Conductor Resistance vs Temperature Calculator
Resistance of a copper or aluminium conductor at its operating temperature. With the formula, a worked example and typical values.
Try your own numbers
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.
Open in the calculator with these values
About this calculator
This page calculates resistance at temperature and change from 20 °C from resistance at 20 °C, operating temperature and conductor material. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
A cable at 90 °C has about 27% more resistance than the cold catalogue value. Voltage drop and loss should use the hot value.
Cross-check: 6 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.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
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 60228, 2023 Ed 4.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Conductor resistance and construction are in scope. The linear temperature coefficient is a material approximation; confirm actual conductor resistance and temperature limits from the cable standard and manufacturer. |
| Cross-check test cases | 6 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 |
|---|---|
| Resistance at 20 °C | 1 Ω |
| Operating temperature (°C) | 70 |
| Conductor material | Copper |
- Factor = 1 + 0.00393 × (70 − 20) = 1.1965
- RT = 1 × 1.1965 = 1.1965 Ω
Assumptions
- Linear temperature coefficient
- Valid across the normal operating range
A cable at 90 °C has about 27% more resistance than the cold catalogue value. Voltage drop and loss should use the hot value.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Resistance at 20 °C | Ω | greater than 0 Ω | |
| Operating temperature (°C) | (number) | -50 or more, up to 250 | |
| Conductor material | (choice) | Copper; Aluminium |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Resistance at temperature | Ω | Shown to 4 decimal places | |
| Change from 20 °C (%) | (number) | Shown to 2 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.
How the result changes with resistance at 20 °c
| Resistance at 20 °C (Ω) | Resistance at temperature (Ω) | Change from 20 °C |
|---|---|---|
| 0.1 | 0.1196 | 19.65 |
| 0.25 | 0.2991 | 19.65 |
| 0.5 | 0.5982 | 19.65 |
| 1 | 1.1965 | 19.65 |
| 2 | 2.393 | 19.65 |
| 4 | 4.786 | 19.65 |
| 10 | 11.965 | 19.65 |
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 resistance at 20 °C, operating temperature (°C) and conductor material. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Linear temperature coefficient. Valid across the normal operating range.
Which calculators should I use next?
Common next steps are Voltage Drop from Conductor Size and Cable Power Loss (I²R). The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 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.
- Conductor Resistance Formulas and Temperature: Conductor resistance formulas: resistance from length, area and resistivity, correction for 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
More Cable Engineering calculatorsAll engineering calculatorsConductor Resistance vs Temperature in Elecmetric