Cable Derating and Ampacity Calculator
Derate a cable for ambient temperature and grouping to get its usable current capacity, and the tabulated ampacity you must start from for a given breaker.
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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 find the current a cable can actually carry once the ambient temperature and grouping are accounted for, or the tabulated ampacity a cable must have to be protected by a given device. It also checks the design current against the device rating and the derated capacity.
Limits of this calculator
Tabulated ampacities are not built in, because they depend on the installation method and come from licensed standards. Enter the value from your cable data sheet or the standard. The calculation bases apply the factors listed in the assumptions below; they are not complete regional table lookups, so check the result against the standard 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: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Temperature factor is a heat-balance approximation, not the tabulated standard factor.
What this calculator checks
- Iz = tabulated rating you enter × temperature factor × grouping factor × other factor.
- Rule Ib ≤ In ≤ Iz for the chosen basis (1.25 × Ib for the NEC-style continuous case).
What remains unverified
- The temperature factor is a heat-balance approximation, not the factor tabulated in the wiring standard.
- The tabulated rating, grouping factor and installation method are yours to take from the data sheet or licensed standard.
- Harmonics, soil thermal resistivity, voltage drop and short-circuit withstand.
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 60364-5-52, 2009 + A1:2024, consolidated Ed 3.1 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Cable current-carrying capacity and correction factors are relevant. The in-app square-root temperature factor is a simplified heat-balance approximation, not a tabulated IEC derating factor; group and other factors are user inputs. |
| Cross-check test cases | 30 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
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 |
|---|---|
| Design current | 160.4 A |
| Protective device rating | 200 A |
| Tabulated ampacity (data sheet, reference conditions) | 300 A |
| Calculation basis | General / IEC-style (70 / 90 °C, 30 °C table ambient) |
| Insulation | XLPE / EPR (90 °C conductor) |
| Ambient temperature (°C) | 40 |
| User-defined conductor limit (°C) | 90 |
| User-defined factor on design current | 1 |
| Reference ambient of the tabulated value (°C) | 30 |
| Grouping factor | 0.8 |
| Other correction factor | 1 |
- θmax = 90 °C
- kt = √((90 − 40) / (90 − 30)) = 0.9129
- Combined factor = 0.9129 × 0.8 × 1 = 0.7303
- Iz = 300 × 0.7303 = 219.09 A
- Required tabulated ampacity = 200 / 0.7303 = 273.86 A
Assumptions
- Approximate temperature factor from a steady-state conductor heat-balance model; it is not a standard-table correction factor
- Tabulated ampacity and grouping factor supplied by the user
- Rule checked: cf × Ib ≤ In ≤ Iz, with cf set by the chosen basis (1.0 General, 1.25 NEC-style continuous)
The temperature multiplier is an engineering approximation, not a substitute for the ambient-temperature correction factor in the applicable wiring standard or cable-manufacturer tables. For final cable selection, use the prescribed factor for the actual cable type, installation method, ambient conditions and grouping. Tabulated ampacity values are not built in because they depend on installation method and may be protected data; read them from your data sheet or licensed standard.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Design current | A | greater than 0 A | |
| Protective device rating | A | greater than 0 A | |
| Tabulated ampacity (data sheet, reference conditions) | A | greater than 0 A | Value from the cable manufacturer or your applicable standard for the installation method |
| Calculation basis | (choice) | General / IEC-style (70 / 90 °C, 30 °C table ambient); IS India (70 / 90 °C, 40 °C table ambient); BS 7671 UK (70 / 90 °C, 30 °C table ambient) - provisional, not checked against the 2026 amendment; AS/NZS 3008 ampacity basis (75 / 90 °C, 40 °C table ambient) - provisional, not checked against the 2025 edition; NEC-style (75 / 90 °C, 125% on continuous load); User-defined (set below in Engineering mode) | |
| Insulation | (choice) | PVC (70 °C; 75 °C under AS/NZS and NEC-style); XLPE / EPR (90 °C conductor) | |
| Ambient temperature (°C) | (number) | -50 or more, up to 80 | |
| User-defined conductor limit (°C) | (number) | 50 or more, up to 250 | Used only with the user-defined basis |
| User-defined factor on design current | (number) | 1 or more, up to 2 | Used only with the user-defined basis; 1.25 for continuous loads under NEC-style rules |
| Reference ambient of the tabulated value (°C) | (number) | -50 or more, up to 80 | 30 °C for cables in air in IEC-based tables; check your data sheet |
| Grouping factor | (number) | 0.1 or more, up to 1 | From the grouping table for your installation method |
| Other correction factor | (number) | 0.1 or more, up to 1.5 | Soil resistivity, depth, harmonics or similar |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Temperature factor (approx.) | (number) | Shown to 3 decimal places | |
| Derated current capacity | A | Shown to 1 decimal place | |
| Tabulated ampacity required | A | Shown to 1 decimal place | |
| Minimum device rating for this basis | A | Shown to 1 decimal place |
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
| Ambient temperature (°C) | Temperature factor | Derated current capacity (A) | Tabulated ampacity required (A) |
|---|---|---|---|
| 30 | 1 | 240 | 250 |
| 35 | 0.957 | 229.8 | 261.1 |
| 40 | 0.913 | 219.1 | 273.9 |
| 45 | 0.866 | 207.8 | 288.7 |
| 50 | 0.816 | 196 | 306.2 |
| 55 | 0.764 | 183.3 | 327.3 |
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 design current, protective device rating, tabulated ampacity (data sheet, reference conditions), calculation basis, insulation, ambient temperature (°C) and grouping factor. Change the unit of a value with its selector and the result stays the same.
Where do I get the tabulated ampacity?
From the cable manufacturer data sheet or the installation table in the standard you work to, for your installation method and conductor. Elecmetric applies the correction factors to that figure; it does not look it up.
How is the temperature factor worked out?
It comes from the heat balance of the conductor, the square root of the margin between the conductor limit and the ambient, over the margin between the conductor limit and the reference ambient of the table. The formula above shows it.
Is cable ampacity a separate calculation from derating?
No. The derated current capacity is the ampacity for your conditions, so this one page covers both questions.
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 Size Formulas: Current and Short-Circuit: Cable sizing formulas for current capacity with correction factors, conductor resistance and temperature
- Cable Loss Formulas: Power and Annual Cost: Cable power loss formulas: I squared R loss for single-phase and three-phase cables, annual energy and cost of the loss
- Busbar Temperature Rise Formulas: Busbar heating formulas: the resistance and heat per metre
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