Short Circuit Current Calculator

Method *

Choose a model that matches your circuit.

Voltage *

Use RMS voltage for AC methods.

Total resistance R *

Includes source and wiring resistance.

Resistance R *

R is the real (loss) part.

Inductive reactance X_L *

X_L = ωL (if you compute it separately).

Capacitive reactance X_C *

X_C = 1/(ωC).

Impedance input

Use line-to-line voltage for V_LL.

R_th *

X_th *

|Z_th| *

Thevenin type

Use open-circuit voltage as V_th.

R_th *

X_th *

Formula used

DC resistive model

I_SC = V / R

AC single-phase impedance model

I_SC = V_RMS / |Z|

|Z| = √(R² + (X_L − X_C)²)

Three-phase symmetrical fault (line-to-line voltage)

I_SC = V_LL / (√3 · |Z_th|)

Thevenin equivalent

DC: I_SC = V_th / R_th | AC: I_SC = V / |Z_th|

How to use this calculator

Select a method that matches your circuit or system.
Enter voltage with the correct unit (use RMS for AC).
Provide resistance and reactance or Thevenin parameters.
Press Calculate to view I_SC above the form.
Download CSV or PDF to store your results.

Example data table

Sample inputs and outputs for quick verification.

Case	Method	Inputs	I_SC (A)
Bench supply	DC Resistive	V = 12 V, R = 0.20 Ω	60
Single-phase load	AC Impedance	V = 230 V, R = 0.15 Ω, X_L = 0.40 Ω, X_C = 0.10 Ω	~541
LV feeder fault	Three-Phase	V_LL = 400 V, R_th = 0.02 Ω, X_th = 0.08 Ω	~2887

Values are illustrative; real systems may require standards-based modeling.

Short circuit current guide

1) What short-circuit current represents

Short-circuit current is the current that flows when a conductive fault creates an unintended low-impedance path. In practice, it is limited by the source, wiring, and machine impedances, not by the load. Engineers use I_SC to size conductors, verify protective device interrupt ratings, and estimate thermal and mechanical stress during faults.

2) Inputs that most strongly affect the result

The dominant driver is the equivalent impedance between the source and the fault. A small change in total impedance causes a large change in current because I is inversely proportional to R or |Z|. For example, at 230 V, reducing impedance from 0.50 Ω to 0.25 Ω doubles the predicted current from 460 A to 920 A.

3) DC resistive estimation for laboratory setups

For DC supplies, battery packs, and resistive circuits, a first estimate uses I_SC = V/R. Here, R should include internal source resistance, contact resistance, wiring, and shunt elements. Milliohm-level differences matter: a 12 V source with 20 mΩ total resistance implies 600 A, which can heat small conductors within seconds.

4) AC impedance method and reactive components

In AC systems, reactance changes the magnitude of current. The calculator uses |Z| = √(R² + (X_L − X_C)²) and I_SC = V_RMS/|Z|. When |X| ≫ R, the current can be large but strongly phase-shifted, affecting real power and the stress seen by protection equipment.

5) Three-phase symmetrical fault level

For balanced three-phase faults, I_SC = V_LL/(√3·|Z_th|). The √3 factor converts line-to-line voltage to per-phase driving voltage. If V_LL = 400 V and |Z_th| = 0.08 Ω, the initial symmetrical current is about 2887 A, matching the example table.

6) X/R ratio, DC offset, and peak current

Real faults include a transient DC component that can increase the first-cycle peak current above the symmetrical RMS value. Higher X/R ratios generally produce slower decay of the offset and higher peak mechanical forces. While this calculator focuses on steady magnitudes, you can use its |Z| and current as a baseline for more detailed transient studies.

7) Protection coordination and interruption ratings

Compare the estimated fault current to breaker or fuse interrupt ratings, and consider the available fault current at the installation point. Also check conductor thermal limits using I²t concepts and time-current curves. If the predicted I_SC is very high, faster clearing times and lower-impedance buswork designs become critical.

8) Practical workflow and safety notes

Start with a conservative impedance estimate, then refine it with measured cable resistance, transformer data, or Thevenin equivalents. Always treat results as planning values, not permission to test by shorting a source. Use rated PPE, proper test equipment, and isolation procedures when validating models in real systems.

FAQs

1) Which voltage should I enter for AC?

Use RMS voltage for single-phase impedance calculations. For three-phase faults, use line-to-line RMS voltage because the formula converts it to per-phase driving voltage internally.

2) What should “total resistance” include in DC mode?

Include source internal resistance, contact resistance, wiring resistance, and any intentional series elements. If you ignore milliohm-level wiring and connectors, you may underestimate current substantially.

3) Can X_L and X_C be zero?

Yes. Set X_L = 0 for non-inductive paths and X_C = 0 when capacitance is negligible. The impedance reduces to R when both are zero.

4) Why does the calculator warn about very high current?

Large I_SC values can exceed conductor ampacity, weld contacts, or exceed interruption ratings. The warning is a safety reminder to verify ratings and protective coordination.

5) What is a Thevenin equivalent in this context?

It replaces a complex network with an equivalent voltage source and an equivalent resistance/impedance seen at the fault point, making short-circuit current estimates straightforward.

6) Does this include motor contribution or transformer saturation?

No. It provides a steady-state magnitude based on the impedances you enter. For detailed power system studies, include machine subtransient reactance and transformer data in your Z estimates.

7) How accurate is the power estimate P ≈ V·I?

It is an idealized magnitude estimate. In AC circuits with significant reactance, real power depends on the phase angle, so V·I may overstate real heating power.