Ground Fault Current Calculator

Inputs

System type

Uses phase voltage for phase-to-ground faults.

Line-to-line voltage (V)

Used for three-phase systems.

Line-to-neutral voltage (V)

Used for single-phase systems.

Source

Source input method

Pick one method based on available data.

Available short-circuit current (A)

Commonly provided by utility or study.

Short-circuit MVA

Converted to an equivalent impedance.

Equivalent source impedance (mΩ)

Use when an Ohmic value is known.

Transformer

Include transformer impedance

Disable if already included in source data.

Transformer rating (kVA)

Nameplate kVA.

Transformer voltage (L-L V)

Voltage where %Z applies.

Transformer impedance (%Z)

Typical distribution values: 4–7%.

Transformer X/R (estimate)

Used to split R and X from %Z.

Feeder and return path

Include conductor loop

Disable for bus faults at the source.

Conductor material

Conductor size

Values are typical; verify for your installation.

Length

One-way length to fault location.

Length units

Loop multiplier

Typical: 2.0 for out-and-back path.

Use custom R and X

Override the table with your own data.

Custom R (Ω per 1000 ft)

Custom X (Ω per 1000 ft)

Notes (optional)

Reset

Example data table

Scenario	Voltage (L-L)	Source (A)	Transformer	Feeder	Estimated Fault Current
Main distribution	480 V	10,000 A	500 kVA, 5.75%	150 ft, 4/0 copper, loop 2.0	Calculated by the tool
Long feeder run	480 V	10,000 A	500 kVA, 5.75%	450 ft, 2/0 aluminum, loop 2.0	Lower due to higher loop impedance
Near-source fault	480 V	15,000 A	500 kVA, 5.75%	50 ft, 4/0 copper, loop 2.0	Higher due to lower conductor impedance

Formula used

The calculator estimates phase-to-ground fault current using an impedance model:

Phase voltage: V_ph = V_LL / √3 (three-phase) or V_ph = V_LN (single-phase).
Total impedance: Z = (R_src + R_tx + R_loop) + j(X_src + X_tx + X_loop).
Magnitude: |Z| = √(R² + X²).
Fault current: I_gf = V_ph / |Z|.
Transformer impedance: Z_tx = (V² / S) × (%Z / 100), where V is L-L voltage and S is in VA.
Conductor loop: R_loop = R′ × (L/1000) × M, X_loop = X′ × (L/1000) × M.

This tool provides an engineering estimate; confirm final protective device settings with project standards and applicable electrical codes.

How to use this calculator

Select the system type and enter the appropriate voltage.
Choose a source input method and fill in that single value.
If the transformer is part of the fault loop, enable it and enter kVA and %Z.
Enable the conductor loop for faults away from the source, then enter length and size.
Adjust the loop multiplier to reflect the return path and bonding.
Click Calculate to view results immediately under the header.
Use Download CSV or Download PDF to save outputs.

Technical article

1) Purpose of ground fault current estimates

Ground faults are among the most frequent electrical incidents on active jobsites. Knowing the expected fault current helps teams select protective devices, verify trip sensitivity, and plan safe work boundaries around temporary power panels, distribution gear, and equipment feeds.

2) What this calculator models

This tool uses an impedance approach: source impedance, optional transformer impedance, and an optional conductor loop are combined to form a total R and X. The phase-to-ground fault current is then estimated from the phase voltage divided by the impedance magnitude.

3) Voltage selection for common construction systems

For three-phase systems, the phase voltage is computed from the line-to-line value (V_LL/√3). Typical temporary distribution may use 208 V or 480 V line-to-line, while single-phase receptacle circuits are often evaluated using line-to-neutral voltage such as 120 V.

4) Source data: three practical input methods

If a utility or study provides an available short-circuit current, the calculator converts it into an equivalent source impedance. If short-circuit MVA is available, it is converted to current and then impedance. If an equivalent impedance is known, it can be entered directly for quick checks.

5) Transformer impact on fault current

Transformer percent impedance (%Z) often governs fault levels on the secondary side. The tool forms Z_tx from the transformer voltage and kVA base. Higher %Z increases impedance and reduces fault current, which can affect coordination and ground-fault protection pickup settings.

6) Feeder and return-path effects

Longer runs increase loop impedance and reduce ground fault current, sometimes enough to delay clearing. The loop multiplier lets you represent the full return path through the equipment grounding conductor, metallic raceway, or bonding network. Use custom R and X when project-specific cable data is available.

7) Reading the output like an engineer

Review both the estimated current and the R/X breakdown. High X/R indicates a more inductive loop and can influence peak asymmetrical current in detailed studies. For field decisions, focus on whether the calculated current comfortably exceeds the protective device’s instantaneous or ground-fault pickup.

8) Applying results to safer site practices

Use conservative assumptions when the installation details are uncertain, document the chosen inputs, and export the report for submittals. Re-run scenarios when changing transformer sizes, relocating panels, or extending feeders. Always validate final settings and labeling against the governing electrical code and project requirements. Record revisions with dates and responsible reviewers.

FAQs

1) Is this the same as bolted three-phase fault current?

No. This calculator estimates phase-to-ground fault current using phase voltage and loop impedance. A three-phase bolted fault uses line-to-line relationships and different fault paths, often producing a higher current.

2) When should I disable the transformer section?

Disable it when your source information already reflects the transformer secondary, such as a short-circuit study value at the panel. Leaving it enabled in that case can double-count impedance and understate current.

3) What loop multiplier should I use?

Use 2.0 for an out-and-back path when the return impedance is similar to the phase conductor path. Increase it if the return path is longer or less conductive. Decrease it only with verified measurements.

4) Why does a longer feeder reduce fault current?

Conductor resistance and reactance add impedance to the fault loop. As length increases, total impedance increases, so current drops. Lower current may affect how quickly protective devices clear the fault.

5) Are the conductor R and X values exact?

They are typical estimates intended for planning. Actual values depend on temperature, installation method, spacing, and cable construction. Use the custom R and X option when manufacturer or study data is available.

6) Can I use this for generator-fed temporary power?

Yes, if you can estimate the generator’s available fault current or equivalent impedance at the terminals. Include transformer and feeder sections as applicable, and use conservative assumptions because generator subtransient behavior varies by model.

7) What should I do if the result seems too low?

Check that voltage and units are correct, confirm whether transformer impedance was counted twice, and review feeder length and size. If uncertain, obtain a short-circuit study or field measurements before final settings.