For medium-voltage (MV) power cables (6kV–35kV), current-carrying capacity (ampere capacity) is fundamental to the design of safe and reliable systems. Inappropriate ampere capacity selection can lead to overheating, insulation aging, partial discharge, and even catastrophic breakdown. This guide, based on the IEC 60287 standard, IEEE research, and field engineering data, details key influencing factors, practical derating rules, and real-world case studies for engineers and construction teams.
1. Core Definition: What is the Current Carrying Capacity of Medium-Voltage Cables?
Current carrying capacity refers to the maximum continuous current a cable can carry under specific conditions, provided that this current does not exceed the temperature limit of its insulation layer. For medium-voltage cables with cross-linked polyethylene (XLPE) insulation:
- Continuous operating temperature: 90°C
- Short-circuit withstand temperature: 250°C (maximum 5 seconds)
- Calculation standard: IEC 60287 series (global reference standard for current carrying capacity of medium-voltage cables)
IET field studies have confirmed that external thermal environments can cause cable temperature rise by up to 70%, therefore, environment and installation methods are the most critical design factors.
2. Key Factors Determining the Current Carrying Capacity of Medium-Voltage Cables
① Conductor Material and Cross-Section
- Conductor Type: Copper (Cu) has approximately 20% higher conductivity than aluminum (Al), thus providing a higher current carrying capacity for the same cross-section.
- Cross-Section Size: Larger conductors reduce resistance and improve heat dissipation, directly increasing current carrying capacity.
- Standard Medium-Voltage Cable Cross-Section Sizes: 25mm², 35mm², 50mm², 70mm², 95mm², 120mm², 150mm², 185mm², 240mm², 300mm².
② Insulation Material (Medium-Voltage cables must use cross-linked polyethylene (XLPE) insulation)
- XLPE insulation has a higher operating temperature and better thermal stability than polyvinyl chloride (PVC) insulation.
- Its continuous operating temperature of 90°C is the benchmark for calculating the current carrying capacity of medium-voltage cables.
③ Laying Methods and Installation Environment
- Air Laying: Open cable trays offer the best heat dissipation → highest current carrying capacity.
- Direct Burial: Soil thermal resistance reduces heat transfer → lower current carrying capacity.
- Dual-Trench Laying: Poor ventilation leads to heat buildup → requires significant reduction in current carrying capacity.
④ Environmental and Soil Thermal Conditions
- High ambient temperatures or high soil thermal resistance (dry/sandy soil) significantly reduce current carrying capacity.
- Moist, compacted soil promotes heat dissipation and can support slightly higher current carrying capacity.
⑤ Cable Grouping and Parallel Installation
- Tightly laid multiple cables can cause mutual heating.
- Typical derating factor: 0.8–0.95, depending on the number of cables and spacing.
⑥ Sheath, Armoring, and Ventilation
- Armored structures (YJV22/YJY23) slightly reduce heat dissipation performance compared to unarmored cables.
- Narrow spaces or poor ventilation further reduce permissible current carrying capacity.
3. Practical Current Carrying Capacity Reference Table (Medium Voltage Cross-linked Polyethylene Cable)
Conditions: Ambient temperature 25°C, soil thermal resistance 1.0 K·m/W
| Cable Type | Voltage Rating | Cross Section | Ampacity (Air Laying) | Ampacity (Direct Burial) |
|---|---|---|---|---|
| YJV/YJY (Cu) | 8.7/10kV | 3*95mm² | 240A | 215A |
| YJV/YJY (Cu) | 8.7/10kV | 3*120mm² | 270A | 245A |
| YJV/YJY (Cu) | 8.7/15kV | 3*150mm² | 305A | 275A |
| YJV22 (Armored) | 26/35kV | 3*185mm² | 340A | 305A |
| YJV22 (Armored) | 26/35kV | 3*240mm² | 390A | 350A |
4. Case Studies in Actual Engineering Projects
Case 1: 10kV Large Motor Power Supply
- Project: 500kW+ Industrial Motor
- Cable: 8.7/10kV YJV 3*120mm² Copper-clad Steel Cross-linked Polyethylene Cable
- Design: Current carrying capacity margin ≥ 2.5 times rated current
- Result: Stable operating temperature < 85°C, no overheating or aging phenomena.
Case 2: Direct Burial Laying in a Dry Industrial Park
- Challenge: High soil thermal resistance (sandy, dry soil)
- Solution: Upgrade to 3*150mm²; adopt a derating factor of 0.9
- Result: Long-term safe operation with extremely low temperature rise.
Case 3: 35kV Wind Farm Collector Line
- Laying Method: Outdoor trench laying, multiple cables connected in parallel
- Solution: YJY23 armored UV-resistant cable; using a derating factor of 0.85
- Result: Reliable performance under heavy load and harsh outdoor conditions.
5. Best Practices for Medium Voltage Cable Current Carrying Capacity Engineering
- For high reliability and high current carrying capacity applications, use copper conductors.
- Medium voltage cables should always use cross-linked polyethylene (XLPE) insulation to meet temperature and safety standards.
- For buried, parallel braided, high-temperature, and poorly ventilated applications, derating factors must be strictly applied.
- Allow a 1.5 to 2.5 times current carrying capacity margin to accommodate impact loads and future expansion.
- For direct burial and harsh environments, select armored cables (YJV22/YJY23).
- Monitor the temperature at joints and terminals to prevent hot spots.
6. Conclusion
For medium-voltage power systems, current carrying capacity is a crucial balance between safety, performance, and cost. By understanding key influencing factors and applying the correct derating rules based on IEC 60287, engineers can avoid overheating, extend cable life, and reduce long-term maintenance costs.
Jinhong Cable offers a full range of 6kV-35kV medium-voltage cross-linked polyethylene (XLPE) power cables with validated current carrying capacity data, compliant with IEC, GB, CE, and RoHS standards, supporting industrial, EPC, and infrastructure projects worldwide.
