Cable Resistance Calculator
Conductor resistance from length, cross-section and material. With the formula, a worked example and typical values.
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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
This page calculates resistance and per unit length from length, cross-section and resistivity at 20 °C. 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
Use the loop (out and back) value for a two-wire circuit.
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 classes, nominal cross-sectional areas and DC resistance are in scope. The calculator uses material resistivity and an approximate temperature correction; verify resistance against the cable datasheet and standard tables. |
| 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 |
|---|---|
| Length | 100 m |
| Cross-section | 16 mm² |
| Resistivity at 20 °C (Ω·mm²/m) | 0.017241 |
| Operating temperature (°C) | 20 |
| Temperature coefficient (per °C) | 0.00393 |
- Temperature factor = 1 + α (T − 20) = 1
- R = ρ × factor × L / A = 0.10776 Ω
Assumptions
- Single conductor, one way
- Linear temperature correction
- DC resistance, no skin or proximity effect
Use the loop (out and back) value for a two-wire circuit.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Length | m | greater than 0 m | |
| Cross-section | mm² | greater than 0 mm² | |
| Resistivity at 20 °C (Ω·mm²/m) | (number) | greater than 0 | Copper 0.017241, aluminium 0.02826 |
| Operating temperature (°C) | (number) | -50 or more, up to 250 | |
| Temperature coefficient (per °C) | (number) | greater than 0, up to 0.01 | Copper 0.00393, aluminium 0.00403 |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Resistance | Ω | Shown to 4 decimal places | |
| Per unit length | Ω/km | Shown to 4 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 length
| Length (m) | Resistance (Ω) | Per unit length (Ω/km) |
|---|---|---|
| 10 | 0.0108 | 1.0776 |
| 25 | 0.0269 | 1.0776 |
| 50 | 0.0539 | 1.0776 |
| 100 | 0.1078 | 1.0776 |
| 200 | 0.2155 | 1.0776 |
| 400 | 0.431 | 1.0776 |
| 1,000 | 1.0776 | 1.0776 |
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 length, cross-section and resistivity at 20 °C (Ω·mm²/m). Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Single conductor, one way. Linear temperature correction. DC resistance, no skin or proximity effect.
Which calculators should I use next?
Common next steps are Voltage Drop and Maximum Cable Length. 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
- Skin Effect Formulas: Skin Depth: Skin effect formulas: skin depth from resistivity and frequency for copper and aluminium
- 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 calculatorsCable Resistance in Elecmetric