IEC 60287 MV Cable Thermal Rating — Beyond the Simple Tables
For MV cables, simple table lookup doesn't work. IEC 60287 requires thermal circuit analysis: soil conditions, burial depth, and mutual heating all calculated explicitly.
February 26, 2026
Why MV Cables Need IEC 60287
Low-voltage cable sizing uses pre-calculated tables (AS/NZS 3008, BS 7671 Appendix 4). These tables make assumptions about installation conditions. For medium-voltage cables (typically 6.6kV–33kV), these assumptions break down:
- Longer cable runs through varying soil conditions
- Higher I²R losses requiring more precise thermal modelling
- Dielectric losses become significant at higher voltages
- Sheath and armour losses affect the thermal circuit
- Economic consequences of over/under-sizing are much greater at MV
IEC 60287 provides the analytical method to calculate current rating from first principles.
The Thermal Circuit Model
IEC 60287 models the cable as a thermal circuit (analogous to an electrical circuit):
- Heat sources: I²R conductor losses (Wc), dielectric losses (Wd), sheath/screen losses (Ws), armour losses (Wa)
- Thermal resistances: T1 (insulation), T2 (bedding), T3 (serving/jacket), T4 (surrounding soil)
- Temperature nodes: conductor, sheath, armour, cable surface, soil
The steady-state thermal equation:
Δθ = (Wc + 0.5Wd) × T1 + (Wc + Wd + Ws) × (T2 + T3) + (Wc + Wd + Ws + Wa) × T4
Where Δθ = maximum conductor temperature − ambient soil temperature.
Solving for the maximum current I that produces Wc = I²Rθ while keeping the conductor at or below its maximum temperature (90°C for XLPE).
Key Input Parameters
Soil Thermal Resistivity (ρ)
The most important external parameter. Typical values:
| Soil Type | ρ (K·m/W) |
|---|---|
| Wet clay | 0.7–1.0 |
| Damp sand | 1.0–1.5 |
| Dry sand | 2.0–3.0 |
| Backfill (thermal concrete) | 0.5–0.8 |
| Rock | 0.5–2.0 |
Doubling ρ can reduce cable rating by 15–25%. Always obtain site-specific measurements.
Burial Depth
Deeper burial = higher T4 (more soil thermal resistance). Standard depth is 0.8–1.0m. Each additional 0.5m of depth reduces rating by approximately 3–5%.
Mutual Heating
Multiple cables in the same trench heat each other. IEC 60287-2-1 provides the method for calculating mutual thermal resistance:
ΔT4_mutual = (W' × ρ) / (2π) × ln(d'/d)
Where d' and d are image and actual distances between cables. For closely spaced cables, this mutual heating can reduce individual cable ratings by 10–30%.
Loss Factors
IEC 60287-1-1 defines loss factors for sheath and armour:
- λ1 (sheath loss factor): 0.02–0.10 for single-core cables, depends on bonding arrangement (solid, single-point, cross-bonded)
- λ2 (armour loss factor): 0.01–0.05 for SWA cables
- Wd (dielectric loss): significant above 33kV, calculated from capacitance and loss angle (tan δ)
The bonding arrangement dramatically affects sheath losses. Cross-bonded sheaths can reduce λ1 to near zero, allowing higher cable ratings.
Practical Application
- Collect site data: soil samples for ρ, actual burial depth, cable spacing
- Select cable construction: conductor, insulation, screen, armour type
- Calculate individual loss components: Wc, Wd, Ws, Wa
- Sum thermal resistances: T1 + T2 + T3 + T4 (including mutual heating)
- Solve for maximum I: iterative calculation (Rθ depends on I, which depends on Rθ)
Run the analysis: Full IEC 60287 thermal rating in the MV Cable Calculator.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
How do I apply derating factors correctly?
Derating factors must be applied multiplicatively: Final Rating = Base Rating × k₁ (ambient) × k₂ (grouping) × k₃ (thermal insulation) × k₄ (ground temp). Each factor comes from specific tables in the relevant standard.
Related Articles
- Mv Cable - Interactive calculator with standards compliance
Try It Yourself
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