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Motor Cable Size & Voltage Drop Calculator
Estimate the minimum copper cable cross-section and voltage drop for a three-phase electric motor circuit from current and cable length.
Design current: — A
Suggested cross-section: — mm²
Voltage drop: — V (—%)
ΔU = √3 · I · L · R · cosφ ; target ≤ 3–5%
Indicative (copper, PVC, ~30°C). Always verify per local standards / an engineer; motor circuits also need protection coordination.
How it works
The design current is the motor current × a sizing factor (≈1.25 for continuous motor duty). The tool picks the smallest standard copper cross-section whose ampacity ≥ design current, then computes the resistive voltage drop over the run. Keep voltage drop within ~3–5%. Related: full-load current calculator.
Copper conductor reference (indicative)
| Cross-section (mm²) | Ampacity (A) | Resistance (Ω/km) |
|---|---|---|
| 1.5 | 18.5 | 12.1 |
| 2.5 | 25 | 7.41 |
| 4 | 34 | 4.61 |
| 6 | 43 | 3.08 |
| 10 | 60 | 1.83 |
| 16 | 80 | 1.15 |
| 25 | 101 | 0.727 |
| 35 | 126 | 0.524 |
| 50 | 153 | 0.387 |
| 70 | 196 | 0.268 |
| 95 | 238 | 0.193 |
| 120 | 276 | 0.153 |
| 150 | 319 | 0.124 |
| 185 | 364 | 0.0991 |
| 240 | 430 | 0.0754 |
Frequently asked questions
How do I choose a motor cable size?
Take the motor full-load current, multiply by a sizing factor (~1.25 for continuous duty), then pick the smallest cable whose current-carrying capacity (ampacity) is at least that value. Finally check the voltage drop is acceptable.
What voltage drop is acceptable?
Commonly ≤3% for feeders and ≤5% total to the load. Long runs may need a larger cross-section than ampacity alone requires.
Why multiply the current by 1.25?
Motor circuits run continuously and must not overheat the cable; codes typically require conductors rated at ~125% of the motor full-load current.
Is this calculation exact?
No — it is indicative for copper PVC at ~30°C. Ampacity depends on installation method, grouping, ambient temperature and standard. Always confirm with an electrical engineer.