Maximum Cable Length Calculator
Longest run that keeps voltage drop within a limit. 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 maximum length from load current, system voltage, allowed drop, cable resistance 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
Check cable ampacity and fault withstand separately.
Cross-check: 14 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
- Longest run that stays within the drop limit for the R and X you enter.
What remains unverified
- Ampacity and short-circuit withstand.
- Earth-fault loop impedance and disconnection time at that length.
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 | Wiring-system selection and voltage-drop context. The calculator solves a simplified voltage-drop equation; it does not verify ampacity, fault protection or installation method. |
| Cross-check test cases | 14 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 | 100 A |
| System voltage (L-L) | 400 V |
| Allowed drop (%) | 3 |
| Cable resistance | 0.1 Ω/km |
| Cable reactance | 0.08 Ω/km |
| Power factor | 0.9 |
| Phase | Three-phase |
| Conductors per phase in parallel | One (single set) |
- Z = R cos φ + X sin φ = 0.1249 Ω/km
- L = 3% × 400 / (√3 × 100 × Z) = 554.8 m
Assumptions
- Balanced load
- R and X are per-km values for one conductor, supplied by the user
- Parallel conductors are identical, the same length and share the current equally
- Single-phase uses out-and-return path
Check cable ampacity and fault withstand separately.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Load current | A | greater than 0 A | |
| System voltage (L-L) | V | greater than 0 V | |
| Allowed drop (%) | (number) | greater than 0, up to 20 | |
| Cable resistance | Ω/km | greater than 0 Ω/km | |
| Cable reactance | Ω/km | 0 Ω/km or more | |
| 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 | |
| Conductors per phase in parallel | (choice) | One (single set); 2 in parallel; 3 in parallel; 4 in parallel; 5 in parallel; 6 in parallel; 7 in parallel; 8 in parallel; 9 in parallel |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Maximum length | m | Shown to 0 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) | Maximum length (m) |
|---|---|
| 10 | 5,548 |
| 25 | 2,219 |
| 50 | 1,110 |
| 100 | 555 |
| 200 | 277 |
| 400 | 139 |
| 1,000 | 55 |
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, system voltage (L-L), allowed drop (%), cable resistance 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. R and X are per-km values for one conductor, supplied by the user. Parallel conductors are identical, the same length and share the current equally. Single-phase uses out-and-return path.
Which calculators should I use next?
Common next steps are Voltage Drop and Cable Resistance. 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. 14 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.
- Voltage Drop Formulas for Cables: Voltage drop formulas for single-phase and three-phase cables, percentage drop
- Cable Size Formulas: Current and Short-Circuit: Cable sizing formulas for current capacity with correction factors, conductor resistance and temperature
More Cable Engineering calculatorsAll engineering calculatorsMaximum Cable Length in Elecmetric