Enter the system voltage and the total equivalent impedance (or resistance in a DC circuit) of the fault path into the calculator to estimate the prospective fault current.

Fault Current Calculator

Estimate available fault current at a transformer or downstream point, or calculate prospective fault current from loop impedance.

Available Fault Current
Fault Loop / PFC
Calculation Method

Choose the input style that best matches the data you have.

Transformer Method
Use transformer kVA, voltage, and percent impedance.
Known Source Current
Start with known available current and add downstream impedance.
System Type
Three Phase
Typical service, panel, MCC, or switchboard calculation.
Single Phase
Single phase source or line-to-neutral style estimate.
Transformer Inputs

This estimates bolted available fault current based on transformer data, plus any added downstream impedance.

Known Source Current Inputs

Use this when you already know available fault current at an upstream point and want a downstream estimate.

Loop Fault Type

This mode converts loop impedance to prospective fault current or prospective short-circuit current.

Single Phase / Line to Neutral
PFC from loop impedance using V ÷ Z.
Three Phase
Prospective short-circuit current using V ÷ (√3 × Z).
Fault Loop Inputs

This calculator provides an estimate for bolted or prospective fault current based on the values entered. Always verify final protection and compliance decisions with applicable standards, utility data, and project engineering.

Fault Current Formula

The following formula (Ohm’s law) can be used to estimate prospective fault current when you know the voltage and the total equivalent impedance seen at the fault point (for DC circuits, impedance is just resistance).

FC = V/Z
  • Where FC is the fault current (amps)
  • V is the applicable voltage at the fault (volts)
  • Z is the total equivalent impedance of the fault path (ohms)

To calculate the fault current, divide the applicable voltage by the magnitude of the total fault-path impedance. In AC power systems, the voltage used (line-to-line vs. line-to-neutral) depends on the type of fault being analyzed (e.g., three-phase vs. line-to-ground), and the impedance typically includes both resistance and reactance.

Fault Current Definition

Fault current is the current that flows when an electrical fault (such as a short circuit) occurs. The available (or prospective) fault current at a point is the maximum current the system can deliver into a fault at that location under stated conditions, and it depends on the source and network impedance and the type of fault.

During a bolted fault the fault-path impedance can be very low, which can make the current very large. In most real power systems, however, the fault current is primarily limited by the total Thevenin/source impedance of the supply (transformers, generators, and conductors), not just the fault’s own resistance.

How to calculate fault current?

Example Problem #1.

First, determine the voltage of the system. For this example, we will use 120 volts as the voltage.

Next, determine the total equivalent impedance (or resistance in a DC circuit) seen by the source at the fault point. For this problem, we will say the impedance magnitude is 0.25 ohms.

Finally, use Ohm's Law to calculate the fault current.

I = V/Z

= 120/0.25

= 480 amps.

Example Problem #2.

In this next example, we will take a look to see how much a smaller impedance affects the current.

Again, we will use the same voltage as above, 120 V.

In this problem, the impedance magnitude is now reduced to only 0.05 ohms.

Using the formula as in example 1:

I = V/Z

= 120/0.05

= 2,400 amps.

This is 5 times the current of example 1.

Fault current calculator