Engineering calculators

Fault Current Calculator

Updated Sep 18, 2026 By Infinity Calculator
Rate Formulas

Simplified Fault Current — 3-Phase Transformer Secondary

Required. Nameplate rating, e.g. 45–2500 kVA. Minimum 1.
Required. Line-to-line secondary volts, e.g. 208, 240, 480. Minimum 1.
Required. Transformer percent impedance from the nameplate, typically 1.5%–6%. Minimum 0.01.
Read-only result — click to select and copy.
Step-by-Step Solution

Advanced Multi-Component System Builder

Step 1 — System Type
Controls the multiplier used in the point-to-point cascade (1.732 for three-phase, 2 for single-phase runs).
Step 2 — Available Primary Fault Current

Do you know the primary fault current available at the source? Selecting “No” assumes an infinite bus — an idealized utility source of unlimited capacity, modeled here as 99,999 A. It gives the most conservative (highest) downstream fault current.

Required. Line-to-line volts at the upstream source, e.g. 480 or 13800.
Read-only while “infinite bus” is selected. Choose “Yes” above to enter a utility-supplied value.
Step 3 — Build the Chain
Components cascade top-to-bottom; order determines the downstream fault current.
Available Fault Current at Each Node

Introduction

A fault is a short circuit. When it happens, a huge amount of current flows for a short time. This fault current calculator tells you how many amps can flow at a spot in your electrical system. You need that number to pick breakers, fuses, and panels that can safely handle the blast.

Use the simple mode to find the available fault current right at a transformer's secondary. Type in the transformer kVA, the secondary voltage, and the percent impedance (%Z) from the nameplate. You get the answer plus each step of the math.

Use the advanced builder when the fault point is farther down the line. Add cable runs, transformers, bus runs, and motors in order. The tool uses the point-to-point method to carry the current down the chain and shows the fault current at every node, with a chart and a diagram.

It works for both 3-phase and 1-phase systems. If you do not know the utility fault current, pick the infinite bus option for the safest, highest answer. Wire size, metal type, and conduit type all change the result, so you can set each one.

How to use our Fault Current Calculator

Enter your transformer size, voltage, and impedance to get the available fault current in amps. You can also build a full system with cables, a transformer, bus runs, and motors to see the fault current at every point in the chain.

3-Phase Mode / 1-Phase Mode tabs: Pick the tab that matches your system. Use 3-phase for most commercial power, and 1-phase for homes and small loads.

Live calculation switch: Leave it on to see results update as you type. Turn it off if you would rather press the Calculate button.

Transformer kVA: Type the kVA rating from the transformer nameplate, like 75 or 300.

Secondary Voltage (V): Type the voltage on the load side of the transformer, like 208, 240, or 480.

% Impedance (%Z): Type the percent impedance from the nameplate. Most transformers fall between 1.5% and 6%.

Available Fault Current (A): This box is the answer. It shows the fault current in amps at the transformer secondary. You can click it to copy the number.

Advanced System Builder

System Type: Choose Three-Phase or Single-Phase. This sets the math used for each cable and bus run.

Do you know the primary fault current: Pick "Yes" if the utility gave you a number. Pick "No" to assume an infinite bus (99,999 A), which gives the highest and safest result.

Source / Primary Voltage (V): Type the voltage at the utility or upstream source, like 480 or 13800.

Available Primary Fault Current (A): Type the amps the utility gave you. This box is locked if you chose the infinite bus.

Component to add: Pick a primary cable, transformer, secondary cable, bus run, or motor, then press Add Component. Parts stack top to bottom in the order power flows.

Length unit system: Choose feet or meters. All lengths switch over and convert for you.

Clear All: Removes every part from the builder so you can start fresh.

Cable Run Inputs

Length of Run: Type the one-way length of the wire from the last point to the fault point.

Conductors Per Phase: Type how many wires run in parallel per phase. Use 1 if there is only one set.

Conductor Size: Pick the wire size in AWG or kcmil.

Conductor Material: Pick copper or aluminum. Copper carries more fault current for the same size.

Conductor Type: Pick single conductors or a three-conductor cable.

Conduit Type: Pick steel, aluminum, or PVC. Steel pipe lowers the fault current a bit.

Transformer Inputs

Primary Voltage (V): Type the voltage feeding the transformer.

Secondary Voltage (V): Type the voltage leaving the transformer.

Transformer kVA: Type the kVA rating from the nameplate.

Impedance (%Z): Type the percent impedance from the nameplate.

Transformer X/R Ratio: Optional. Add it to see the asymmetrical fault current too.

Source Type and Secondary Winding Type (1-phase mode): Pick how the transformer is fed and how the secondary is wired. Line-to-neutral on a center-tapped unit uses a 1.5 factor.

Bus Run Inputs

Bus Impedance: Type the impedance per foot or per meter from the busway data sheet.

Bus Length: Type how long the bus run is.

Bus Voltage Rating (V): Type the voltage the busway runs at.

Motor Inputs

Motor Voltage (V): Type the motor nameplate voltage.

Phase: Pick 1-phase or 3-phase.

Motor Power Rating: Type the motor size and pick hp or kW.

Motor Power Factor: Type a decimal like 0.85, or a percent like 85.

Motor Efficiency: Type a decimal like 0.92, or a percent like 92.

Locked-Rotor Multiple: Type how many times full-load current the motor feeds into a fault. The customary value is 4.

Press Calculate System Fault Current to see the bolted fault current, a step-by-step solution, a system schematic, and a chart of the fault current at each node.

What Is Fault Current?

Fault current is the large burst of electricity that flows when a wire touches another wire or ground by mistake. It is often called short-circuit current or available fault current. Normal loads may pull 20 or 50 amps. A fault can pull 10,000 amps or more in a fraction of a second. That much power can melt metal, blow apart gear, and start a fire or an arc flash.

Why Fault Current Matters

Every breaker, fuse, and panel has a rating that tells how much fault current it can safely stop or hold. Breakers use an AIC rating (amps interrupting capacity). Panels and control gear use an SCCR rating (short-circuit current rating). If the real fault current at that spot is higher than the rating, the device can fail, explode, or weld shut instead of tripping. The NEC requires you to know the available fault current and to mark it on service equipment. So you must do the math before you pick parts.

How Fault Current Is Found

The simple way starts at a transformer. You take the full-load current on the secondary side, then divide by the percent impedance (%Z) from the nameplate. A low %Z means less push-back inside the transformer, so more fault current comes out. A 5% impedance transformer can put out about 20 times its full-load current.

  • Three-phase: full-load amps = (kVA × 1000) ÷ (1.732 × volts)
  • Single-phase: full-load amps = (kVA × 1000) ÷ volts
  • Fault current: full-load amps × (100 ÷ %Z)

The Point-to-Point Method

Real systems have wire, busway, and more than one transformer between the utility and the fault. Each piece adds impedance, so fault current drops as you move away from the source. The point-to-point method walks down the chain one step at a time. At each step it finds a factor f from the length, the conductor size, and the voltage, then a multiplier M = 1 ÷ (1 + f). Multiply the upstream current by M to get the current at the next point.

Wire size and type change the answer. Copper carries more fault current than aluminum of the same size. Steel conduit adds reactance and lowers the current a bit compared to PVC. Running two or three sets of conductors per phase raises the fault current, because parallel paths mean less impedance.

Infinite Bus

If you do not know how much fault current the utility can supply, you can assume an infinite bus. This pretends the utility has unlimited power, so the transformer impedance is the only thing holding the current back. It gives the highest, safest-to-design-for answer. If the power company gives you a real number, use it instead for a closer result.

Motor Contribution

Spinning motors act like small generators for a few cycles after a fault. They feed current back into the fault. A common rule adds about 4 times the motor full-load current. On a plant floor with many motors, this extra current can be thousands of amps, so do not skip it.

Bolted vs. Arcing Faults

A bolted fault means the conductors are solidly joined, with no arc. This gives the highest current and is the number used for picking AIC and SCCR ratings. An arcing fault has an air gap, so the current is lower but the heat and blast can be worse. Arc flash studies use the bolted value as the starting point.

X/R Ratio and Asymmetrical Current

The X/R ratio compares reactance to resistance in the circuit. A high X/R ratio means the first half cycle of the fault has a big DC offset, pushing the peak current well above the symmetrical value. That higher number is the asymmetrical RMS current. Some gear is rated for it, so check it on large services.

Good Practice

Use nameplate data whenever you can, not guesses. Recheck the numbers any time a transformer is swapped, a service is upsized, or the utility changes its supply. Label the service with the available fault current and the date, as the NEC asks. These results are estimates for planning; a licensed engineer should sign off on the final design.

Formulas used

Transformer secondary full-load current (3-phase)
I_{FLA} = \frac{kVA \times 1000}{\sqrt{3} \times V_{sec}}
Transformer secondary full-load current (1-phase)
I_{FLA} = \frac{kVA \times 1000}{V_{sec}}
Available fault current at transformer secondary terminals
I_{SC} = I_{FLA} \times \frac{100}{\%Z}
Point-to-point cable factor and multiplier
f = \frac{k \times L_{ft} \times I}{C \times n \times V}, \quad M = \frac{1}{1+f}, \quad k = \begin{cases} 1.732 & \text{3-phase} \\ 2 & \text{1-phase} \end{cases}
Fault current downstream of a cable or bus run
I_{out} = I_{in} \times M
Transformer cascade factor and secondary fault current
f = \frac{I_{pri} \times V_{pri} \times k \times \%Z}{100 \times kVA \times 1000}, \quad I_{sec} = I_{pri} \times \frac{V_{pri}}{V_{sec}} \times \frac{1}{1+f} \times w
Bus run factor from impedance per unit length
Z_{bus} = z_{\ell} \times L, \quad f = \frac{k \times I \times Z_{bus}}{V_{bus}}
Motor full-load current and fault contribution
I_{FLA} = \frac{P_{W}}{k \cdot V_{m} \cdot PF \cdot \eta}, \quad I_{total} = I_{in} + LR \times I_{FLA}

Frequently asked questions

How much fault current does a 75 kVA transformer put out?

It depends on the voltage and the nameplate impedance. For a 75 kVA, 208V three-phase unit with 5.75% impedance:

  • Full-load amps = 75,000 ÷ (1.732 × 208) = 208 A
  • Multiplier = 100 ÷ 5.75 = 17.4
  • Fault current = 208 × 17.4 = about 3,620 A

The same 75 kVA at 240V single-phase with 1.6% impedance gives about 19,500 A. Low impedance means much higher fault current.

Where do I find the percent impedance on a transformer?

Look at the metal nameplate on the side of the transformer. It is printed as %Z, %IZ, or "Impedance." Most dry-type units read between 1.5% and 6%.

If the plate is missing or unreadable, ask the maker with the serial number. Do not guess. A wrong %Z can throw the fault current off by thousands of amps.

Is a 10,000 AIC breaker enough for a house?

Usually yes, but not always. Most homes fed by a 25 kVA or 50 kVA utility transformer see under 10,000 amps at the panel.

Homes close to a large pad-mount transformer, or with short, heavy service wires, can top 10,000 amps. Then you need 22 kAIC breakers or a series-rated setup. Run the numbers before you buy.

What is the difference between AIC and SCCR?

Both are short-circuit ratings, but they cover different things.

  • AIC (amps interrupting capacity) is for one device that opens the circuit, like a breaker or fuse. It says how much current that device can safely break.
  • SCCR (short-circuit current rating) is for a whole assembly, like a panel, control panel, or motor starter. It says how much current the gear can take without becoming a hazard.

Both must be equal to or higher than the available fault current at that spot.

What happens if fault current is higher than a breaker's AIC rating?

The breaker can fail instead of tripping. Contacts may weld shut, the case can crack, and the arc can blow out of the panel. That means fire, blast, and no protection for the wire downstream.

It is also a code violation. Every device must be rated for the current available where it is installed.

Does fault current get lower the farther you are from the transformer?

Yes. Every foot of wire adds impedance, and impedance chokes the current down. The drop is biggest with small wire and long runs.

For example, 100 feet of #6 copper can cut the fault current at a subpanel by more than half. That is why you should figure the fault current at each panel, not just at the service.

How do you reduce available fault current?

You add impedance or limit the current. Common ways:

  • Use a transformer with higher percent impedance
  • Use longer or smaller feeder conductors (watch voltage drop)
  • Install current-limiting fuses or current-limiting breakers
  • Add a line reactor ahead of the gear
  • Avoid parallel conductor sets where they are not needed

Often it is cheaper to buy gear with a higher rating than to change the system.

Can you measure available fault current with a meter?

No. You cannot safely create a short circuit to measure it. Available fault current is always calculated from the utility data, the transformer nameplate, and the wire between them.

Loop impedance testers can estimate it on small circuits, but the calculated value is what the NEC and equipment labels use.

Does the NEC require a fault current label?

Yes. NEC 110.24 says service equipment in other than dwelling units must be field marked with the maximum available fault current and the date it was figured.

The label must be updated when the system changes, such as a new transformer or a bigger service, so the marked value stays correct.

What is the C value in a point-to-point fault calculation?

C is a constant for each wire size, metal, and conduit type. It rolls resistance and reactance into one number. Bigger wire and copper give a bigger C.

A bigger C means less voltage lost during the fault, so more fault current reaches the far end. For example, 4/0 copper in steel conduit has a C of about 15,082, while #2 copper is only about 5,906.

How much fault current does a generator make compared to a transformer?

Much less. A generator usually supplies about 3 to 5 times its rated full-load current into a fault, and that drops off within a few cycles.

A transformer fed by the utility can push 15 to 25 times its full-load current and hold it. So gear that is fine on generator power may still be undersized on utility power.

Is fault current higher on the primary or secondary side of a transformer?

In amps, the secondary is usually higher because the voltage is lower. A step-down transformer trades volts for amps.

But in power terms the transformer cuts the fault down hard. The impedance of the winding is the main limit, which is why %Z matters so much.

What is a series rated system?

It is a tested pair: a strong upstream breaker or fuse protects a weaker downstream breaker. The upstream device clears fast enough that the small breaker never sees the full fault.

You can only use combinations the maker has tested and listed, and the panel must be labeled with them. You cannot mix and match on your own.

How do I get the available fault current from the utility?

Call the power company and ask for the available short-circuit current at your service point. Give them the address, the service size, and the voltage.

They usually reply with a value in kA plus the X/R ratio. If they will not give a number, assume an infinite bus for a safe, high estimate.

What is the difference between fault current and full-load current?

Full-load current is what the circuit draws when everything runs normally. Fault current is what flows when a short circuit happens, and there is no load to hold it back.

Full-load current sets the wire size and breaker trip size. Fault current sets the AIC and SCCR ratings. You need both numbers to design a safe system.

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