Selecting Your Medium Voltage (MV) Cable
A guide to understanding Medium Voltage cable sizing for safety, efficiency, and reliability in power distribution networks.
Medium voltage cables are the critical arteries of power distribution networks, typically operating at voltages from 3.3kV up to 33kV. They are essential for linking primary substations to smaller, local transformers, powering industrial sites, and supporting large-scale commercial infrastructure.
Choosing the correct MV cable is a critical engineering decision that ensures the safety, efficiency, and long-term reliability of a power network. Using an undersized cable can lead to overheating, significant energy loss, and catastrophic failure. An oversized cable, while safe, is not cost-effective and can present installation challenges.
Medium Voltage Cable Size Chart (BS 6622)
The chart below provides an indicative current rating for common single-core copper (Cu) XLPE/AWA cables manufactured to BS 6622. These ratings are based on a standard installation method and set environmental conditions.
| Conductor Size (mm²) | Voltage Rating | Approx. Current Rating (Amps)* |
|---|---|---|
| 50mm² | 6.35/11kV | 215 A |
| 70mm² | 6.35/11kV | 260 A |
| 95mm² | 6.35/11kV | 310 A |
| 120mm² | 6.35/11kV | 355 A |
| 150mm² | 6.35/11kV | 400 A |
| 185mm² | 6.35/11kV | 450 A |
| 240mm² | 6.35/11kV | 520 A |
| 300mm² | 6.35/11kV | 585 A |
| 400mm² | 6.35/11kV | 665 A |
| 120mm² | 19/33kV | 380 A |
| 150mm² | 19/33kV | 425 A |
| 185mm² | 19/33kV | 480 A |
| 240mm² | 19/33kV | 555 A |
| 300mm² | 19/33kV | 630 A |
| 400mm² | 19/33kV | 720 A |
Disclaimer:
*This chart is for guidance only. Ratings are indicative for single-core, treble-cleated cables laid direct in the ground, based on 90°C conductor temp, 15°C ground temp, and soil thermal resistivity of 1.2 K·m/W. Always consult a qualified engineer and use the manufacturer's rating data (calculated to IEC 60287) for design. BS 7671 covers low voltage installations only.
Critical Factors Influencing Cable Sizing
The current-carrying capacity (ampacity) is not a fixed number. It's highly dependent on the environment and installation method. Values must be adjusted using correction factors based on:
1. Installation Method
- Direct in Ground: Depth and soil thermal properties are key.
- In Ducts: Reduced airflow traps heat, lowering the rating.
- In Air: Spacing on trays or cleats is essential for heat dissipation.
2. Ambient Temperature
Standard ratings assume specific ground (15°C) or air (30°C) temperatures. Higher temperatures lower the cable's ampacity.
3. Cable Grouping
When multiple cables are laid together, their mutual heat raises the local temperature, requiring a de-rating factor to be applied.
4. Conductor Material
- Copper (Cu): Higher conductivity.
- Aluminium (Al): Lighter and more cost-effective but needs a larger size for the same rating.
How to Calculate Medium Voltage Cable Size
MV cable sizing is a short calculation followed by a check against the manufacturer's rating data. Work through these steps and pick the largest size any of them gives you.
1. Work out the full load current
For a three-phase circuit, current (A) = power (VA) ÷ (√3 × line voltage). A 1,500kVA transformer fed at 11kV draws about 1,500,000 ÷ (1.732 × 11,000) = 79A. The same transformer fed at 33kV draws about 26A. If you have kW rather than kVA, divide by the power factor first.
2. Apply rating factors
Divide the load current by the correction factors for your installation: ground or air temperature, soil thermal resistivity, depth of lay, ducts and grouping with other circuits. The result is the minimum tabulated rating the cable needs, which you then compare with the chart above or the manufacturer's datasheet.
3. Check the short-circuit rating
At 11kV and 33kV the fault level often decides the conductor size rather than the load. Use the adiabatic equation S = I√t ÷ k, where I is the prospective fault current, t is the protection clearing time in seconds and k is 143 for copper or 94 for aluminium conductors with XLPE insulation. A 10kA fault cleared in 1 second needs at least 10,000 × 1 ÷ 143 = 70mm² copper, even if the load current is small. The metallic screen must also be rated for the earth fault current.
4. Check voltage drop and confirm
Voltage drop is rarely the limiting factor at MV, but check it on long feeders. Then confirm the chosen size against the manufacturer's datasheet, which is calculated to IEC 60287. For low voltage circuits up to 1kV, use our free cable size calculator instead.
Send us your load, voltage, fault level and route details and we will size the cable, check stock and return a price with the datasheet attached. We also supply the matching MV joints and terminations.
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