Cable Size for Voltage Drop Calculator
Find the smallest standard cable cross-section that keeps voltage drop within your limit, from current, length, voltage, power factor and conductor type.
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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 when a long run, not the current rating, is likely to decide the cable size. Enter the load, the route length and the voltage drop you will accept, and it returns the minimum cross-section and the next standard size with the drop that size gives.
Limits of this calculator
This sizes for voltage drop only; it is not a cable rating. Check the derated current capacity and the short-circuit withstand before choosing a cable.
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: Sizes for voltage drop only from the IEC 60228 size list; ampacity is a separate check.
What this calculator checks
- Smallest IEC 60228 size that keeps voltage drop within the limit you set, with resistance at the full-load conductor temperature.
What remains unverified
- Current-carrying capacity (no ampacity table is embedded).
- Installation method, grouping, ambient and soil correction.
- Short-circuit withstand and protective-device coordination.
- Cable manufacturer AC resistance and reactance (a constant reactance is assumed).
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; IEC 60228, IEC 60364-5-52:2009 + A1:2024 Ed 3.1; IEC 60228:2023 Ed 4.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Wiring-system voltage-drop design and conductor dimensions/resistance are relevant. The calculator is a first-pass size estimate; confirm the actual cable data, installation method and permissible drop. |
| 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 |
|---|---|
| Load current | 400.94 A |
| Route length | 180 m |
| System voltage (L-L) | 400 V |
| Allowed drop (%) | 3 |
| Power factor | 0.9 |
| Conductor | Copper |
| Insulation | XLPE / EPR (90 °C) |
| Cable reactance | 0.08 Ω/km |
| Phase | Three-phase |
- Operating temperature 90 °C, ρθ = 0.021984 Ω·mm²/m
- Allowed drop = 3% × 400 V = 12 V
- Reactive part = 4.359 V
- A min = 323.67 mm²
- Next standard size = 400 mm², drop 10.54 V (2.64%)
Assumptions
- Resistivity taken at the full-load conductor temperature (70 or 90 °C)
- Reactance treated as constant, independent of size
- Standard sizes from the IEC 60228 preferred series, 1.5 to 630 mm²
- Copper 0.017241 and aluminium 0.02826 Ω·mm²/m at 20 °C
This sizes for voltage drop only. Check the result against derated ampacity and short-circuit withstand before selecting a cable.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Load current | A | greater than 0 A | |
| Route length | m | greater than 0 m | |
| System voltage (L-L) | V | greater than 0 V | |
| Allowed drop (%) | (number) | greater than 0, up to 20 | |
| 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. |
| Conductor | (choice) | Copper; Aluminium | |
| Insulation | (choice) | PVC (70 °C); XLPE / EPR (90 °C) | |
| Cable reactance | Ω/km | 0 Ω/km or more | Typical 0.07 to 0.09 Ω/km for multicore cable |
| Phase | (choice) | Three-phase; Single-phase |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Minimum cross-section | mm² | Shown to 1 decimal place | |
| Standard size | mm² | Shown to 1 decimal place | |
| Drop at standard size | V | Shown to 2 decimal places | |
| Drop as % of system voltage | (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.
Typical values
| Route length (m) | Minimum cross-section (mm²) | Standard size (mm²) | Drop as % of system voltage |
|---|---|---|---|
| 25 | 30.1 | 35 | 2.6 |
| 50 | 63.7 | 70 | 2.76 |
| 100 | 143.4 | 150 | 2.9 |
| 180 | 323.7 | 400 | 2.64 |
| 250 | 577.7 | 630 | 2.88 |
| 300 | 870.5 | — | — |
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, system voltage (L-L), allowed drop (%), power factor, conductor and insulation. Change the unit of a value with its selector and the result stays the same.
Is the size it gives enough for the current?
Not necessarily. Voltage drop and current capacity are separate checks and the larger size governs. Use the cable derating calculator for the current capacity, and check the short-circuit withstand as well.
What voltage drop limit should I use?
The limit comes from the wiring rules or specification you work to and differs by circuit type and country. Enter that figure; Elecmetric does not decide it for you.
Which standard sizes does it choose from?
The IEC 60228 preferred series from 1.5 to 630 mm², as listed in the assumptions below. If you buy another series, use the minimum cross-section and select upward from your own list.
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
- Voltage Drop Formulas for Cables: Voltage drop formulas for single-phase and three-phase cables, percentage drop
- Circuit Breaker Formulas: Rating and Trip: Circuit breaker formulas: rating from design current and cable capacity, the 1.45 overload rule, trip ranges of B
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