In short. A cable must carry its load current without overheating, keep voltage drop within limits and survive the fault until protection clears it. Screening checks all three. The largest of the three requirements sets the size. Final selection must follow the applicable cable standard and the installation conditions.
Three checks
| Check | Question |
|---|---|
| Current rating | Can it carry the load current after derating for installation? |
| Voltage drop | Is the drop within the allowed limit at full load and during motor start? |
| Short-circuit withstand | Can it survive the fault until the protective device clears it? |
Load current
A 75 kW motor at 415 V, power factor 0.88 and efficiency 0.93 draws I = 75,000 ÷ (1.732 × 415 × 0.88 × 0.93) ≈ 128 A.
Current rating with correction factors
Cable tables give a current rating for reference conditions. Apply factors for ambient temperature, grouping with other cables, burial depth or soil resistivity, and installation method. The corrected rating must exceed the design current, which is the load current as seen by the protective device.
Voltage drop
Voltage drop (%) = Voltage drop ÷ V × 100
R and X are the cable’s resistance and reactance per length from the manufacturer’s tables. Many designs limit steady-state drop to around 3 to 5 per cent at the load end, subject to code and equipment needs, and check motor starting separately, when the drop is larger because the current is several times higher at low power factor.
Worked example (illustrative)
The 75 kW motor load (128 A) is 120 m from the panel. Suppose a 70 mm² copper cable has R about 0.32 Ω/km and X about 0.08 Ω/km at the operating temperature, with cos φ = 0.88 (sin φ = 0.475). Voltage drop = 1.732 × 128 × 0.12 × (0.32 × 0.88 + 0.08 × 0.475) = 26.6 × (0.2816 + 0.038) ≈ 8.5 V, about 2.0 per cent of 415 V. That passes a 3 per cent limit. In motor starting at six times current and power factor 0.3, the drop is several times higher, and should be checked against the motor and contactor tolerance. Values here are examples from typical tables, not data for a specific cable.
Short-circuit withstand
Isc is the fault current in amperes, t the clearing time in seconds and k a constant depending on conductor and insulation, found in the standard. This check often governs for short, heavily loaded cables close to a strong source.
Generators change the picture
- Generator fault current is lower, so clearing times may be longer.
- Voltage is more sensitive to drop because generator regulation also has limits.
- Long cables to rental connection points can drop significantly.
When to stop screening
For parallel cables, high-ambient or buried routes, harmonic currents, large motors, or high fault levels, use a full calculation according to the applicable standard. See cable sizing and voltage-drop studies.
A three-step cable screen
- Current rating: choose a size whose derated capacity exceeds the design current.
- Voltage drop: check at full load and at starting.
- Short-circuit and earth fault: check withstand and loop impedance.
Voltage drop (three-phase) ≈ √3 × I × L × (R cosφ + X sinφ), R and X in ohm/km, L in km
Worked example (illustrative)
A 100 A load at 0.85 PF over 80 m of 50 mm² copper (R ≈ 0.494, X ≈ 0.08 ohm/km). R cosφ = 0.42, X sinφ = 0.042, sum 0.462. ΔV = 1.732 × 100 × 0.08 × 0.462 ≈ 6.4 V, or 1.5 percent of 415 V. Acceptable under typical limits of 3 to 5 percent.
| Derating factor | Value (indicative, use the standard) |
|---|---|
| Ambient 45 °C vs 30 °C | About 0.87 to 0.9 |
| Four circuits grouped | About 0.65 to 0.75 |
| Buried in ground | Depends on soil thermal resistivity |
Generator note
Voltage drop on the generator feeder subtracts from the voltage dip budget at motor start. Keep generator cables short and sized for starting.
Limits of the screen
It does not cover harmonics, parallel runs, or short-circuit withstand in detail. See cable sizing and cable studies.
Common mistakes
- Selecting on current rating alone.
- Ignoring grouping and temperature factors.
- Forgetting the motor-start voltage drop.
- Ignoring short-circuit withstand.
What to send us
Send load, voltage, length, route and installation method. Send your application details.
Questions engineers ask
What voltage drop is acceptable?
Codes and equipment set limits; many designs aim for a few per cent at steady load. Check the applicable code and the equipment’s tolerance.
Why does motor starting need a separate check?
Starting current is several times higher at low power factor, so the drop is larger, and may stall the motor or drop contactors.
Copper or aluminium?
Both are used. Aluminium needs larger cross-section and careful terminations. Choose by cost, space and code.
Is this enough for a final design?
No. It is screening. Final design follows the cable standard and a verified fault level.
Values are illustrative. Use manufacturer cable data and applicable standards for design.