Cable Short-Circuit Calculator (I²t = K²S²) – Free

How to use

Minimum conductor size for a prospective short-circuit current and clearing time, and what a cable withstands, to AS/NZS 3008.1.1:2025.

Conductor

Insulation

Conductor size

Fault

Prospective short-circuit current

kA

The maximum, at the origin of the circuit (Clause 2.3.3(a))

Clearing time

s

From the protective device's curve at that current; at most 5 s

Short-Circuit Withstand

Enter the fault current and clearing time.

How it's calculated

The adiabatic short-circuit check

A conductor must survive the fault until the protective device clears it, without its insulation passing the short-circuit temperature limit.

Adiabatic equation (Equation 5.3)

: prospective short-circuit current, A
: clearing time, s
: constant from Table 5.1
: conductor cross-section, mm²

Minimum conductor size

Withstand current for a time

Withstand time at a current

K depends on the conductor (copper or aluminium), the initial temperature (the insulation's maximum operating temperature) and the final temperature (the insulation's short-circuit limit).

Standards referenced

AS/NZS 3008.1.1:2025 Clause 2.3.3 (short-circuit performance) and Clause 5.3, Equation 5.3
AS/NZS 3008.1.1:2025 Table 5.1 (values of K) and Table 5.2 (short-circuit temperature limits)
AS/NZS 3000:2018 Clause 2.5.4.2(b) (protection against short-circuit current)

Worked example: 16 mm² copper at 6 kA

A 16 mm² copper V-75 cable on a board with a 6 kA prospective fault level, protected by a device that clears in 0.1 s.

1. K (Table 5.1)
2. Let-through energy
3. Conductor withstand
4. Minimum size
5. Withstand current

The fault lets through more energy than 16 mm² can absorb: it needs at least 17.1 mm², so the next standard size, 25 mm², or a faster device. At 0.088 s or less the 16 mm² cable would pass.

Frequently Asked Questions

Short-Circuit Rating Questions Answered

What is the adiabatic equation?

I²t = K²S². During a short circuit the conductor heats up too quickly to lose heat to its surroundings, so all the energy I²t goes into raising its temperature. K²S² is the energy that takes the conductor from its operating temperature to the insulation's limit. AS/NZS 3008.1.1:2025 Clause 5.3 gives it as Equation 5.3, and the minimum size follows as S = I√t / K.

Where does K come from?

AS/NZS 3008.1.1:2025 Table 5.1 tabulates K by conductor material, initial temperature and final temperature. The calculator starts the fault at the insulation's maximum operating temperature (75 °C for V-75 PVC, 90 °C for X-90) and ends it at the insulation's short-circuit limit (160 °C for PVC up to 300 mm², 140 °C above; 250 °C for XLPE). Copper V-75 gives K = 111.2.

Which fault current should I use?

The maximum prospective short-circuit current at the origin of the circuit (AS/NZS 3008.1.1:2025 Clause 2.3.3(a)), because a fault can occur right at the start of the cable. Your network operator or the upstream switchboard's fault level gives it.

What clearing time should I enter?

The time the protective device takes to clear that current, read from its time-current curve. Circuit breakers clearing in the instantaneous region often take 10 to 20 ms, in which case use the manufacturer's let-through energy (I²t) instead of current squared times time, since the adiabatic equation is only reliable down to about 0.1 s.

Does the earth conductor need the same check?

Yes, with the earth fault current and copper K. AS/NZS 3000:2018 Clause 5.3.3.1.1 requires the earthing conductor to carry the fault current for the device's clearing time. The earth cable size calculator does that check together with Table 5.1.

Reviewed by Suheib Sherif, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

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