assumptions must agree.

Fault Current and Voltage Regulation: The Core Tradeoff

For a balanced three-phase terminal fault with an ideal stiff source, Ifault = Ifull-load × 100 / %Z. A 1000 kVA, 480 V line-to-line transformer has approximately 1202.813 A full-load current. At 5%Z this model gives 24,056.261 A; at 6%Z it gives 20,046.884 A, a 16.667% reduction. Upstream and feeder impedances and other current sources are omitted. See the ABB handbook for the impedance model and system-study context.

Voltage regulation depends on actual resistance, reactance, loading and power factor. Percent-impedance magnitude alone does not determine regulation or justify a preferred design range. This calculator does not compute regulation; selection requires the complete system design.

X/R Ratio and Asymmetrical Fault Current

System X/R affects the transient DC offset and asymmetrical peak current after a fault. In a simple single-frequency R–L equivalent, the decay time constant is τ = (X/R)/(2πf): at 60 Hz, X/R of 10 gives about 26.5 ms and X/R of 20 gives about 53.1 ms. This calculator does not evaluate transient or peak current; transformer-only X/R may differ from the complete network.

Use actual system resistance/reactance and the applicable fault-study method. X/R cannot be assigned reliably from rating alone. Symmetrical RMS current here does not establish breaker peak or asymmetrical duty, or arc-flash energy.

Parallel Transformer Operation and %Z Matching

Under equal no-load secondary voltages, compatible phase relation, equal impedance angles and negligible unequal leads, load shares are proportional to each rating divided by its own-rating per-unit impedance. A 1000 kVA unit at 6%Z and a 500 kVA unit at 4%Z share 4:3. At 1500 kVA total they carry about 857.143 and 642.857 kVA; the smaller reaches 500 kVA at about 1166.667 kVA combined load. Proportional-to-rating sharing requires compatible own-rating impedances and manufacturer-approved conditions. Unequal load sharing and no-load circulating current from voltage/phase mismatch are different effects. This calculator does not assess parallel-operation suitability.

IEEE and IEC Manufacturing Tolerances

Impedance tolerance depends on the specified standard and edition, construction, winding pair, tap and procurement guarantee. Blanket IEEE ±7.5% or IEC ±10% statements do not apply to every unit. As arithmetic examples only, 6%Z with an explicitly applicable ±7.5% tolerance gives 5.55–6.45%; ±10% gives 5.4–6.6%. This calculator uses entered impedance without an allowance; a full study must apply project-specific limits.

IEC 60076-5:2006 uses two seconds as a thermal short-circuit calculation duration unless a different duration is specified, with conditions and exceptions. This is not a promise that every transformer survives any two-second fault and does not establish a universal minimum %Z. Use manufacturer withstand data and complete protective coordination.

Applications in Protection Coordination and Arc Flash

Protective-device assessment needs available fault current at each location, including the actual upstream network, transformer, feeders and other sources, together with applicable code requirements and equipment ratings. This transformer-only estimate is one input to a qualified study and is insufficient alone to select interrupting ratings or settings.

Arc-flash energy depends on the arcing-current model, protective clearing time, electrode configuration, enclosure, gap and working distance. A percentage change in transformer-only bolted current does not imply a fixed percentage change in incident energy. Lower current can increase clearing time and energy, as discussed in Eaton’s paper. This calculator does not include motor contribution, peak current, arc-flash energy or a hazard-category determination.