EleCalculator

Safety & Protection calculator

Short Circuit Current Calculator

This short circuit current calculator is a U.S.-focused available-fault-current screen. Enter transformer kVA, system voltage, percent impedance, feeder data, or known source impedance to estimate symmetrical RMS fault current for the point you are reviewing. The result panel reports available fault current in amperes and kA, equivalent impedance, and fault MVA so you can compare the project condition with breaker interrupting ratings, equipment SCCR labels, and coordination assumptions. The page is intentionally scoped to screening current duty and SCCR review, not to replace a full coordination or arc-flash study.

Updated July 16, 2026

Enter transformer kVA, system voltage, percent impedance, feeder data, or known impedance to screen available fault current at the review point.

Transformer screen: calculate full-load current, divide by per-unit impedance, then compare the result panel with SCCR and interrupting ratings.

Choose transformer, feeder, or known-impedance mode below to screen available fault current before SCCR, interrupting-rating, or coordination review

Calculator Inputs

Field notes

Calculation Results

Enter values above to see calculation results

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Calculation history

Example Calculations

1500 kVA transformer at 480VScreen the available 3-phase fault current at the transformer secondary terminals.InputsCalculation Mode: Transformer secondary terminalsSystem Voltage: 480Transformer Rating: 1500Transformer Impedance: 5.75
480V panel 100 ft from the transformerAdd feeder impedance to estimate the available fault current at a remote panel fed by one 4/0 copper conductor per phase.InputsCalculation Mode: Feeder end from transformerSystem Voltage: 480Transformer Rating: 1500Transformer Impedance: 5.75Cable Length: 100Cable Size: 4/0Cable Material: CopperCurrent-Carrying Conductors: 1
More examples. Open to review 1 additional calculation example.
Known impedance at a 208V busUse the total source impedance directly when it has already been developed elsewhere.InputsCalculation Mode: Known total 3-phase impedanceSystem Voltage: 208Total Impedance: 0.012

How to Use

How to use the short circuit current calculator

  1. Select the calculation mode that matches the data you actually have: transformer secondary, feeder end, known 3-phase impedance, or known 1-phase loop impedance.
  2. Enter the system voltage. Use line-to-line voltage for 3-phase modes and line-to-neutral voltage for the single-phase loop mode.
  3. For transformer-based screens, enter the transformer kVA and percent impedance.
  4. For remote feeder screens, add the one-way feeder length, conductor size, material, and any parallel sets.
  5. Review the available fault current in amperes and kA, the fault level, and the equivalent impedance.
  6. Compare the result against breaker interrupting ratings, SCCR labels, and the assumptions used for the actual project.

What this calculator does well

  • It gives a quick transformer-secondary available-fault-current screen from nameplate data.
  • It shows how feeder impedance lowers the fault current at a downstream panel or equipment point.
  • It supports prospective short-circuit-current checks when a total source or loop impedance is already known.

What this calculator does not model

  • Utility source variation, motor contribution, generator contribution, or inverter-limited source behavior.
  • Project-specific X/R ratios, breaker let-through, or full protective-device coordination.
  • Arc-flash incident energy, clearing time, or a permit-level fault study package.

Quick screening references used on this page

Screen Modeled Basis Why It Matters
Transformer secondary Isc = FLA ÷ Zpu Fastest way to screen available fault current from transformer nameplate data.
Remote feeder point Transformer impedance + modeled feeder impedance Shows how conductor length and size reduce the duty seen at downstream equipment.
Known 3-phase impedance Isc = VLL ÷ (√3 × Z) Useful when a study or manufacturer already gives the total source impedance at the bus.
Known 1-phase loop Isc = VLN ÷ Zloop Helpful for branch-circuit prospective-fault-current screens when loop impedance is known.

Use the result panel for the actual transformer-secondary, feeder-end, or known-impedance result. Keep the calculated available fault current tied to the entered kVA, voltage, percent impedance, feeder length, conductor data, and source assumptions before comparing it with SCCR or interrupting ratings.

Next checks after available fault current

Common Applications

Checking whether a panel or disconnect sees 10 kA, 22 kA, 42 kA, or 65 kA class duty
Screening SCCR exposure for industrial panels, starters, and control equipment
Comparing a transformer-terminal result against a downstream feeder-end result
More applications. Open to review 2 additional use cases.
Reviewing prospective short-circuit current before breaker or fuse selection
Documenting an early-stage available-fault-current assumption before a formal study

Frequently Asked Questions

What is available fault current?
Available fault current is the maximum current a source can deliver into a fault at a specific location under the assumptions used for that location. It is the number commonly compared against breaker interrupting ratings, fuse ratings, and equipment SCCR labels.
How do you calculate transformer secondary short circuit current?
A common screening method uses transformer full-load current divided by the transformer per-unit impedance. In practical terms, first compute full-load current from transformer kVA and voltage, then divide by percent impedance expressed as a decimal. That gives a quick available-fault-current estimate at the transformer secondary terminals.
Why is the feeder-end result lower than the transformer-terminal result?
Because conductor resistance and reactance add impedance between the transformer and the faulted point. As total impedance increases, available fault current drops. That is why the downstream panel often sees less duty than the transformer secondary lugs.
Does this calculator give me the SCCR of the equipment?
No. SCCR is an equipment rating assigned by design, testing, or component combination rules. This calculator estimates the available fault current you must compare against that rating. If the available current exceeds the equipment SCCR, the design needs correction.
Can I use this result directly for arc-flash labeling?
Not by itself. Fault current is one important arc-flash input, but clearing time, equipment type, working distance, protective-device settings, and the project study method all matter. Use this tool only as a screening step before a project-specific arc-flash review.

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