Free tool · Electrical
Available Fault Current Calculator
Enter the utility’s available fault current and the transformer, then add each panel and say what feeds it: the transformer or another panel. For every feeder choose the wire size, copper or aluminum, the sets in parallel, the length and the conduit. If a panel feeds motors, add them by horsepower and their contribution to a fault is included. The calculator shows the available fault current at every panel, draws a one-line diagram with the wire and conduit on each feeder, and prints a technical report for an engineer.
Type the interrupting (AIC) rating from a panel’s label and it tells you whether the available current is above it. It is an estimate for review by a qualified engineer, not an approval.
The numbers stay in your browser: we do not store or receive them.
Start with the utility and the transformer, then add each panel and say what feeds it.
Utility
Transformer
Enter the utility available fault current at the transformer primary, in amps.
Panels
Feeder to this panel
Motors on this panel (optional)
Enter the feeder length in feet.
One-line diagram
Complete the utility, the transformer and at least one panel to print.
An estimate for review by a qualified engineer, not an approval. Being below a rating does not make a design safe. The rating comes from the equipment’s own label.
Why the utility figure is an input
The fault current at a panel depends on how strong the utility source is, and only the utility can tell you that. Ask for the available fault current at the transformer primary and enter it. Without it the answer would have to assume an infinitely strong source and would overstate every number. The calculator also shows what the transformer alone would give on an infinite source, so you can see the difference.
The method
Utility: impedance = primary voltage ÷ (1.732 × utility fault current), referred to the secondary side by the square of the voltage ratio.
Transformer: impedance = percent impedance ÷ 100 × secondary voltage² ÷ (kVA × 1,000).
Feeder: resistance and reactance per 1,000 ft × length ÷ 1,000 ÷ sets in parallel.
At a panel: fault current = secondary voltage ÷ (1.732 × the combined impedance from the source to that panel). Impedances are added as complex numbers (resistance plus reactance), and a panel fed from another panel carries every feeder above it.
A worked example
A 500 kVA, 480 V transformer with 5.75 percent impedance on a 50,000 A, 12,470 V utility gives 10,376 A at its secondary (10,459 A if the source were infinitely strong).
A main panel fed by three sets of 4/0 copper in steel conduit, 60 ft, sees 9,985 A. A subpanel off it through 100 ft of 1/0 copper in PVC sees 8,142 A, and one through 300 ft of 6 AWG copper in steel sees 2,086 A. A distribution panel fed from the transformer by two sets of 500 kcmil aluminum in steel, 40 ft, sees 10,007 A. If its label says 10,000 A, the available current is above the rating.
Press Try an example to load these numbers.
Motors
A motor that is running when a fault happens feeds current back into it for the first few cycles. Add the motors on each panel by horsepower. The calculator looks up the full-load amps in the NEC table for the system voltage (1/2 to 200 hp at 208, 240, 480 and 600 V), or you can enter the nameplate amps instead. An induction motor is taken as feeding in 4 times its full-load amps and a synchronous motor 5 times, and you can change both multiples. The motor option is for three-phase systems.
The utility and every motor group are solved together as one network, so a motor on a subpanel also adds to the main panel through the feeder between them. The report shows every result with and without the motors.
For example, two 50 hp induction motors (65 A each, about 520 A into a fault) on the main panel raise it from 9,985 A to 10,505 A, and the distribution panel on the other side of the transformer from 10,007 A to 10,489 A.
The NEC table is NFPA’s published data and the 4 and 5 multiples are common working values read from secondary guidance, so check both against the standards for a formal study.
Where the wire data come from
Resistance and reactance for 8 AWG to 1000 kcmil copper and 6 AWG to 1000 kcmil aluminum are Southwire’s own published values for THHN/THWN-2 conductors: SPEC 10000 (copper) and SPEC 10012 (aluminum). Southwire’s reactance is for non-magnetic conduit with one conductor diameter between centers.
Steel conduit adds reactance. Comparing the steel-conduit column of NEC Chapter 9 Table 9 with Southwire’s values gives a ratio of 1.23 to 1.27 at 4 AWG, 2 AWG, 1/0, 250 kcmil and 500 kcmil, so the calculator multiplies the reactance by 1.25 for steel conduit. By default the conductor resistance is the cold, 25 °C value, which gives the higher and more cautious fault current. You can switch to the 75 °C value.
What this does not do
It does not include bus and connection impedance, fuses or breakers that limit the current, series-rated combinations, arcing or ground faults, or a voltage change other than the one transformer. It does not calculate arc-flash incident energy, which is a job for an engineer. Single phase is line to line only; on a center-tapped transformer the line-to-neutral fault can be higher. For other cable types use the cable maker’s data.
The comparison with a rating is arithmetic. The rating comes from the equipment’s own label or listing, and being below it is not an approval.
References
The printed report lists every formula with the numbers substituted, and cites these sources:
• Southwire, SIMpull THHN/THWN-2 Copper, SPEC 10000, and SIMpull THHN Aluminum, SPEC 10012: electrical properties tables (resistance and reactance per 1,000 ft).
• NFPA 70, National Electrical Code, Chapter 9, Table 9: steel-conduit reactance, compared with Southwire’s non-magnetic values to get the 1.25 factor.
• NFPA 70, National Electrical Code, 110.9 and 110.10: interrupting rating and short-circuit current ratings of equipment.
• IEEE Std 551 (Violet Book), IEEE Std 141 (Red Book) and IEEE Std 399 (Brown Book): the impedance method of calculating short-circuit current. They cover far more than this calculator does and should be consulted for a full study.
• NFPA 70, National Electrical Code, Table 430.250: full-load current of three-phase induction motors, 1/2 to 200 hp.
• Motor contribution multiples, 4 times full-load amps for induction and 5 times for synchronous motors: common working values in low-voltage short-circuit practice.
FAQ
What does AIC mean?
AIC is the amps interrupting capacity: the highest fault current a breaker or fuse is rated to interrupt. The available fault current where it is installed must not be higher than that rating. The National Electrical Code covers this in 110.9 and 110.10.
Where do I get the utility fault current?
Ask the utility for the available fault current at the transformer primary. It is usually given in amps or in MVA. If it is MVA, the current in kA is MVA ÷ (1.732 × the line voltage in kV).
Why does steel conduit change the answer?
Steel is magnetic, so it adds reactance to the feeder. More impedance means less fault current at the panel.
Does it calculate arc flash?
No. Arc-flash incident energy needs the protective device’s clearing time and a licensed method (IEEE 1584), and the result decides what PPE people wear, so it is left to an engineer. The fault current this calculator gives is one of the inputs an engineer would use.
Do motors add to the fault current?
Yes, for the first few cycles. Add the motors on each panel by horsepower and the calculator includes their contribution and shows the result with and without them.
Does this replace a short-circuit study?
No. A full study accounts for detailed motor data, current-limiting devices and series ratings, which this quick estimate does not. This is a quick estimate, and the printed report is made to be handed to the engineer.
Is anything I type saved?
No. The calculation runs in your browser and nothing is stored or sent to us.
Pulling the feeders as well? The free wire pull tension calculator gives the pulling force for each run, and PoPs Estimating measures conduit and wire from your plans.
Questions?
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