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PROTECTION

Short Circuit Calculator

Fault current · MVA · Transformer SC · Cable impedance — IEC & NEC

I = V / Z MVA = √3 × V × I

I = V / Z

Enter voltage and impedance to find fault current

V
Ω
—
%
Fault current — A
MVA fault level — MVA
Fault current (min) — A
Fault current (max) — A
ISC = V / Z  |  MVA = √3 × V × I / 1,000,000

Fault current vs impedance

Z ↑ I ↓ I
Fault current decreases as impedance increases

Safety levels

  • < 10 kA — Low
  • 10–25 kA — Medium
  • 25–50 kA — High
  • > 50 kA — Very High
Risk level —

Transformer SC

Calculate secondary fault current from transformer specs

kVA
V
%
—
V
Full load current — A
Fault current — A
MVA fault level — MVA
X/R ratio (est.) — ratio
IFL = kVA / (√3 × V)  |  ISC = IFL / (Z% / 100)

Transformer schematic

Vp Vs ⚡ FAULT Z% = 5.75%
Fault current on secondary: ISC = IFL / (Z% / 100)

Cable impedance

Calculate cable impedance from length and gauge

AWG
ft
—
runs
Total resistance (R) — Ω
Reactance (X) — Ω
Total impedance (Z) — Ω
Voltage drop (per 100A) — V
Z = √(R² + X²)  |  R = ρ × L / A

Impedance vs length

Length ↑ Z ↑ Z
Impedance increases linearly with length