Available Fault Current Calculator
Use this tool to accurately calculate a transformer's secondary fault current.
How to use the fault current calculator
Three phase formula
Single phase formula
Calculator variables & definitions
Below are the definitions for the different terms that this calculator uses
What is Voltage?
Electric potential, measured in volts (V), represents the amount of electrical potential energy per unit charge at a specific point in the circuit. Voltage is the driving force that causes electric charges to move within a circuit.
What is kVA?
kVA stands for kilovolt-ampere, which is a unit of apparent power in an electrical system. It measures the total power, both real and reactive, in an alternating current (AC) circuit. The symbol for kilovolt-ampere is "kVA."
What are Amps?
Amps, short for amperes, is a unit of electric current. It is a measure of the rate at which electric charges flow through a conductor in an electrical circuit.
What is a transformer?
A transformer is an electrical device that takes a given input voltage and changes it to a different output voltage. This change can either be an increase or a decrease in voltage.
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When to use the Available Fault Current Calculator
Common applications for our Available Fault Current Calculator:
• Sizing circuit breakers and switchgear for new installations
• Verifying protective device interrupting ratings during electrical system upgrades
• Completing NEC-required fault current labels on service equipment
• Evaluating existing systems after adding load or changing transformer capacity
Need help with complex fault current scenarios? Our application engineers can assist with multi-source systems, high-fault applications, or custom transformer impedance specifications.
Sizing circuit breakers and switchgear for new installations
Verifying protective device interrupting ratings during electrical system upgrades
Completing NEC-required fault current labels on service equipment
Evaluating existing systems after adding load or changing transformer capacity
What to do with your fault current calculation
You'll use your available fault current calculations in several key areas:
• Match interrupting ratings: Your circuit breakers, fuses, and switchgear must have an interrupting rating, measured in kAIC (kiloamperes interrupting capacity), that exceeds the available fault current.
• Specify protective devices: Your protective devices should generally have interrupting ratings 20-25% above calculated fault current to account for utility system changes, measurement tolerances, and future modifications.
• Account for transformer impedance: Lower-impedance transformers deliver higher fault current. Custom impedance specifications can help manage fault current levels.
• Coordinate protection: Your transformer, switchboard, and feeder breakers must be properly coordinated so that faults clear at the appropriate level without nuisance tripping or cascading failures.
• Adhere to label requirements: NEC 110.24 requires the maximum available fault current to be field-marked at service equipment.
Working on a high-fault application? Giga's engineering team can help. Schedule a consultation to discuss your specific project requirements.
Match interrupting ratings: Your circuit breakers, fuses, and switchgear must have an interrupting rating, measured in kAIC, that exceeds the available fault current.
Specify protective devices: Your protective devices should generally have interrupting ratings 20-25% above calculated fault current to account for utility system changes, measurement tolerances, and future modifications.
Account for transformer impedance: Lower-impedance transformers deliver higher fault current. Custom impedance specifications can help manage fault current levels.
Coordinate protection: Your transformer, switchboard, and feeder breakers must be properly coordinated so that faults clear at the appropriate level without nuisance tripping or cascading failures.
Adhere to label requirements: NEC 110.24 requires the maximum available fault current to be field-marked at service equipment.
What else affects available fault current?
The Available Fault Calculator focuses on transformer-limited fault current, but real-world systems have additional variables:
• Utility contribution
• Cable impedance
• System voltage
• Parallel transformers
Need a more advanced fault current study for your build? Our engineering team can recommend transformer specifications that work for your application. Get in touch for project-specific guidance.
Utility contribution
Cable impedance
System voltage
Parallel transformers
Available fault current FAQs
What's the difference between available fault current and full-load current?
Available fault current is the maximum current during a short-circuit condition. Full-load current is your unit's normal operating current.
Do I need to recalculate fault current after system changes?
Yes. Whenever you change transformer capacity, add parallel sources, or modify the system configuration, recalculate available fault current and update your labels.
Is this calculator suitable for utility-side fault current?
No. This calculator determines transformer secondary fault current only. For primary-side or utility contribution calculations, contact your utility provider.
Can I reduce fault current if it's too high for my equipment?
Yes. You can reduce fault current by specifying higher-impedance transformers, selecting switchgear and breakers with higher interrupting ratings, or taking a number of other steps. Our engineering team can help you evaluate trade-offs for your specific application.