Fault Current Calculator
Transformer available short-circuit current — 3-phase & 1-phase
This fault current calculator estimates the available fault current (maximum short-circuit current) at a transformer's secondary using the infinite-bus method. Enter the transformer's kVA, secondary voltage, and percent impedance (%Z) from the nameplate to get the prospective fault current in amps for either a three-phase or single-phase transformer. Use the result to confirm your switchgear, breakers, and fuses have an interrupting rating (kAIC) above the available fault current.
Your result is the minimum interrupting rating your equipment must exceed. Shop equipment rated above your available fault current:
Medium-Voltage Switchgear · Low-Voltage Switchboards · Circuit Breakers · Transformers
How to Use the Calculator
Three phase formula
Available Fault Current = ((kVA × 1000) / 1.732 / Secondary Voltage) / (IZ / 100)
Single phase formula
Available Fault Current = ((kVA × 1000) / Secondary Voltage) / (IZ / 100)
Worked Example
1,000 kVA, 480 V, 5.75 %Z, three-phase
FLA = (kVA × 1,000) ÷ (√3 × V) = 1,000,000 ÷ (1.732 × 480) = 1,203 A
AFC = FLA ÷ (%Z ÷ 100) = 1,203 ÷ 0.0575 = 20,920 A ≈ 20.9 kA
Result: protective devices need an interrupting rating above ~21 kA — use the next standard rating with margin, e.g. 25 kAIC.
Formulas
Three-phase:
Available Fault Current = ((kVA × 1,000) ÷ 1.732 ÷ Vsecondary) ÷ (%Z ÷ 100)
Single-phase:
Available Fault Current = ((kVA × 1,000) ÷ Vsecondary) ÷ (%Z ÷ 100)
The three-phase and single-phase formulas differ only by the √3 (1.732) factor in the full-load amps step. Select the matching mode in the calculator above.
Typical Transformer %Z
| Transformer Type / Size | Typical %Z |
|---|---|
| Single-phase, 25–167 kVA | 1.5 – 3.5% |
| Three-phase dry-type, ≤ 500 kVA | 4 – 6% |
| Three-phase, 750 – 2,500 kVA | 5.75 – 6.5% |
| Three-phase, ≥ 2,500 kVA | 6.5%+ |
Always use the actual nameplate %Z when available — these are typical ranges only.
About the Infinite-Bus Method
This calculator uses the infinite-bus (worst-case) method: it assumes an unlimited source upstream and that only the transformer's impedance limits the fault current. It does not account for source impedance, conductor impedance, or motor contribution, so the real value at a downstream panel will be lower. The result is conservative for equipment-rating selection but is not a substitute for a full short-circuit/coordination study, and is generally not used for arc-flash incident-energy calculations.
Frequently Asked Questions
What is available fault current?
The maximum current that can flow during a short circuit at a given point. Protective devices must have an interrupting rating (kAIC) above it.
How do I calculate transformer fault current?
Divide full-load amps by per-unit impedance: AFC = FLA ÷ (%Z ÷ 100). For three-phase transformers, full-load amps = (kVA × 1,000) ÷ (1.732 × secondary voltage).
Where do I find %Z?
On the transformer nameplate, listed as "Impedance" or "%IZ". If unavailable, use the typical ranges in the table above as a starting estimate — but always verify against the actual nameplate before sizing protective equipment.
Why does available fault current matter?
Breakers, fuses, and switchgear must be rated to safely interrupt it. Exceeding a device's kAIC/SCCR rating during a fault can cause catastrophic failure, including arc flash, fire, or explosion.
Does this include cable or motor contribution?
No — the infinite-bus method ignores source impedance, conductor impedance, and motor contribution, giving a conservative worst-case value. The actual fault current at a downstream panel will be lower.
Three-phase vs. single-phase — what's different?
The formulas differ by the √3 (1.732) factor used to calculate full-load amps for a three-phase transformer. Select the matching mode in the calculator — entering the wrong phase type will produce an incorrect result.