Cable Short-Circuit Withstand Calculator
Find the minimum cable cross-section that withstands a short-circuit from fault current, clearing time and conductor and insulation material.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this to check that a cable will not be damaged by heat while a fault is cleared. It applies the adiabatic equation, which assumes all the fault heat stays in the conductor, and returns the smallest cross-section and the next standard size above it.
Limits of this calculator
The method is valid for clearing times up to five seconds and uses the material constant for the conductor and insulation you choose. Use the clearing time of the protective device at the prospective fault current, not its trip setting. Confirm the constant against the standard edition you work to.
Cross-check: 30 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed. Basis: Adiabatic k values published in IEC 60364-4-43 / 60364-5-54; edition not revalidated.
What this calculator checks
- Minimum conductor area S = I√t / k with the built-in adiabatic k values.
What remains unverified
- The k values against the current edition of the standard (initial and final temperatures, size range, insulation).
- The real clearing time at the fault level: take it from the protective-device curve.
- Let-through energy of the breaker, and the withstand of screens and armour.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60364-4-43; IEC 60949, IEC 60364-4-43:2023 Ed 4.0; IEC 60949:1988 Ed 1.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Overcurrent protection and thermally permissible cable short-circuit current are relevant. The calculator uses S = I√t/k; confirm the correct k value, initial/final temperatures and adiabatic applicability in the project edition. |
| Cross-check test cases | 30 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Fault current (rms) | 25 kA |
| Clearing time | 1 s |
| Conductor and insulation | Copper / XLPE (k = 143) |
- k = 143
- S = 25,000 × √1 / 143 = 174.83 mm²
- Next standard size = 185 mm²
Assumptions
- Adiabatic heating (clearing time up to 5 s)
- k values at the normal starting temperature, as published in IEC 60364-4-43 and IEC 60364-5-54. For PVC the standard lowers k for conductors above 300 mm² (copper 103, aluminium 68), and this calculator applies that when the required area is above 300 mm²
- Fault current is the prospective rms value at the cable
- IEC-type k values apply on every basis. AS/NZS 3008.1.1:2025 uses its own K and final-temperature tables (for XLPE a higher K, reported as 167.4 for 250 °C), so do not use this result as an AS/NZS 3008 check
Use the clearing time of the protective device at the fault level, not its trip setting.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Fault current (rms) | kA | greater than 0 kA | |
| Clearing time | s | greater than 0 s, up to 5 s | |
| Conductor and insulation | (choice) | Copper / PVC (k = 115, 103 above 300 mm²); Copper / XLPE (k = 143); Aluminium / PVC (k = 76, 68 above 300 mm²); Aluminium / XLPE (k = 94) |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Minimum cross-section | mm² | Shown to 1 decimal place | |
| Standard size | mm² | Shown to 1 decimal place | |
| Material constant k | (number) | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Fault current (rms) (kA) | Minimum cross-section (mm²) | Standard size (mm²) | Material constant k |
|---|---|---|---|
| 6 | 42 | 50 | 143 |
| 10 | 69.9 | 70 | 143 |
| 16 | 111.9 | 120 | 143 |
| 25 | 174.8 | 185 | 143 |
| 40 | 279.7 | 300 | 143 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60364-5-52 (selection and erection of wiring systems, current-carrying capacity)
- IEC 60364-4-43 (protection against overcurrent, short-circuit withstand k factors)
- IEC 60228 (conductor sizes)
- IEC 60287 (calculation of cable current rating)
Ampacity and grouping factors come from your cable data sheet or your licensed copy of the standard; see STANDARDS.md for the method constants Elecmetric does use. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter fault current (rms), clearing time and conductor and insulation. Change the unit of a value with its selector and the result stays the same.
Why does the clearing time matter so much?
The required area grows with the square root of the time the fault lasts. A slower protective device needs a larger cable, and a current-limiting device can allow a smaller one.
Which fault current should I use?
The prospective rms fault current at the cable, usually the highest value, at the point where the cable starts. Use the clearing time of the device at that current.
Does this replace checking current-carrying capacity?
No. It checks only thermal withstand during a fault. Current rating and voltage drop are separate checks, covered by the cable size and cable derating calculators.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 30 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Cable I²t Withstand Check Formulas: Cable short-circuit withstand formulas: the I²t limit of a conductor
- Short-Circuit Formulas: Fault Level by MVA: Short-circuit formulas using the MVA method: transformer fault level, adding the source, three-phase fault current
- Fusing Current Formulas: Wire Fuse Estimate: Fusing current of a bare wire from Preece’s rule: current that melts a copper or aluminium wire in free air
- Conductor Force Formulas: Short-Circuit Forces: Force between parallel conductors from current and spacing: force per metre for a short circuit, total force on a span
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