Short-circuit current calculator (line-to-neutral loop)
For a breaker to clear a short circuit instantly, the fault current at the farthest point of the line must be high enough. The calculator estimates the line-to-neutral loop current from the line's and supply's resistance, and compares it with the breaker's trip threshold.
Input data
Result
Fill in the fields — the verdict will appear here
A green ratio (≥ the upper bound: B×5, C×10, D×20) means the breaker is guaranteed to clear the fault instantly. Red means instant tripping isn't guaranteed — check the time-current curve, or use a smaller cross-section/rating. Reactive and contact resistances are ignored — for a real project, calculate fully per GOST 28249.
How it's calculated
- Add up the loop resistance: the part up to the panel (transformer and trunk line), plus the phase and neutral conductor resistance to the fault point.
- Divide the phase voltage by this resistance to get the single-phase short-circuit current.
- Compare it with the breaker's rating. For the instantaneous trip to fire, guaranteed, use the curve's upper bound: B — ×5, C — ×10, D — ×20.
Example: 220 V, 0.1 Ω up to the panel, 2.5 mm² copper over 30 m → a 0.52 Ω loop, 423 A short-circuit current. For a C16 breaker, that's 26.4× the rating against the needed 10× — tripping is guaranteed.
Isc = Uph/(Zext + Zloop), Zloop ≈ 2·ρ·L/S. Guaranteed instant tripping happens at Isc ≥ k·In at the upper edge of the curve (k: B=5, C=10, D=20, GOST IEC 60898). Method per GOST 28249, requirement — PUE 1.7.79 / GOST R 50571.
This is an estimate
Verify final values against current standards, the project design, and equipment nameplates.