What is this cable size tool used for?
This tool calculates the minimum conductor cross-section that satisfies a chosen voltage-drop limit, based on line current, cable length, conductor material and allowable voltage-drop percentage.
It can be used as a first-pass cross-section estimate for panel feeder circuits, motor and drive connections and remote field-equipment feeds.
Formula used
- General: A = k x I x L x ρ / ΔU
- This is the voltage-drop formula (ΔU = k x I x L x ρ / A) solved for cross-section.
Variables and meaning
- I
- Line current.
- L
- One-way cable length.
- ρ
- Conductor resistivity (approximate values at 20°C: copper 0.0175, aluminum 0.028 Ω·mm²/m).
- ΔU
- Allowable voltage drop, computed from source voltage and the allowed percentage.
- k
- Return-conductor or three-phase factor depending on system type.
- A
- Calculated required minimum cross-section.
Standard cross-section table
The result is rounded up to the first value at or above the calculated minimum from these common commercial cross-sections (mm²):
- 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300
Unit reference tables
Voltage units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Millivolt (mV) | mV | 0.001 V | Sensors and low-level signals |
| Volt (V) | V | 1 V | Base SI voltage unit |
| Kilovolt (kV) | kV | 1,000 V | High-voltage transmission |
Current units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Milliampere (mA) | mA | 0.001 A | Electronic circuit currents |
| Ampere (A) | A | 1 A | Base SI current unit |
| Kiloampere (kA) | kA | 1,000 A | Industrial power systems |
Length units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Millimeter (mm) | mm | 0.001 m | Thin layers and small diameters |
| Centimeter (cm) | cm | 0.01 m | Short dimensions |
| Meter (m) | m | 1 m | Base SI length unit |
| Kilometer (km) | km | 1,000 m | Very large distances and long layers |
| Inch (in) | in | 0.0254 m | Imperial/US thickness measurements |
| Foot (ft) | ft | 0.3048 m | Imperial/US length measurements |
Worked examples
400 V, 20 A, 50 m, 3% allowable drop, copper, three-phase
ΔU_allowed = 400 x 0.03 = 12 V. A = 1.732 x 20 x 50 x 0.0175 / 12 which gives about 2.53 mm². The nearest standard cross-section is 4 mm².
230 V, 16 A, 25 m, 3% allowable drop, copper, single-phase
ΔU_allowed = 230 x 0.03 = 6.9 V. A = 2 x 16 x 25 x 0.0175 / 6.9 which gives about 2.03 mm². The nearest standard cross-section is 2.5 mm².
Typical applications
- Panel feeder circuits
- Motor and drive connections
- Remote field-equipment feeds
- Quick cross-section estimates during project estimation
Assumptions and limitations
- This calculation recommends a cross-section based on the voltage-drop limit only; current-carrying capacity (ampacity), installation method, grouping and ambient-temperature corrections are not included.
- The recommended cross-section must be verified separately against ampacity tables in the applicable electrical code (e.g. IEC, national or local regulations).
- The resistivity values used (copper 0.0175, aluminum 0.028 Ω·mm²/m) are approximate 20°C reference values.
- If the calculated cross-section exceeds the 300 mm² standard table, the tool does not suggest a standard value; a custom cross-section or parallel conductors should be evaluated.