Cable Power Loss Calculator
Heat dissipated in a cable run and the share of the load power it wastes. 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 cable loss and loss as % of load power from load current, route length, conductor resistance, system voltage and 1 more input. 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
Cable loss is continuous heat. Over a year it is often worth a larger conductor than the voltage-drop limit alone requires.
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 | Not applicable: I²R loss is a textbook circuit identity. Cable resistance and installation data should be taken from the relevant conductor standard and manufacturer, but the equation itself has no separate governing calculation standard. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| 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; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Load current | 100 A |
| Route length | 1,000 m |
| Conductor resistance | 0.2 Ω/km |
| System voltage (L-L) | 400 V |
| Power factor | 0.9 |
| Phase | Three-phase |
- Conductor resistance = R × L = 0.2 Ω
- Loss = 3 × 100² × 0.2 = 6,000 W
- Load power = 62.35 kW, loss share = 9.623 %
Assumptions
- Balanced load
- Resistance is the AC value at operating temperature
- Neutral current ignored
Cable loss is continuous heat. Over a year it is often worth a larger conductor than the voltage-drop limit alone requires.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Load current | A | greater than 0 A | |
| Route length | m | greater than 0 m | |
| Conductor resistance | Ω/km | greater than 0 Ω/km | At operating temperature, from the cable data sheet |
| System voltage (L-L) | V | greater than 0 V | |
| Power factor | (number) | greater than 0, up to 1 | Rough starting point only. Resistive heating and incandescent loads are 1.0; a fully loaded induction motor is typically 0.8 to 0.88 and falls sharply when lightly loaded; discharge lighting depends on its ballast or driver. For real work use the nameplate or a measured value. |
| Phase | (choice) | Three-phase; Single-phase |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Cable loss | W | Shown to 1 decimal place | |
| Loss as % of load power | (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 load current
| Load current (A) | Cable loss (W) | Loss as % of load power |
|---|---|---|
| 10 | 60 | 0.96 |
| 25 | 375 | 2.41 |
| 50 | 1,500 | 4.81 |
| 100 | 6,000 | 9.62 |
| 200 | 24,000 | 19.25 |
| 400 | 96,000 | 38.49 |
| 1,000 | 600,000 | 96.23 |
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 load current, route length, conductor resistance, system voltage (L-L) and power factor. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Balanced load. Resistance is the AC value at operating temperature. Neutral current ignored.
Which calculators should I use next?
Common next steps are Voltage Drop and Appliance Running Cost (Energy and Cost). 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.
- 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
- Conductor Resistance Formulas and Temperature: Conductor resistance formulas: resistance from length, area and resistivity, correction for temperature
- Voltage Drop Formulas for Cables: Voltage drop formulas for single-phase and three-phase cables, percentage drop
- Cable Sheath Circulating Current Formulas: Sheath circulating current formulas for single-core cables bonded at both ends: the mutual reactance in trefoil
- Cable Charging Current Formulas: Cable charging current formulas: the capacitance of a length of cable
More Cable Engineering calculatorsAll engineering calculatorsCable Power Loss (I²R) in Elecmetric