An 11kV cable is a High Voltage (HV) electrical cable used for power distribution, rated for a nominal voltage of 11,000 volts. Selecting the correct 11kV cable requires calculating the appropriate size (in mm²) based on your electrical load (in kW or Amps) and ensuring it meets installation standards for conductor type, core count, and burial depth.
How to Choose the Right 11kV Cable: 3 Key Factors
To choose the right 11kV cable, you must first determine your required conductor material (Aluminium or Copper), the number of cores (typically 3-core for balanced loads), and finally, the conductor size (mm²) based on your total power demand in Amps.
Aluminium vs. Copper Conductors
Aluminium (XLPE/AWA) Conductors:
- Lower conductivity than copper
- Lighter weight – ideal for overhead installations
- Lower cost – typically 30-40% cheaper than copper
- Less flexible – requires larger bend radius
- Typical use: Power distribution networks, overhead lines
Copper (XLPE/SWA) Conductors:
- Higher conductivity – better for high-load applications
- Heavier weight – more robust for underground use
- Higher cost – premium option for critical installations
- More flexible – easier to install in tight spaces
- Typical use: Industrial installations, underground networks, high-load applications
Best for Budget & Overhead: Choose Aluminium conductors for long-distance power distribution where weight and cost are primary concerns.
Best for Industrial & Underground: Choose Copper conductors for high-load industrial applications and underground installations where conductivity and durability are critical.
Determining Core Count
The number of cores in your 11kV cable depends on your system configuration:
Is your system a three-phase balanced load?
- Yes: A 3-Core cable is the standard and most efficient choice
- No: For single-phase systems or those requiring a separate earth conductor, consider a 1-Core or 4-Core cable. Consult an electrical engineer for unbalanced loads.
How to Calculate 11kV Cable Size for Your Electrical Load
To calculate 11kV cable size, first convert your load in kW to Amps using the formula:
Amps = (kW × 1000) / (1.732 × 11,000V × Power Factor)
A typical power factor is 0.8. Then, use the current rating chart below to match the calculated Amps to the correct cable size in mm².
11kV Cable Sizing & Current Rating Table (XLPE/SWA Copper)
| Cable Size (mm²) | Current Rating in Ground (Amps) | Current Rating in Air (Amps) |
|---|---|---|
| 70 | 205 | 220 |
| 95 | 240 | 260 |
| 120 | 270 | 295 |
| 150 | 300 | 330 |
| 185 | 340 | 375 |
| 240 | 390 | 430 |
| 300 | 440 | 485 |
Sizing an 11kV Cable for a 500 kW Load – Step by Step
- Calculate Amps: Amps = (500 × 1000) / (1.732 × 11000 × 0.8) = 32.8 Amps
- Apply Derating Factors: Consider factors like ambient temperature, soil thermal resistivity, and cable grouping. Common derating factors can reduce current capacity by 10-30%.
- Select Cable Size: Based on the chart, even the smallest standard size (70mm²) can handle this load, but engineering best practices might require a larger size for voltage drop considerations over distance.
Quick Sizing Tip: Ensure your calculated cable size has a current rating at least 25% higher than your operational load. This provides a safe operating margin.
How Deep Should 11kV Cables Be Buried?
The standard burial depth for 11kV cables is a minimum of 900mm (0.9 metres) from the ground surface to the top of the cable. This depth can increase depending on the surface type, such as under roadways, where it is typically 1200mm (1.2 metres) or more.
Installation Depth Checklist
- Cable trench excavated to a minimum of 1050mm
- 75mm layer of sand bedding laid and compacted
- Cable laid in the trench
- Another 75mm layer of sand laid over the cable
- Cable protection tiles or warning tape placed at ~300mm above the cable
- Backfill with subsoil, ensuring no sharp rocks
- Compact the surface
Common Installation Mistakes
Problem: Insufficient depth
Solution: Re-excavate. Failure to meet depth standards can lead to cable damage and severe safety risks.
Problem: Using sharp backfill
Solution: Remove and replace with sifted soil or sand to prevent sheath damage.
Required Resources
You will need:
- Warning Tape (marked “DANGER HIGH VOLTAGE CABLE BELOW”)
- Cable Tiles for mechanical protection
- Fine-grain sand for bedding
What is the Difference Between 11kV and 33kV Cable?
The primary difference is their voltage rating. 11kV cables are designed for a maximum of 11,000 volts, while 33kV cables handle 33,000 volts. This difference in voltage capacity necessitates thicker insulation and greater physical dimensions for the 33kV cable.
Key Takeaway: Use 11kV cables for primary distribution to substations or large industrial facilities. Use 33kV cables for higher-level power transmission over longer distances before it’s stepped down to 11kV.
Real-World Applications
11kV Example: Powering a large factory, a hospital, or a residential neighbourhood from a local substation.
33kV Example: Transmitting power from a wind farm to a regional grid substation.
In Summary
Selecting and installing 11kV cables requires careful consideration of conductor material, core count, and proper sizing based on your electrical load. Whether you’re choosing between aluminium and copper conductors, calculating the appropriate cable size, or ensuring proper burial depth, following these guidelines will help ensure a safe and efficient high voltage distribution system. Always consult with qualified electrical engineers like the team at Wilken when working with high voltage installations.
