Available Fault Current and SCCR Screen
Estimate symmetrical bolted-fault current at transformer terminals and downstream equipment, then compare the result separately with protective-device interrupting rating and assembly SCCR.
Source and transformer
Infinite-bus mode sets upstream source impedance to zero, producing a conservative higher-current estimate.
Use the transformer nameplate impedance.
Use manufacturer data when available. The example value must be replaced with project data.
Conductor path to equipment
Enter the AC resistance for one conductor at the applicable temperature.
Use data for the actual conductor arrangement and raceway.
Use the fuse or circuit breaker's marked interrupting rating at the system voltage. Enter 0 to skip the AIC comparison.
Use the assembly's marked SCCR at the system voltage. Enter 0 to skip the comparison.
Estimated available fault current
About this calculator
Estimate symmetrical bolted-fault current at transformer secondary terminals and after an entered conductor path. The screen combines upstream source impedance, transformer impedance, and conductor resistance and reactance.
It compares the equipment-point estimate separately with a protective device's interrupting rating and an equipment assembly's short-circuit current rating (SCCR). AIC and SCCR are different ratings and are not interchangeable.
Inputs and assumptions
Use the transformer nameplate kVA, winding voltages, impedance, and X/R ratio. If utility fault data is available, enter symmetrical RMS fault current and X/R at the transformer primary. The conductor values must match the installed material, temperature, raceway arrangement, one-way length, and parallel-set count.
The prefilled numbers are an example, not a default installation. Replace them with utility, transformer, conductor, and equipment data before using the result. Infinite-bus mode assumes zero upstream impedance and reports a higher screening current.
Method and assumptions
For known utility data, the calculator derives the source equivalent impedance at the transformer primary from the entered fault current, splits it into resistance and reactance using X/R, and reflects it to the secondary using the winding-voltage ratio squared.
Transformer impedance magnitude is the nameplate percent impedance multiplied by the base impedance V²/kVA. The X/R ratio separates transformer impedance into resistance and reactance. Source, transformer, and conductor impedances are then added as series complex impedances.
For single-phase faults, select a full-winding line-to-line fault or a center-tapped half-winding line-to-neutral fault. Both use a two-conductor loop; the half-winding option adjusts transformer resistance by 1.5 and reactance by 1.2. For a balanced three-phase fault, the calculator uses line-to-line voltage, the √3 relationship, and one conductor path per phase. Parallel conductor sets divide conductor impedance equally.
The result is a symmetrical RMS bolted-fault estimate at the entered location. It does not account for arc impedance, line-to-ground faults on three-phase systems, motor/generator backfeed, equipment tolerance, switching states, or protective-device clearing time.
Example
The example inputs use a 75 kVA, 480 V to 208 V, three-phase transformer with 5.75% impedance, a 10 kA utility source at the primary, and 100 ft of entered conductor path. The calculated transformer-terminal fault current is about 3.13 kA; after the conductor impedance, the equipment-point estimate is about 2.13 kA.
With the example ratings, a 3 kA protective-device interrupting rating exceeds the estimate, while a 2 kA assembly SCCR is below it. This comparison illustrates the arithmetic only; replace all example ratings and source data with equipment-specific values.
Interpret the results
Use the transformer-terminal value to understand the source available at the secondary. Use the equipment-point value for a location after the entered conductor path. A longer path generally adds impedance and lowers the calculated current; more parallel sets reduce conductor impedance.
The two rating comparisons are independent. A protective device can have an interrupting rating above available fault current while the assembly SCCR remains below it, or the reverse. Confirm the complete equipment listing, overcurrent-device coordination, system grounding, utility data, and adopted requirements with the responsible engineer or authority.
FAQs
What should I enter for utility fault current?
Enter the available symmetrical fault current at the transformer primary at the listed primary voltage. Use the utility-provided value and source X/R ratio for the actual service.
What does infinite-bus source mean?
It sets upstream source impedance to zero. This produces a higher fault-current estimate than a finite source and can serve as a conservative screening case for equipment ratings. It is not a utility study.
What is the difference between AIC and SCCR?
The protective-device interrupting rating applies to a fuse or circuit breaker. Assembly SCCR applies to the listed equipment assembly. They are separate ratings, so the calculator compares them independently. Use the marked ratings for the system voltage and the exact device or assembly.
How should I model a 120/240 V single-phase secondary?
For a line-to-line fault across the full 240 V winding, select full winding and enter 240 V. For a line-to-neutral fault on a center-tapped 120 V half winding, select half winding and enter 120 V. This does not model a line-to-ground fault on a three-phase system.
How should I enter conductor impedance?
Enter AC resistance and reactance per 1,000 ft for one conductor at the applicable operating temperature and actual raceway arrangement. The calculation doubles the one-way length for a single-phase loop and divides conductor impedance by the number of parallel sets.
Does a passing comparison establish code compliance?
No. The estimate depends on the source, transformer, and conductor data entered. It does not verify the equipment listing, system grounding, protective-device coordination, series ratings, utility requirements, or every applicable installation rule.
Does this calculate arc-flash energy or ground-fault current?
No. It calculates a symmetrical RMS bolted fault with a series-impedance model. It does not calculate arc-flash incident energy, ground faults, motor or generator contribution, asymmetrical peak current, or a complete short-circuit study.
Sources
- NFPA 70, National Electrical Code, 2026 edition — Sections 110.9, 110.10, and 110.24 address interrupting ratings, short-circuit current ratings, and available-fault-current marking. Confirm the adopted edition and local amendments.
- Eaton, Basics of Power System Design — short-circuit current calculations, including the resistance and reactance adjustments for a center-tapped transformer's half winding.
- NFPA Code-Making Panel 1 Second Draft Report, preliminary report — public code-development material for equipment short-circuit and available fault-current provisions; it is not a substitute for the adopted final text.
- Obtain utility fault data and manufacturer transformer, conductor, overcurrent-device, and assembly ratings for the actual installation.