ISC = (c * U) / √(3 * ZSC)
ZSC = √((R₀ + RL)² + (X₀ + XL)²)
L1-L2-L3
ISC = (c * U) / √(3 * ZL)
L-L
ISC = (c * U) / (2 * ZL)
L-N
ISC = (c * U) / (√3 * (ZL + ZN))
L-PE
ISC = (c * U) / (√3 * (ZL + ZPE))
Where：
ISC = Maximum short-circuit current (A)
c = Voltage factor = 1.05
U = Concatenated voltage [L-L] (V)
ZL = Impedance of phase (Ω)
ZN = Impedance of neutral (Ω)
ZPE = Impedance of PE (Ω)
R = Resistance (Ω)
X = Reactance (Ω)
Calculate maximum prospective short-circuit current (Isc) at any point in a low-voltage AC circuit per IEC 60909. Supports 3-phase, L-L, L-N, and L-PE faults based on source impedance and conductor data.
Designed for electrical engineers, solar designers, electricians, and facility managers who need to verify equipment safety and code compliance. Whether you're sizing breakers for a commercial panel, designing a PV system, or performing an arc flash study, this calculator helps ensure your installation can safely withstand fault conditions.
The maximum short-circuit current is determined by the total impedance between the source and the fault location, including:
Available short-circuit current at the origin (kA)
System voltage (V) – with 5% overvoltage factor (c = 1.05)
Line length (m/ft/yd)
Conductor material (Copper or Aluminum)
Conductor cross-section (mm² or AWG)
Cable type (Unipolar, Multicore, etc.) affecting reactance
Number of parallel conductors per phase/neutral/PE
Type of fault:
Three-phase (L1-L2-L3)
Phase-to-phase (L-L)
Phase-to-neutral (L-N)
Phase-to-protective earth (L-PE)
Longer lines, smaller cross-sections, aluminum conductors, or higher-resistance installations reduce the short-circuit current at the load end due to increased line impedance (R + jX).
Three-phase fault: I SC = (1.05 × U) / (√3 × Z L)
Phase-to-phase fault: I SC = (1.05 × U) / (2 × Z L)
Phase-to-neutral fault: I SC = (1.05 × U) / (√3 × (Z L + Z N))
Phase-to-PE fault: I SC = (1.05 × U) / (√3 × (Z L + Z PE))
Where ZL, ZN, and ZPE are the total impedances of phase, neutral, and protective earth conductors, respectively—adjusted for length, material, cross-section, and parallel paths.
Circuit breaker & fuse selection: Verify that the device’s rated breaking capacity (Icn or Icu) exceeds the calculated ISC at its installation point—essential for meeting NEC and IEC SCCR requirements.
Protection coordination: Ensure selective tripping between upstream and downstream devices by comparing actual fault currents against time-current curves.
Arc flash risk assessment: Estimate incident energy levels to determine if arc-resistant switchgear or PPE is required.
Conductor thermal withstand: Check that cables can endure short-circuit heating without damage (using I²t or adiabatic equation).
Solar PV system design: Confirm that MPPT trackers and inverters are rated for the maximum short-circuit current from your PV strings—critical for avoiding equipment damage under fault conditions.
• Source short-circuit current: 10 kA
• System voltage: 220 V (single-phase) or 400 V (three-phase)
• Conductor material: Copper
• Phase size: 1.5 mm² (≈ 16 AWG)
• Neutral & PE size: 1.5 mm²
• Line length: 10 meters
• Fault type: Phase-to-earth (L-PE)
Copper conductors achieve higher admissible I²t than aluminum under identical conditions. Larger cross‑section brings higher thermal capacity and higher allowable let‑through energy. Small‑size cables have low I²t rating and risk insulation damage during short‑circuit, so proper protective device matching is required.
Different insulation materials have different maximum temperature limits and define short‑circuit thermal‑stability performance. PVC, XLPE/EPR are common organic insulations. Mineral‑insulated cables offer superior high‑temperature resistance. Different service conditions such as touch‑accessible areas and fire‑risk zones have distinct I²t limits, so select insulation type correctly.
Pass condition: the actual let‑through I²t of protective device must be lower than cable admissible I²t. PE protective conductors require short‑circuit thermal‑stability verification as well, not only phase conductors. This calculator supports phase conductor, single‑core PE and multi‑core‑cable PE conductor calculation to prevent PE conductor burnout during faults.
It’s derived from system voltage and total impedance: ISC = (1.05 × U) / Ztotal, where Ztotal includes both source and line impedances. This tool automates the full calculation—including conductor resistance, reactance, parallel paths, and fault type.
SSCR (Short-Circuit Current Rating) is the maximum fault current a device can safely interrupt. Your calculated ISC must be ≤ the SCCR of all downstream equipment—otherwise, catastrophic failure can occur during a fault.
Yes—for continuous loads, NEC Article 210.20 requires overcurrent devices to be rated at least 125% of the load current. While this rule governs normal operation, your short-circuit analysis ensures the same device can also handle abnormal fault conditions.
Absolutely. Solar designers use this tool to verify that the short-circuit current from PV strings does not exceed the input rating of inverters or MPPT trackers—a common requirement in UL 1741 and IEC 62109.
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