How to use the cable short-circuit calculator

What the cable short-circuit calculator does

During a short circuit the current through a cable can be hundreds of times its rating. The protective device clears it in a fraction of a second, but until it does, the conductor heats up fast enough to damage the insulation. The free cable short-circuit calculator checks whether a conductor survives that heat, using the adiabatic equation of AS/NZS 3008.1.1:2025:

The energy the fault lets through (I²t) must not exceed what the conductor can absorb before the insulation reaches its limit (K²S²). The calculator gives the pass or fail, the minimum conductor size and how long the cable can carry the fault.

Use the cable short-circuit calculation when you:

• connect a small cable close to a large transformer or main switchboard, where fault levels are high;
• reuse an existing cable on a new, stronger supply;
• choose between V-75 and X-90 insulation, or between two protective devices with different clearing times.

Before you start: fault current and clearing time

• Prospective short-circuit current at the origin of the circuit, in kA: from the switchboard's fault level, the network operator, or an upstream fault calculation.
• Clearing time in seconds: from the protective device's time-current curve at that current.
• The conductor: copper or aluminium, insulation type and size in mm².

How to check cable short-circuit withstand, step by step

1. 1

   Describe the conductor

   In the Conductor card choose the Conductor material (Copper or Aluminium) and the Insulation: Thermoplastic V-75 (PVC, 75 °C), XLPE or elastomer X-90 (90 °C) or Fire rated (110 °C). The insulation sets both the starting temperature and the limit temperature, and so K. Then pick the Conductor size, 1 mm² to 630 mm².

   

2. 2

   Enter the fault

   In the Fault card enter the Prospective short-circuit current in kA. The hint reminds you to use the maximum, at the origin of the circuit. Enter the Clearing time in seconds, read from the device's curve; the field accepts 0.01 s to 5 s.

3. 3

   Read the withstand check

   The Short-Circuit Withstand card updates as you type. The next section explains each row.

   

Reading the short-circuit withstand result

The headline reads Conductor withstands the fault when it passes, or 1 check fails when it does not, with the size, material, current and time underneath. The rows are:

• Minimum size (I√t / K): the smallest cross-section that survives, to two decimals.
• Smallest standard size: the next size on the ladder at or above the minimum. It turns red, saying the chosen size is too small, when the check fails.
• K (Table 5.1) and Temperature rise: the constant and the start and limit temperatures it is read for.
• Let-through I²t and Withstand K²S²: the two sides of the equation in A²s.
• Withstand current for the entered time, and Withstand time at the entered current: how much headroom the cable has, either way.

Worked example: a 16 mm² cable at 6 kA

A 16 mm² copper cable with Thermoplastic V-75 (PVC, 75 °C) insulation is fed from a board with a prospective fault of 6 kA, and the upstream device clears it in 0.1 s. With those values entered the card shows 1 check fails:

The 16 mm² V-75 conductor needs at least 17.06 mm², so the smallest standard size is 25 mm². It can only carry 6 kA for 0.088 s, less than the 0.1 s the device takes. Two changes in the calculator fix it:

• Set Conductor size to 25 mm². The withstand rises to 7.73 × 10⁶ A²s and the cable lasts 0.215 s at the fault current.
• Keep 16 mm² and change Insulation to X-90. K rises to 142.9 because the limit temperature is higher, the minimum falls to 13.28 mm², and the cable passes with 0.145 s of withstand.

A faster device is the third option: enter its clearing time and see whether the original size passes.

Best practice and the adiabatic equation in AS/NZS 3008

• Take the clearing time from the device. Read it from the time-current curve at the prospective fault current, not from a disconnection-time table.
• Use let-through energy for very fast devices. Below about 0.1 s, compare the manufacturer's let-through I²t with the Withstand K²S² row instead of entering a time.
• Check at the start of the cable. The fault at the origin is the largest; the far end has the most impedance and the lowest current.
• Run it inside the cable sizing. The cable size calculator does this check for the active and the earth together with current rating, voltage drop and fault loop.

Common short-circuit mistakes

• Entering 0.4 s or 5 s by default

  Those are disconnection-time limits. The device usually clears a large fault much faster, or much slower near its overload region. Use the curve.

• Using the fault current at the far end

  The adiabatic check uses the maximum prospective current at the origin of the circuit.

• Starting the fault cold

  A conductor at ambient temperature has a higher K. The calculator starts at the insulation's maximum operating temperature; do the same in a hand check.

• Picking the minimum size instead of a standard size

  The Minimum size row is a calculated cross-section. Install at least the Smallest standard size shown below it.

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