Bolted Fault Current Calculator

Enter system data

The page stays single-column, while the input area uses 3 columns on large screens, 2 on medium screens, and 1 on mobile.

Advanced electrical model

Bus voltage (kV, line-to-line)

Example: 0.48, 4.16, 11, 33

Voltage factor

Use 1.00 to 1.10 for a realistic operating range.

Source short-circuit level (MVA)

Available fault level seen at the source side.

Source X/R ratio

Used to split source impedance into resistance and reactance.

Transformer size (kVA)

Set to 0 if no transformer is included.

Transformer impedance (%)

Typical values often range from 4% to 8%.

Transformer X/R ratio

Used for peak-current estimation.

Cable data basis

Choose the matching basis before entering feeder impedance.

Cable length

Use ft with the first basis, or km with the second.

Cable resistance

Ohm per 1000 ft or ohm per km.

Cable reactance

Ohm per 1000 ft or ohm per km.

Additional resistance (ohm)

Use for busway, reactor, switchgear, or custom items.

Additional reactance (ohm)

Add any extra series reactance not captured elsewhere.

Motor contribution (kA)

Approximate initial contribution from connected motors.

Clearing time (s)

Used for the I²t thermal-duty indicator.

Example data table

The sample below shows a low-voltage bus with transformer and feeder impedance included.

Item	Sample value	Unit	Notes
Bus voltage	0.48	kV	Three-phase line-to-line bus
Voltage factor	1.05	-	Assumes slightly elevated operating voltage
Source short-circuit level	500	MVA	Utility/source stiffness
Transformer size	1500	kVA	Secondary tied to the selected bus
Transformer impedance	5.75	%	Nameplate value
Cable length	120	ft	Using per 1000 ft basis
Cable R / X	0.018 / 0.015	ohm per 1000 ft	Feeder impedance data
Motor contribution	1.8	kA	Initial motor backfeed estimate
Symmetrical RMS current	25.297	kA	Calculated bolted fault current
Total available current	27.097	kA	Includes motor contribution
Fault duty	22.528	MVA	At the selected bus
Peak current estimate	50.826	kA	Based on the equivalent X/R ratio

Formula used

1. Source impedance
Zsource = V² / MVA_sc

2. Transformer impedance
Ztx = (Z% / 100) × (V² / MVA_tx)

3. Cable impedance
Rcable = Rrate × Length and Xcable = Xrate × Length using the selected basis

4. Total impedance
Rtotal = ΣR, Xtotal = ΣX, Ztotal = √(Rtotal² + Xtotal²)

5. Symmetrical bolted fault current
Ibf = (V × Voltage Factor) / (√3 × Ztotal)

6. Total available current
Itotal = Ibf + Imotor

7. Fault duty
Fault MVA = √3 × V × Itotal

8. Peak current estimate
κ = 1.02 + 0.98e^-3/(X/R), then ipeak = κ × √2 × Ibf

Voltage is entered in kV line-to-line. Current results are shown in kA. Impedance is handled in ohms on the chosen bus base.

How to use this calculator

Enter the line-to-line bus voltage in kV.
Set a voltage factor that reflects expected operating voltage during the fault.
Provide the available source short-circuit MVA and source X/R ratio.
Enter transformer kVA, percent impedance, and X/R if a transformer feeds the bus.
Select the cable basis, then enter feeder length and impedance values.
Add any extra resistance or reactance from reactors, busway, or custom equipment.
Estimate motor contribution if motors on the bus can backfeed the fault.
Enter clearing time to review a simple I²t duty indicator.
Press the calculate button to display results above the form.
Use the CSV or PDF buttons to export the calculated summary.

FAQs

1) What is bolted fault current?

Bolted fault current is the maximum available current when conductors are shorted together with negligible fault resistance. It helps determine interrupting and withstand duties for electrical equipment.

2) Why are source and transformer values both needed?

The source sets upstream strength, while transformer impedance often limits current heavily on the secondary bus. Using both gives a more realistic available fault level.

3) Does motor contribution really matter?

Yes. Large motors can feed current back into a nearby fault for the first few cycles. That extra contribution can affect interrupting rating checks and protective device coordination.

4) Is this calculator for ground faults too?

No. This page is arranged for three-phase bolted faults using equivalent positive-sequence impedance. Single-line-to-ground studies need sequence networks and different fault equations.

5) Which voltage should I enter?

Enter the nominal line-to-line voltage at the bus where you want the fault current. For low-voltage switchboards, that is often 0.48 kV or 0.415 kV.

6) How accurate is the peak current estimate?

It is an engineering estimate based on the equivalent X/R ratio. It is useful for screening studies, but final equipment verification should follow the governing standard and manufacturer data.

7) Can longer cables reduce available fault current?

Yes. Added feeder resistance and reactance increase total impedance, which lowers the symmetrical RMS fault current seen at the downstream bus.

8) How should I use the result for breaker selection?

Compare the calculated duty against the breaker interrupting rating and any required withstand or peak limits. Also confirm coordination, voltage class, and the applicable standard for your system.