Cable Selection and Sizing: Top 20 Engineering Questions

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At Industry Solutions, we often receive inquiries from plant engineers and maintenance teams about how to perform proper Cable Selection and Sizing for their electrical systems. Choosing the right cable isn’t just about current rating—it directly affects equipment reliability, safety, and operational efficiency.
Whether it’s a motor feeder, MCC panel, or underground power line, selecting the correct cable size and type can prevent costly downtime, overheating, and energy losses.

In this guide, we’ve compiled the 20 most common questions we get from industrial clients about cable selection and sizing—each answered with practical insights and engineering reasoning to help you make the right decisions for your facility.

1. What is the difference between cable selection and cable sizing?

Cable selection refers to choosing the right cable type and construction (e.g., PVC, XLPE, armoured, single-core), while cable sizing means determining the correct cross-sectional area of the conductor to carry the required current safely without overheating or excessive voltage drop.

2. What factors affect cable selection and sizing?

Key factors include:

  • Load current

  • Voltage level (LV, MV, HV)

  • Installation method (buried, in conduit, in air)

  • Ambient temperature

  • Grouping of cables

  • Voltage drop limit

  • Short circuit capacity

  • Derating factors

  • Type of load (motor, lighting, heating)

3. How is current carrying capacity determined?

The current carrying capacity (ampacity) is based on the conductor material, insulation type, ambient conditions, and installation method. Standards like IEC 60364 or NEC Table 310.16 provide reference values that must be adjusted using derating factors.

4. What is voltage drop and how much is acceptable?

Voltage drop is the reduction in voltage along the length of the cable due to resistance and reactance.
Typical limits:

  • 3% for lighting circuits

  • 5% for power circuits
    Excessive voltage drop can cause equipment malfunction or energy loss.

5. How do I calculate voltage drop?

Voltage drop (Vd) = √3 × I × (R × cosφ + X × sinφ) × L
Where:

  • I = current

  • R = resistance per km

  • X = reactance per km

  • L = length in km
    Software tools or manufacturer tables can simplify this process.

6. Which cable insulation should I choose (PVC, XLPE, EPR)?

  • PVC: cost-effective, limited temperature (70°C)

  • XLPE: higher thermal rating (90°C), better for industrial use

  • EPR: excellent flexibility and thermal stability, ideal for harsh environments

7. When should I use armoured cables?

Use armoured cables when mechanical protection is required—such as underground installations or areas exposed to physical damage. Steel Wire Armoured (SWA) or Aluminium Wire Armoured (AWA) cables are common in LV and MV systems.

8. Copper vs Aluminium – which is better?

  • Copper: higher conductivity, smaller size, more expensive

  • Aluminium: lighter and cheaper but larger in size for the same ampacity
    Copper is preferred for critical circuits; aluminium suits long-distance power distribution.

9. How does ambient temperature affect cable sizing?

Higher temperatures reduce the current-carrying capacity. For example, if the standard ampacity is rated at 30°C, and your site has 45°C, apply a temperature derating factor (typically around 0.87).

10. What is a derating factor and why is it important?

A derating factor adjusts the rated current capacity to match actual site conditions.
Factors include:

  • Ambient temperature

  • Number of cables grouped

  • Soil thermal resistivity (for buried cables)

11. How to select a cable for motors or VFDs?

For motor feeders, consider:

  • Full load current + starting current

  • Ambient temperature

  • Cable length and voltage drop

  • Type of starting (DOL, Star-Delta, Soft Starter, VFD)
    For VFD-fed motors, always use shielded cables to minimize EMI and bearing currents.

12. Do I need to consider motor starting current in cable sizing?

Yes. Motor starting current (6–8× FLA for DOL) can cause a temporary voltage dip. Ensure the cable can handle the transient current without excessive voltage drop or overheating.

13. What standards are used for cable selection and sizing?

Common standards include:

  • IEC 60287 – Calculation of current rating

  • IEC 60364 – Electrical installations

  • BS 7671 – IET Wiring Regulations

  • NEC (NFPA 70) – U.S. standard for cable ampacity

14. What is short-circuit rating and why is it important?

Cables must withstand the thermal and mechanical stress caused by a short circuit until the protective device trips.
The formula:
Isc = √(Ad × K² × t)
Where K depends on material and insulation, and t is fault duration.

15. How do grouping and spacing affect ampacity?

When multiple cables run together, heat dissipation decreases, requiring grouping derating factors. The more cables grouped, the lower each cable’s ampacity.

16. What about underground cable installations?

For buried cables, consider:

  • Soil thermal resistivity

  • Installation depth

  • Number of circuits

  • Ambient soil temperature
    Derating factors from IEC 60287-2-1 are applied to adjust current rating.

17. What is the difference between single-core and multi-core cables?

  • Single-core: better cooling, lower reactance, easier for high current systems

  • Multi-core: easier handling and protection, used for control or low-power loads

18. How do harmonics affect cable selection?

Non-linear loads like VFDs and UPSs cause harmonics that increase cable heating.
Use oversized conductors or derating (by ~10–15%) to handle additional losses.

19. How do I verify that the selected cable is correct?

Verification includes:

  • Checking ampacity vs actual load

  • Confirming voltage drop < allowed limit

  • Verifying short-circuit withstand capacity

  • Ensuring compliance with protection device rating

20. How do I document cable sizing in a project?

Create a Cable Schedule including:

  • Source and destination

  • Cable type and size

  • Length and installation route

  • Voltage drop and derating calculations

  • Reference standards

This documentation ensures transparency, traceability, and easier maintenance later.

Conclusion

Proper cable selection and sizing ensures reliability, safety, and efficiency in any industrial electrical system. By following international standards and considering installation conditions, you can avoid overheating, energy loss, and unplanned downtime.
For complex systems with multiple loads, using advanced software and field verification ensures long-term performance and compliance.


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