Fault Current Calculator
Calculate available fault current with the point-to-point method, from the source through transformers to every node, and get the values you need for NEC 110.24 field marking. Subscribe for unlimited fault current calcs.
How is this tool working for you? .
Address
Job #
Source transformer
Secondary voltage configuration
Output to load
AFC on the 208V 3Ø branch (kA)
Wire run
Length (ft)
Conductor size
Material
Parallel runs
Phase conductors
Conduit
Panel 1
Type
Min. kAIC
208V 3Ø
Additional note for PDF report
Fault current summary
| Location | Length | Size | Material | Qty | Phase cond. | Conduit | AFC |
|---|---|---|---|---|---|---|---|
| Source (208V 3Ø) | — | — | — | — | — | — | |
| Wire run | — | 4/0 AWG | Al | 1 | Singles | Non-steel | — |
| Panel 1 | — | — | — | — | — | — |
Next: Finalize report
Customize a 1-page PDF with your shop info
Assumptions & Limitations
Fault current calculations are based on the point-to-point method, together with the assumptions described below. Center-tapped systems estimate L-N fault current at 1.5× the L-L value at the transformer terminals, and L-N branches model the neutral at phase-conductor impedance. Single-phase branches from three-phase systems use bolted-fault ratios: L-L legs at 86.6% of the three-phase symmetrical value, L-N legs at 100% per the near-transformer assumption; actual L-N bolted faults normally fall between 25% and 125% of the three-phase value. Calculations assume nominal utility voltage and ungrounded delta windings; corner-grounded delta is not modeled, and motor contribution is excluded. Where source available fault current is calculated rather than user-specified, the utility source is assumed to have zero source impedance (an infinite bus).
Kopperfield is not a licensed engineering firm. This tool is intended to assist in the preparation of electrical documentation. Where required by applicable law or regulation, final engineering review and approval remain the responsibility of the licensed electrical professional.
Printing of this page is disabled. Please proceed to the next step to access the full PDF of your fault current calculation.
What is available fault current?
Available fault current, also called available short-circuit current or prospective fault current, is the maximum current that would flow at a given point during a bolted short circuit. It's measured in amps or kA and drops as you move downstream, because every foot of conductor adds impedance. You need it to confirm your equipment can survive a fault, and to field-mark service equipment.
Start with the fault current at the source, then reduce it through each conductor run. If the utility gave you an available fault current or a meter reading, that's your starting value and use it as-is. If not, calculate it from the transformer's kVA and nameplate impedance. Kopperfield's calculator allows either input.
Full-load amps
Three-phase — FLA = (kVA × 1000) ÷ (V × 1.732)
Single-phase — FLA = (kVA × 1000) ÷ V
Fault current at the secondary
AFC = FLA ÷ (%Z ÷ 100)
Nameplate impedance carries a manufacturing tolerance, so to be conservative, use %Z × 0.90 (UL allows ±10%). On a 120/240V center-tapped secondary, run the numbers at the 240V winding — the 120V half-winding delivers 1.5× that value. On a three-phase system, a line-to-line branch (say the 480V pair of a 480/277V wye) sees 0.866× the three-phase value.
f factor
Three-phase — f = (1.732 × L × I) ÷ (C × n × V)
Single-phase — f = (2 × L × I) ÷ (C × n × V)
Multiplier and result
M = 1 ÷ (1 + f)
AFC at end of run = I × M
The factor of 2 applies to any single-phase branch — line-to-line or line-to-neutral. Repeat this for each run, feeding the result of one into the next as your new I.
L length in feet · I fault current at the start of the run · C conductor constant · n conductors per phase · V branch voltage
Where the conductor data comes from. Kopperfield's fault current calculator uses industry standard C values derived from conductor resistance and reactance published in IEEE Std 241-1990 and IEEE Std 242-1986. Pick your material, raceway, and cable type and Kopperfield looks up the number.
Downstream transformers work differently. A transformer partway down the system doesn't get the infinite-bus treatment above — its secondary fault current is limited by whatever is actually available at its primary, combined with its own impedance. The calculator allows you to specify when there is a downstream transformer and calculates accordingly.
The point-to-point method calculates fault current one segment at a time. You take the value at the start of a run, reduce it for the length, size, and material of the conductor, and get the value at the end. Repeat for every segment and you have a number at every point. It's the standard field method, and it's what Kopperfield's calculator runs.
Any time you're installing or modifying equipment that has to interrupt or withstand a fault. In practice that covers most commercial work and some residential work, plus any job where an inspector asks for the number or the service equipment needs a label.
- Installing a new service, panel, or switchboard
- Upgrading an existing service or swapping the transformer
- Adding significant load to an existing system
- Specifying breakers, fuses, or switchgear for a project
- Field-marking service equipment to meet NEC 110.24
- Responding to a plan reviewer or inspector asking for the value
The number is only useful once you compare it against your equipment's rating: the AIC or the SCCR, depending on the equipment type. The rating has to be at least as high as the available fault current at that point. If it falls short, the equipment isn't suitable for that location.
| Rating | What it is | Where to find it | Code |
|---|---|---|---|
| AIC — Interrupting rating | How much fault current a breaker or fuse can safely interrupt | Breaker or fuse nameplate, at the circuit voltage | NEC 110.9 |
| SCCR — Short-circuit current rating | How much fault current a panelboard, switchboard, or assembly can withstand | Equipment nameplate or listing label | NEC 110.10 |
- Calculate the available fault current at that point off your calculation.
- Compare the AIC or SCCR to the rating on the equipment and confirm the equipment can support the same or greater fault current.
- Repeat at every point where equipment is installed.
If the available fault current is higher than the rating, the equipment isn't rated for that location. You can specify gear with a higher AIC or SCCR, use a series-rated combination listed for the application, or bring the available fault current down with a higher-impedance transformer or a longer or smaller feeder. Which route makes sense depends on the job. High-fault and multi-source systems generally warrant an engineered short-circuit study.
The calculator provides the available fault current at each point. Verifying it against your equipment ratings remains your responsibility.
NEC 110.24 requires service equipment in other than dwelling units to be legibly field-marked with the maximum available fault current. The marking has to include the date the calculation was performed and be durable enough for the environment. Kopperfield's report prints the per-point value and the date, so you can create the label easily.
What goes on the label
- Maximum available fault current at the service equipment
- Date the calculation was performed
When it has to be redone. A transformer swap, a service upgrade, or a change on the utility side can all change the maximum available fault current. Any of them triggers a re-verification or recalculation.
