Enter the voltage and resistance of a short circuit into the calculator to determine the fault current.

Fault Current Formula

The following formula is used to calculate a fault current.

FC = V/R
  • Where FC is the current (amps)
  • V is the voltage (volts)
  • R is the resistance (ohms)

To calculate the fault current, divide the voltage at fault by the resistance.

Fault Current Definition

A fault current is defined as the maximum current available at any given point in a distribution system.

In the case of a fault, the resistance gets very small which causes the current to become extremely large. The example problems below show this effect.

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 resistance. In practice, this would be based on the total resistance in the distribution system. For this problem, we will say the resistance is .25 ohms.

Finally, use the formula from Ohm’s Law to calculate the fault current.

I = V/R

= 120/.25

= 480 amps.

Example Problem #2.

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

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

In this problem, the resistance is now reduced to only .05 ohms.

Using the formula as in example 1:

I = V/R

= 120/.05

= 2,400 amps.

This is nearly 5 times the current of example 1.

Fault current calculator