A short circuit study calculates the maximum fault current available at each bus and piece of equipment in an electrical system. That number tells you two things: whether your breakers and fuses can safely interrupt a fault, and how much current the arc flash analysis needs to calculate incident energy.
You cannot run an arc flash analysis without short circuit study results. The IEEE 1584 2018 methodology requires available bolted fault current as a direct input to the incident energy calculation. It is not an estimate or a default value. It is an engineering calculation that depends on the actual characteristics of your system: the utility service, the transformer, the cables, and the equipment between them.
Short circuit studies are almost always performed alongside electrical coordination studies and arc flash studies. The system model built for the short circuit study is the same model used for the coordination study and the arc flash calculations. You build it once and run all three analyses.
What a short circuit study calculates
The study calculates available fault current at every bus in the system under worst-case fault conditions. The result is reported in amperes or kiloamperes (kA) at each location. That value decreases as you move further from the source transformer, because cable impedance reduces the current available at each downstream point.
At the transformer secondary, fault current can be extremely high. A 1500 kVA transformer at 480V with 5.75% impedance can deliver over 35,000 amperes of three-phase fault current at the secondary terminals. By the time that fault current has traveled through 200 feet of cable and a panel main, it may be 20,000 amperes at the panel bus. A branch circuit 100 feet further may see 15,000 amperes.
Each of those values matters. The breaker or fuse protecting each location must be rated to interrupt the available fault current at that point. And the arc flash analysis needs the fault current value at each specific bus to calculate incident energy accurately.
Types of faults analyzed
| Fault type | Description | Primary use in the study |
|---|---|---|
| Three-phase bolted fault | All three phases shorted together with zero impedance | Maximum fault current; worst case for equipment interrupting ratings and arc flash calculations |
| Line-to-line fault | Two phases shorted together | Common in cable insulation failures; typically about 87% of three-phase fault current |
| Line-to-ground fault | One phase shorted to ground | Most frequent fault type in practice; magnitude depends on system grounding |
| Double line-to-ground fault | Two phases shorted to ground simultaneously | Used in relay coordination and ground protection analysis |
The three-phase bolted fault produces the highest fault current in most systems. It is the primary value used for equipment interrupting rating checks and the primary input for IEEE 1584 arc flash calculations. Line-to-ground fault analysis is critical for ground fault protection coordination, particularly on high-resistance grounded or resistance grounded systems where ground fault currents behave differently.
Why fault current matters for arc flash
The IEEE 1584 arc flash calculation is a function of arcing fault current, the gap between conductors, the equipment type, the working distance, and the time the protective device takes to clear the fault. Arcing fault current is derived directly from available bolted fault current using empirical equations in the standard.
Higher available fault current generally means more energy available to sustain an arc. But the relationship is not linear, and clearing time matters just as much. A high-fault-current bus protected by a current-limiting fuse that clears in a fraction of a cycle can have lower incident energy than a lower-fault-current bus protected by a slow-clearing breaker.
Arc flash and fault current: More available fault current does not always mean higher incident energy. Current-limiting protective devices can use high available fault current to their advantage, clearing faster as current rises. The combination of fault current magnitude and protective device clearing time determines incident energy, not either factor alone.
This is why the short circuit study, the coordination study, and the arc flash analysis must be done together. Using assumed or default fault current values in the arc flash calculation produces results you cannot rely on.
How fault current varies through the system
Transformer impedance
Transformer impedance is the single largest factor controlling fault current at the transformer secondary. A transformer with 5.75% impedance limits fault current at its secondary terminals to approximately 17 times its full-load current. A transformer with 4% impedance allows nearly 25 times full-load current through under a bolted fault. Getting the actual impedance from the nameplate, not from a table of typical values, is essential. Impedance varies by manufacturer and vintage.
Cable impedance
Every foot of cable adds resistance and reactance that reduces available fault current downstream. Long cable runs in large industrial facilities can cut available fault current significantly compared to what is available at the transformer secondary. The study model needs accurate cable data: conductor size, material (copper or aluminum), length, and number of conductors per phase. These are field measurements, not design assumptions.
Motor contributions
Motors connected to the system contribute to fault current for the first few cycles after a fault occurs. They act briefly like generators. On systems with large motor loads, motor contribution can add meaningfully to the available fault current at certain buses. The short circuit study accounts for this when motor data is available. For arc flash analysis, motor contributions increase the arcing current used in the calculation.
Equipment interrupting ratings
Every circuit breaker and fuse in the system has an interrupting rating, expressed in kiloamperes of symmetrical current (kAIC) at a specific voltage. That rating is the maximum fault current the device is tested to safely interrupt. If the available fault current at a device's location exceeds its interrupting rating, the device can fail catastrophically during a fault. It may not open. The arc continues. The equipment can explode.
The short circuit study compares available fault current at each location against the interrupting rating of the protective device at that location. Devices found to be underrated require replacement. This is a separate finding from the arc flash study but often surfaces at the same time.
ANSI/IEEE C37 standards govern interrupting ratings for circuit breakers. The rating must be checked at the actual system voltage, not at the nominal voltage on the device nameplate, because fault current changes with voltage.
Relationship between short circuit and coordination studies
The short circuit study and the coordination study use the same system model. The short circuit study calculates fault current at each bus. The coordination study uses those fault current values to plot time-current curves and verify that protective devices trip in the correct sequence.
You need the short circuit study results to draw the coordination plots accurately. The minimum fault current at the end of a long feeder and the maximum fault current at the transformer secondary bracket the range of currents the coordination study must evaluate. Without those numbers, the TCC plots are drawn at assumed currents, and the coordination conclusions may not hold in the real system.
Data needed for a short circuit study
The short circuit study model requires specific field data. Assumed or typical values produce unreliable results.
- Utility available fault current at the service entrance: this comes from the serving utility and should be confirmed in writing. Most utilities will provide it as a maximum and minimum value.
- Transformer data: kVA rating, primary and secondary voltage, impedance percentage, connection type (delta-wye or other), and whether the neutral is solidly grounded or resistance grounded
- Cable data: conductor size in AWG or kcmil, conductor material (copper or aluminum), conduit type (magnetic or non-magnetic), length of each run, and number of conductors per phase
- Bus and panel identifications: every location in the system where fault current will be reported needs a unique identifier that matches the one-line diagram
- Motor data: nameplate kW or HP, voltage, and full-load amperes for significant motor loads
- Breaker interrupting ratings: frame size, trip rating, and listed kAIC rating from the nameplate or manufacturer data
Download the free arc flash field data collection checklist for a complete data point list organized by equipment type.
How 70Ez speeds up data collection for short circuit studies
Transformer nameplate data and cable information are the two data sets that take the most field time in a short circuit study. Transformer nameplates are dense with information: kVA, voltage, impedance, connection type, manufacturer, and serial number. Cable data means measuring run lengths, reading conductor sizes from panelboard directories or cable tags, and verifying conductor material.
70Ez gives field technicians a faster way to capture nameplate data. The AI reads transformer nameplates directly from photographs, pulls the impedance, voltage, and rating data, and organizes it by project. Technicians verify and export the data into SKM PowerTools, ETAP, or EasyPower without re-keying every field.
See how arc flash data collection works for the full field-to-export workflow. The data requirements for a short circuit study and an arc flash study overlap almost entirely, which is why collecting both at once in the field saves significant time.
Frequently asked questions
Is a short circuit study the same as an arc flash study?
No. A short circuit study calculates available fault current at each bus. An arc flash study uses those fault current values, along with protective device clearing times, to calculate incident energy. The short circuit study is a required input to the arc flash analysis. They are separate calculations that are almost always performed together using the same system model.
Can I use manufacturer-published typical impedance values for transformers?
Using typical values introduces error into the fault current calculation and, by extension, the arc flash analysis. Impedance varies between units of the same size and model. The nameplate impedance is the correct value to use. If the nameplate is inaccessible or unreadable, the transformer manufacturer may be able to provide the factory test report for the specific unit by serial number.
How does a short circuit study affect equipment replacement decisions?
When the study finds breakers or fuses with interrupting ratings below the available fault current at their location, those devices must be replaced. The study report identifies each underrated device. Facilities sometimes discover underrated equipment that has been in service for years. Addressing those findings is a separate scope from the arc flash study but is commonly done at the same time.
How often should a short circuit study be updated?
The NFPA 70E requirement to review arc flash studies every five years applies to the short circuit study underlying them. Any change that affects available fault current, including a new transformer, a change in utility service, or new cables of different size or length, requires a review of the short circuit study results.
What software is used to run short circuit studies?
Short circuit studies are performed using power system analysis software. The three most widely used platforms in North America are SKM PowerTools, ETAP, and EasyPower. All three perform ANSI-method short circuit calculations and produce the bus fault current values needed for arc flash analysis.