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Calcimator

Short Circuit Current Calculator

Calculate available fault current, X/R ratio, and interrupting duty at a point downstream of a transformer.

Inputs

kVA
%Z

ANSI C57.12.00: 75–333 kVA: 4.0–5.5%Z; 500–1,000 kVA: 5.5–6.0%Z; 1,000–5,000 kVA: 5.75–7.5%Z

V
ft
mm²
phase

Results

Available Fault Current

14,648 A

Min. Withstand Rating

15,000 A

X/R Ratio0.91
Interrupting Duty14,648 A
How to Use This Calculator
  1. Enter the Transformer KVA rating and its Percent Impedance (%Z) from the transformer nameplate — typical 5–6% for 1,000+ kVA units.
  2. Enter the Secondary System Voltage in volts (e.g., 208 or 480 V).
  3. Enter the Cable Length in meters from the transformer to the fault point and the Cable Size in mm².
  4. Select the Number of Phases (1 or 3).
  5. Read the Available Fault Current in amperes — use this to verify breaker and switchgear interrupting ratings.
  6. Use the Minimum Withstand Rating in amperes to confirm buses, lugs, and equipment can survive a bolted fault without damage.

How the result changes with Transformer Impedance

Transformer ImpedanceAvailable Fault CurrentMin. Withstand Rating
2.424,743 A25,000 A
5.914,386 A15,000 A
109,652 A10,000 A
147,307 A8,000 A

What each input means

Transformer KVA
Rated KVA of the upstream transformer.
Transformer Impedance
Transformer percent impedance from nameplate per ANSI/IEEE C57.12.00. Typical values: 15–75 kVA: 2.0–3.5%; 75–333 kVA: 4.0–5.5%; 500–1,000 kVA: 5.5–6.0%; 1,000–5,000 kVA: 5.75–7.5%. Higher %Z = lower fault current.
System Voltage
Secondary system voltage (e.g., 208, 480V).
Cable Length to Fault
Distance from transformer to the point of fault calculation.
Cable Size
Cross-sectional area of the cable conductor in mm².
Number of Phases
Select 1 for single-phase or 3 for three-phase.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Transformer KVA = 1000, Transformer Impedance = 5.75, System Voltage = 480, Cable Length to Fault = 30 = 6 input(s) provided
  2. Calculate Available Fault Current
    Available Fault Current
    14648 = 14648
  3. Calculate Min. Withstand Rating
    Min. Withstand Rating
    15000 = 15000
  4. Calculate X/R Ratio
    X/R Ratio
    0.91 = 0.91
  5. Calculate Interrupting Duty
    Interrupting Duty
    14648 = 14648

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