Short-Circuit Current Calculator

Estimate available fault current from source voltage and total impedance on a common equivalent-circuit basis, with an optional screening multiplier for preliminary review.

PDF worksheet
Inputs
Result

Formulas

  • \(I_{\text{AFC}} = \frac{V}{Z}\)
  • \(I_{\text{adjusted}} = I_{\text{AFC}} \times M\)
  • \(I_{\text{AFC,kA}} = \frac{I_{\text{AFC}}}{1000}\)
  • \(I_{\text{adjusted,kA}} = \frac{I_{\text{adjusted}}}{1000}\)
  • \(\%Z_{100\text{ A}} = \frac{100\text{ A} \times Z}{V} \times 100\)
  • available fault current = source voltage / total impedance
  • source voltage and total impedance must be on the same equivalent-circuit basis
  • available fault current in kA = available fault current / 1000
  • adjusted fault current in kA = adjusted fault current / 1000
  • equipment rating boundary = no interrupting rating, SCCR, AIC, withstand rating, series rating, fuse, breaker, switchgear, panelboard, transformer, conductor, or equipment selection is made by this calculator

What this calculator estimates

This tool estimates available fault current from source voltage and total impedance on a single equivalent-circuit basis. It is a screening aid for early-stage review, not a substitute for a full short-circuit study.

Formula basis

Available fault current: \(I_{\text{AFC}} = \frac{V}{Z}\)

Adjusted screening current: \(I_{\text{adj}} = I_{\text{AFC}} \times M\)

Impedance percent at a 100 A base: \(\%Z = \frac{100\text{ A} \times Z}{V} \times 100\)

Where:

  • V = source voltage in volts
  • Z = total impedance of source and circuit conductors in ohms
  • M = screening multiplier supplied by the user
  • Ibase = 100 A fixed reference base for the %Z figure

The screening multiplier is a user-controlled margin for conservative screening only. It is not a code-defined safety factor, equipment rating, or compliance result.

Worked example

Given the default inputs of 240 V, 0.08 ohm, and 1.1:

Variable Value
Source voltage (V) 240 V
Total impedance (Z) 0.08 ohm
Screening multiplier (M) 1.1

Step 1 – Available fault current: \(I_{\text{AFC}} = \frac{240}{0.08} = 3{,}000\text{ A}\)

Step 2 – Adjusted screening current: \(I_{\text{adj}} = 3{,}000 \times 1.1 = 3{,}300\text{ A}\)

Step 3 – Impedance percent at a 100 A base: \(\%Z = \frac{100 \times 0.08}{240} \times 100 = 3.3333\%\)

All three outputs are derived directly from the entered values. No lookup-table constants or utility fault-current tables are used here.

Use and limitations

This calculator uses simplified Ohm’s law screening only. It does not model utility contribution, transformer X/R ratio, motor contribution, protective-device coordination, arc-flash conditions, or equipment ratings. Use an engineered fault-current study when the result must support labeling, verification, or final design decisions.

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FAQs

Can this replace an engineered fault-current study?

No. It is a simple impedance screening calculation and does not model utility contribution, motor contribution, transformer details, X/R ratio, protective-device coordination, equipment interrupting ratings, SCCR/AIC, arc-flash conditions, manufacturer limits, or AHJ requirements.