Short-Circuit Current Calculator
Use this preliminary short-circuit current calculator for same-basis V/Z screening arithmetic before engineered fault studies, equipment ratings, protective-device coordination, arc-flash review, and utility data review. It is not a short-circuit study, not equipment selection, and not code-compliance.
- Available fault current
- A
- Adjusted fault current
- A
- Available fault current
- kA
- Adjusted fault current
- kA
- Impedance percent at 100 A base
- %
Calculation details
- Impedance used
- ohm
- Screening multiplier used
- x
- Calculation basis
- Available fault current boundary
- Utility/study boundary
- Equipment rating boundary
- Protective-device boundary
- Arc-flash boundary
- Code-compliance boundary
Recent results
Formulas
- Available fault current = source voltage / total impedance
- Source voltage and total impedance must be on the same equivalent-circuit basis
- Adjusted fault current = available fault current x screening multiplier
- Available fault current in kA = available fault current / 1000
- Adjusted fault current in kA = adjusted fault current / 1000
- 100 A screening-base voltage-drop percent = total impedance x 100 A / source voltage x 100
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: IAFC = V / Z
Adjusted screening current: Iadj = IAFC × M
Impedance percent at a 100 A base: %Z = (100 A × Z / V) × 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: IAFC = 240 / 0.08 = 3,000 A
Step 2 – Adjusted screening current: Iadj = 3,000 × 1.1 = 3,300 A
Step 3 – Impedance percent at a 100 A base: %Z = (100 × 0.08 / 240) × 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.
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.