I2t cable withstand calculator

What this calculator checks

This calculator screens whether an entered cable conductor or protective conductor has enough adiabatic short-circuit withstand for the fault current, clearing time and k value entered by the user. It answers a narrow engineering worksheet question: if the conductor area is S, the material factor is k, and the protective device clears the entered fault current in the entered time, is the calculated fault energy less than or equal to the calculated conductor withstand?

The calculation is intentionally transparent. It does not choose the cable, nominate a protective conductor, read a device curve, set a disconnection time, copy a standard table or close project review. It calculates I2t and compares it with k2S2. That boundary is important because the adiabatic equation is sensitive to inputs that usually come from somewhere else: prospective fault current, protective-device clearing time, conductor material, insulation temperature limits, and the correct k value for those conditions.

Australian context matters because the terms used on site and in project records are usually cable, conductor area, protective conductor, earthing conductor, switchboard, protective device, fault current, clearing time and manufacturer data. The default supply context remains 230/400 V a.c. at 50 Hz, but voltage is not an input on this page because the energy check starts after the fault current has already been established. Use the short-circuit current calculator or project study to establish current before using this page.

The page includes screening k presets for copper and aluminium conductors under named PVC or XLPE insulation temperature assumptions. These presets are not a substitute for the current standard or manufacturer data. They are included so the calculation can be explored without leaving the k value invisible. A professional worksheet should record the actual k value source, the conductor construction, the insulation limits, the clearing time source and the project location where the fault current applies.

Input basis

The prospective fault current is entered in kiloamperes. It should represent the point where the conductor is being checked. A current at the transformer terminals is not automatically the same current at a downstream distribution board, final subcircuit or equipment terminal. If the available current has been calculated elsewhere, carry across the same assumptions and record the source.

The clearing time is entered in seconds. This value should come from protective-device data, a coordination study, a manufacturer curve, project documentation or another verified method. Do not use the labelled current rating of a circuit breaker as if it were a clearing time. The thermal energy check is about the duration of the fault before the protective device clears, not the normal load current of the circuit.

The conductor area is entered in mm2. The calculator treats that value as the conductor area being screened. It does not check current-carrying capacity, voltage drop, grouping, installation method, conductor operating temperature under load, mechanical protection or minimum earthing conductor rules. Those matters still belong in the wider cable-selection workflow.

The k value is the material and temperature factor used in the adiabatic relationship. It depends on conductor material and the initial and final temperature limits assumed for the insulation system. Presets can support a fast review, but the correct value for a real project must come from the current standard, manufacturer data or the documented project basis.

Calculation workflow

Use the workflow as a record discipline as much as a calculation sequence. The result is easy to reproduce, but only if the inputs are tied to the same point in the installation and the same protective-device condition.

1. Identify the conductor being checked and whether it is an active conductor, protective conductor or earthing conductor.
2. Confirm the calculation point, such as main switchboard, distribution board, final subcircuit origin or equipment terminal.
3. Establish prospective fault current for that point using project data, a short-circuit study or a related calculator.
4. Establish the protective-device clearing time for that current and device setting.
5. Enter the conductor cross-sectional area in mm2.
6. Select a k preset only for screening, or enter the verified k value manually.
7. Add device let-through energy only when an applicable A2s value is available.
8. Compare fault energy, conductor withstand, required area and margin.
9. Record the k-value source, clearing-time source, conductor data and follow-up checks required by the project.

This order avoids a common error: solving the adiabatic equation first and then looking for inputs that make the result appear acceptable. The better approach is to decide the calculation point, current source, device data and conductor assumptions first. The calculator then makes the arithmetic consequence clear.

Formula interpretation

The main comparison is I2t <= k2S2. The left side is the thermal energy associated with the entered fault current and clearing time. The right side is the conductor's adiabatic withstand for the entered area and selected k value. If I2t is less than or equal to k2S2, the conductor is within the entered adiabatic withstand on this worksheet. If I2t is greater, the entered combination should not be applied without changing the project basis or reviewing conductor and protection data.

The minimum area by equation is Smin = sqrt(I2t) / k. This value is useful because it shows how much conductor area the entered current and time imply under the selected k value. It is not a cable-selection decision. Cable selection may still be governed by current-carrying capacity, voltage drop, installation conditions, mechanical requirements, earthing requirements, thermal grouping, standards requirements or manufacturer instructions.

Fault current has a strong effect because it is squared. Doubling the current creates four times the I2t energy for the same clearing time. Clearing time also matters directly. A protective device that clears quickly at a high fault current can produce a lower I2t than a device that clears more slowly at a lower current, depending on the actual curve. This is why device data matters.

k value review

The k value is not a decorative input. It carries assumptions about conductor material and temperature limits. Copper and aluminium have different k values. PVC and XLPE insulation systems use different temperature assumptions. A protective conductor that is bare, insulated, part of a cable, separate from a cable or installed in another arrangement may need different treatment under the applicable method.

The preset names are therefore written as material and insulation assumptions rather than as universal answers. A copper PVC preset and a copper XLPE preset should not be swapped just because the cable appears to be copper. The insulation system, initial operating temperature, final short-circuit temperature and standard method need to be consistent.

Manual k mode is often the better option for professional records. It forces the worksheet to show the exact k value used. The record should then state where that number came from. If a later reviewer changes the conductor material, insulation type, installation method or standard edition, the calculation can be repeated without guessing which hidden assumption was originally used.

Worked example: protective conductor

A reviewer checks a 16 mm2 copper protective conductor using a k value of 115. The entered prospective fault current is 5.5 kA and the clearing time is 0.1 seconds. The fault energy is 5,500 squared multiplied by 0.1, which gives 3,025,000 A2s. The conductor withstand is 115 squared multiplied by 16 squared, which gives 3,385,600 A2s.

The arithmetic comparison is within the entered adiabatic withstand. The minimum area implied by the equation is about 15.12 mm2, so the entered 16 mm2 conductor is above that arithmetic minimum. The result should still be recorded as a worksheet output, not as project closure. The record needs the selected k value source, the clearing-time source and the point where 5.5 kA applies.

Worked example: entered let-through energy

A second worksheet checks a 10 mm2 copper conductor with k equal to 115. The entered prospective fault current is 3 kA and the entered clearing time is 0.1 seconds. Current squared times time gives 900,000 A2s. The conductor withstand is 115 squared multiplied by 10 squared, or 1,322,500 A2s. The current-squared-time comparison is within the calculated conductor withstand for the entered current and time basis.

If the reviewer also enters 1,500,000 A2s as device let-through energy from manufacturer or project device data, the optional device comparison changes the review. That entered device energy is above the calculated conductor withstand. The correct response is not to force the calculator to produce a cable size. The correct response is to review the protective device, conductor, clearing-time basis, fault-current basis and applicable project requirements.

Relationship to other calculators

Use the short-circuit current calculator when the missing input is prospective fault current. That page estimates current from source impedance or transformer kVA and impedance data. Use the fault loop impedance calculator when the starting point is a measured or calculated loop impedance value. Use this I2t page only after the fault current and clearing time basis are known.

Use the voltage drop calculator when the concern is voltage loss under load. Voltage drop and short-circuit withstand are different criteria. A cable can sit within one entered check and still need review on the other. A complete cable-selection workflow may need current-carrying capacity, voltage drop, short-circuit temperature rise, protection, installation conditions and documentation. This page owns only the adiabatic withstand arithmetic.

Australian standards and project context

Australian electrical work should be checked against current standards, state or territory obligations, DNSP conditions where relevant, product information and project documentation. AS/NZS 3000 provides installation context, while AS/NZS 3008.1.1 is commonly used in Australian low-voltage cable-selection work. The public calculator does not reproduce protected tables or make a standards decision.

This conservative treatment is deliberate. The adiabatic equation can be written on one line, but the professional decision behind it is not one line. It depends on the right current, the right device clearing time, the right conductor data, the right k value and the right installation context. If any one of those inputs is uncertain, the result should be treated as a prompt for further review.

Manufacturer data can override a simple worksheet. Current-limiting devices, fuses, electronic trip units, settings, service conditions and device combinations may require curve or let-through data rather than a plain current-squared-times-time shortcut. The optional let-through field lets the user record an A2s comparison value, but the calculator does not decide whether that value is applicable.

Common errors

• Using the protective-device current rating as though it were fault current.
• Entering fault current in amperes while the field expects kiloamperes.
• Assuming a k value without checking conductor material and insulation temperature limits.
• Using a transformer-terminal fault current for a downstream conductor without considering the downstream path.
• Applying a device let-through value outside the conditions where the manufacturer data applies.
• Treating the minimum area by equation as cable-selection closure.
• Ignoring current-carrying capacity, voltage drop, grouping and installation method after one entered I2t check is within its arithmetic criterion.
• Failing to record the source of the clearing time and k value.

These errors usually occur because the equation is simple and the input sources are not. A good calculation record makes every input traceable. It should state the conductor, location, fault-current basis, protective-device clearing basis, k-value basis, entered area, result and required follow-up.

Minimum export record

Export only after the worksheet has a valid result and a traceable project reference. The modal should stay short; the calculation basis belongs in the project record and in the values captured on the result panel.

A strong record might read: "Distribution board protective conductor review, 5.5 kA prospective fault current at board, 0.1 s clearing time from device data, 16 mm2 copper protective conductor, k=115 from project cable basis, calculated I2t 3.025 MA2s and conductor withstand 3.386 MA2s." That sentence gives another reviewer enough information to repeat the calculation and question the inputs.

A weak record would only give a one-word outcome. That does not say where the current was taken, which protective device cleared the fault, which k value applied, what conductor was checked or what standard basis was used. This calculator is designed to produce the useful numbers, but the engineering record still belongs to the person reviewing the project.