Prospective Short Circuit Current Calculator

What Is Prospective Short Circuit Current? The prospective short circuit current (Isc) is the maximum fault current that would flow at a given point in an electrical system if a zero-impedance short circuit were applied there. It is the worst case: the current that circuit breakers and fuses must be able to interrupt safely. Every protective device carries a rated breaking capacity, and it must exceed the Isc wherever it is installed.

Why It Matters for Safety Put a breaker rated to interrupt 6 kA where the fault current is 15 kA and it does not simply fail to open. It fails catastrophically, and the result is an arc flash, a fire, or an exploding panel. Calculating Isc at every board is required by IEC 60364, the NEC, and effectively every national code. The good news is that Isc falls as you move away from the transformer, because more cable means more impedance. That is why a sub-board can use ordinary MCBs while the main switchboard cannot.

The Transformer Impedance Transformers are specified with a percentage impedance (Z%), typically 4–6% for distribution transformers. This represents the voltage that must be applied to the primary to cause rated current to flow with the secondary short-circuited. Transformer impedance in ohms: Zt = V² × (Z/100) / S Where V is the secondary voltage and S the rated apparent power in VA.

Which voltage to enter for three-phase. Use the line-to-line value, 400 V or 480 V rather than 230 V or 277 V, together with the transformer’s full three-phase kVA. That pairing gives the correct per-phase impedance directly. (Line-to-neutral voltage with one third of the kVA gives the identical answer, since (V/√3)² ÷ (S/3) = V²/S, but mixing the two conventions produces an answer wrong by a factor of three.)

Cable Impedance The cable adds resistance between the transformer and the fault point. Cable resistance: Rc = ρ × L / A Where ρ for copper = 0.0172 Ω·mm²/m, L is length in metres, A is cross-section in mm². For single-phase circuits, the total cable path is 2× the one-way length (out and back). For three-phase, only one conductor length is used in the simplified formula (the fault loop is more complex but this gives a conservative estimate).

The Fault Current Calculation Single-phase: Z_total = Zt + 2 × Rc Isc = V_supply / Z_total

Three-phase: the phase-to-phase voltage applies: Isc = V_line / (√3 × Z_total) Or equivalently using phase voltage: Isc = V_phase / Z_total

Cable inductive reactance also matters at high fault currents, but resistance dominates on short runs, and leaving reactance out understates the total impedance. That biases Isc upward, which is the safe direction when you are choosing a breaking capacity.

Interpreting the Result Common circuit breaker Icu (ultimate breaking capacity) ratings: 6 kA, 10 kA, 16 kA, 25 kA, 36 kA, 50 kA. The breaker rating at any point must exceed the calculated Isc at that point. Main distribution boards close to the transformer may see Isc of 20–50 kA, requiring high-capacity breakers. Sub-boards and final circuits are typically 3–15 kA due to cable impedance, allowing standard MCBs.

The Importance of Accurate Data Transformer impedance data comes from the nameplate (or supplier). Default 5% is a common starting point. Cable cross-section must match actual installed cable. Underestimate Isc and you fit a breaker that cannot clear the fault, so always assume the smallest credible impedance when choosing breaking capacity. Overestimate it and you pay for switchgear you did not need. Accurate nameplate data matters in both directions.