In short. A cable size is correct only when it passes every test: it carries the design current after derating, keeps voltage within limits in running and starting, survives the fault until protection clears, and lets the earth-fault device operate. Amps from a table are only the first check.
The sequence
- Design current. For a balanced three-phase load the current follows from power, voltage, power factor and efficiency.
I = P ÷ (√3 × V × PF × η)
- Installation method. Tray, conduit, buried, duct, free air, grouped.
- Ampacity with derating. Table value multiplied by correction factors for ambient temperature, grouping and soil. See ambient derating.
- Voltage drop. Check at running and starting current.
- Short-circuit withstand. Adiabatic check against fault current and clearing time.
- Earth-fault loop. Confirm the protective device operates within the required time.
- Mechanical and environmental. Bend radius, pulling, UV, oil, fire performance.
- Terminations. Lugs and glands that fit the conductor and equipment.
- Growth allowance.
Voltage drop
ΔV = √3 × I × L × (R cosφ + X sinφ)
Use consistent units and conductor data. The drop at the motor terminals during starting is often the governing condition for long runs, because starting current is several times running current and power factor is low. A cable that passes at running current can fail the starting check. See motor starting studies.
Short-circuit withstand
S ≥ (I × √t) ÷ k
Here S is conductor area, I the fault current, t the clearing time and k a material and insulation constant. Check with both maximum fault current (for energy) and minimum (for clearing time). See cable protection and coordination.
Parallel conductors
Parallel cables must be the same length, size and type, with matched terminations, and arranged to share current. Poor sharing can overload one conductor while the total looks acceptable. Measure individual currents at commissioning.
Generator-fed and temporary circuits
- Distances from rental sets can be long. Check voltage drop with the generator’s source impedance included.
- Lower generator fault current may not trip the breaker fast enough at the remote end.
- Temporary cables see handling, weather and repeated connection, so inspect and test them each deployment. See rental power.
Harmonics and neutrals
Nonlinear loads add heating and triplen harmonic currents in the neutral. A neutral the same size as phases may be insufficient. See harmonics.
Illustrative example
A 90 kW pump, 415 V, 0.88 power factor, 94 percent efficiency, draws about 150 A. A 70 mm² cable on a tray looks adequate. The 180 m run gives a running voltage drop within limits, but the starting drop with six times current exceeds the motor’s allowance when fed from the generator. The study moves to a larger conductor, or a soft starter, and rechecks the fault-loop limit.
Four checks every cable must pass
- Current-carrying capacity after derating for ambient temperature, grouping and installation method.
- Voltage drop at full load and at motor starting.
- Short-circuit withstand for the clearing time.
- Earth-fault loop impedance so protection trips quickly enough.
Voltage drop formula (three-phase)
ΔV ≈ √3 × I × L × (R cosφ + X sinφ) ÷ 1000, with R and X in ohm/km, L in km
Worked example (illustrative)
A 90 A motor at 0.85 power factor, 120 m of 35 mm² copper (R about 0.668 ohm/km, X about 0.08 ohm/km). R cosφ = 0.568, X sinφ = 0.042; sum 0.61. ΔV = 1.732 × 90 × 0.12 × 0.61 ≈ 11.4 V, which is about 2.7 percent of 415 V. Acceptable under a typical limit of 3 to 5 percent; during starting at six times current the drop would be far greater, so check starting separately.
Derating inputs (indicative)
| Factor | Effect |
|---|---|
| Ambient 45 °C versus 30 °C reference | Rating drops by roughly 10 to 15 percent |
| Grouped circuits on a tray | Further reduction by grouping factor |
| Harmonic neutral current | Neutral may need to be full size |
Use tabulated values from the governing cable standard and manufacturer data.
Generator feeders
Voltage drop between generator and board reduces the voltage at the load and consumes part of the generator’s voltage-dip allowance. See cable sizing.
Common mistakes
- Stopping at ampacity.
- Ignoring the starting case.
- No derating for grouping.
What to send us
Send the cable schedule, lengths and load data. Send your application details. See also cable sizing and voltage drop.
Questions engineers ask
What voltage drop is acceptable?
Limits come from standards, equipment and the process. Define them before sizing.
Is aluminium acceptable?
Yes, with proper terminations and sizing. Compare installed cost and reliability, not just price.
Why does the earth-fault loop matter?
The protective device needs enough fault current to disconnect in time, and long cables reduce it.
Do I need a cable register?
For larger sites it helps track size, length, tests and history.
Values are illustrative. Follow the applicable standards for ratings and factors.