Transformer Impedance Calculator

A transformer impedance calculator converts nameplate Transformer impedance into per-unit and ohmic values, then calculates the transformer-limited current at the secondary terminals. For a balanced three-phase transformer, this current is the symmetrical current level implied by transformer kVA, secondary voltage, and percent impedance before downstream conductor, raceway, connection, and utility-source impedance are included.

The primary output for fault-current workflow is Transformer-limited current. It provides the transformer contribution used when checking the starting point for available fault current at a transformer secondary. It is not a conductor ampacity calculation, voltage-drop calculation, or final overcurrent-device interrupting-rating determination.

Use this transformer impedance workflow for per-unit arithmetic checks. It does not replace a fault-current study or equipment rating review.

Updated August 22, 2026

Transformer Secondary Fault Current

Transformer percent impedance, shown on a nameplate as %Z, represents the transformer’s internal impedance as a percentage of its rated base impedance. Lower percent impedance produces higher transformer-limited current; higher percent impedance produces lower current.

A transformer supplying a secondary switchboard, feeder, or motor-control lineup may have substantial available current even when its normal-load current is relatively modest. The transformer impedance calculation establishes the current available directly at the transformer secondary based on the transformer alone.

The calculated current can support a broader electrical review involving:

• Secondary overcurrent protective-device interrupting ratings
• Switchboard, panelboard, disconnect, and bus equipment ratings
• Feeder and branch-circuit available fault-current evaluation
• Conductor and connection impedance used in downstream fault-current calculations
• Motor contribution and system fault-current studies
• Utility and upstream source impedance review

It does not determine required conductor AWG or kcmil size, ampacity, raceway fill, voltage drop, terminal rating, insulation temperature rating, or adjustment factor. Those design decisions require their own load, installation, and code review.

Input Values

Input	Electrical purpose
Transformer size	The transformer apparent-power base in kVA
Secondary voltage	The secondary voltage base in V used for the impedance conversion
Phase	Selects single-phase or balanced three-phase base math
Transformer impedance	The transformer nameplate or manufacturer impedance in %Z

For a balanced three-phase calculation, Secondary voltage is used as the line-to-line voltage base. The transformer kVA and voltage must describe the same secondary winding and operating configuration.

Use the actual nameplate Transformer impedance whenever available. A generic assumed impedance can produce a materially different transformer-limited-current result, especially where equipment interrupting ratings or available fault-current labels are being reviewed.

Calculation Method

For Balanced three-phase, the calculator uses the following base quantities.

Base Current

\(I_\text{base}=\frac{\text{Transformer size} \times 1{,}000}{\sqrt{3}\times\text{Secondary voltage}}\)

Where:

• \(I_\text{base}\) is Base current in amperes
• Transformer size is entered in kVA
• Secondary voltage is entered in line-to-line volts

Base Impedance

\(Z_\text{base}=\frac{V^2}{\text{Transformer size} \times 1{,}000}\)

Where:

• \(Z_\text{base}\) is Base impedance in ohms
• (V) is Secondary voltage in volts

Per-Unit and Ohmic Impedance

\(Z_\text{pu}=\frac{\text{Transformer impedance}}{100}\)

\(Z_\text{transformer}=Z_\text{pu}\times Z_\text{base}\)

The calculator reports Per-unit impedance as a decimal value. For example, 5.75%Z becomes 0.0575 pu.

Transformer-Limited Current

\(I_\text{transformer-limited}=\frac{I_\text{base}}{Z_\text{pu}}\)

This result represents the current limited by the transformer impedance alone at the selected secondary voltage base.

Calculation Example

A 75 kVA transformer has a 480 V balanced three-phase secondary and a nameplate impedance of 5.75%Z.

Field	Value
Transformer size	75 kVA
Secondary voltage	480 V
Phase	Balanced three-phase
Transformer impedance	5.75%Z

The base current is:

\(I_\text{base}= \frac{75{,}000}{\sqrt{3}\times480} =90.211\text{ A}\)

The base impedance is:

\(Z_\text{base}= \frac{480^2}{75{,}000} =3.072\text{ ohm}\)

The per-unit transformer impedance is:

\(Z_\text{pu}=\frac{5.75}{100}=0.0575\text{ pu}\)

The transformer impedance in ohms is:

\(Z_\text{transformer}=0.0575\times3.072 =0.1766\text{ ohm}\)

The transformer-limited current is:

\(I_\text{transformer-limited}= \frac{90.211}{0.0575} =1568.8866\text{ A}\)

Result: 1,568.8866 A transformer-limited current at the transformer secondary, based on the entered 75 kVA, 480 V, balanced three-phase, and 5.75%Z values.

Field Verification

The calculated Transformer-limited current is located at the transformer secondary terminals in the calculator’s simplified impedance model. Fault current at a downstream panel, disconnect, motor controller, or branch-circuit load will generally change when secondary feeder conductors, raceway arrangement, conductor material, conductor size, length, splices, terminations, and other impedance sources are included.

A complete available-fault-current and equipment-rating review may also require:

• Actual upstream source and transformer configuration
• Utility or service-source impedance
• Primary protective-device and conductor information
• Secondary conductor length, material, AWG or kcmil size, and installation path
• Parallel conductors and raceway routing
• Motor contribution where applicable
• Equipment short-circuit current rating and overcurrent-device interrupting rating
• Project-specific requirements of the AHJ

Do not use transformer-limited current alone to establish equipment suitability or a final fault-current value at remote distribution equipment. Verify the final installation conditions, manufacturer data, and applicable code requirements separately.