PROTECTION · CALCULATOR

Short-circuit current at a point

Free Short-circuit current at a point: get prospective fault current from voltage and verified equivalent source + line impedance. With the formula shown.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Voltage

Verified equivalent source + line impedance

Supply

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Isc = U / Z (1-phase); ULL / (√3 × Z) (3-phase)

Simplified impedance method; use phase-neutral for 1-phase. Does not check X/R, peak current, DC offset or breaker interrupting capacity.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Voltage: 120 V · Verified equivalent source + line impedance: 0.15 Ω · Supply: Single phase

Prospective fault current: 800 A

This example uses the stated inputs; results for the entered values appear above.

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