35 kV Cable Selection Guide: XLPE Construction, Ampacity & Buying Tips

Specifying a 35 kV cable for a new substation feeder or an industrial distribution line is a decision engineers rarely get to make twice. The cable will carry fault current, see soil heat, and stay in service for decades. The right approach is to settle three things first: the system voltage, the required current-carrying capacity under the actual installation method, and the short-circuit thermal rating. Once those are fixed, the choice of conductor, insulation, metal shield, and outer sheath follows from a short list of practical options. For most 35 kV projects, the economical and reliable answer is an XLPE-insulated cable with a metallic shielding layer and a robust sheath matched to the soil or corridor conditions.

What “35 kV Cable” Means in Design Terms

On the nameplate, a 35 kV cable is usually described by its rated voltage U0/U, not by the nominal system voltage alone. In a 35 kV three-phase system, the cable rating should be 26/35 kV. That means the rated voltage between conductor and earth is 26 kV, while the rated voltage between phases is 35 kV. The equipment maximum operating voltage for this class is 40.5 kV, and the insulation thickness and electrical distance values in the standard are based on that. Both IEC 60502-2 and GB/T 12706.2 follow the same logic.

From the outside in, a 35 kV cable consists of a conductor, semi-conductive conductor screen, XLPE insulation, semi-conductive insulation screen, metallic screen, and an outer sheath. XLPE cross-linked polyethylene is the industry standard because it withstands higher operating temperatures than PVC. Conductor temperatures of 90°C are allowed under continuous load, and emergency overloads up to 130°C can be tolerated for short periods.

XLPE Insulated Power Cable for 3.6/6kV to 26/35kVThis medium- and high-voltage cable features cross-linked polyethylene insulation, allowing operation at 90°C and short-circuit resistance up to 250°C. It suits underground, trench, tunnel, or overhead installation, ensuring safe and reliable power transmission.View Product →

Conductor, Insulation, Shield and Sheath: The Four Decisions That Matter

• Conductor material: choose copper when space is tight, when the cable route is short, or when maximum thermal efficiency is needed. Choose aluminum on long overhead or buried routes where cost and weight are the main drivers. Both are covered by Class 2 stranded conductor requirements in IEC 60228.
• Insulation compound: XLPE is the standard for 26/35 kV. The typical nominal insulation thickness for this voltage class is around 10.5 mm, depending on the standard edition and the short-circuit requirements.
• Metallic screen: copper tape or copper wire screen. The screen cross-section must be large enough to withstand the single line-to-ground fault current for the protection clearing time. Copper tape screens work for smaller conductors and lower fault levels; copper wire screens are preferred for larger conductors and higher fault currents.
• Outer sheath: PVC is stable in air, easy to terminate, and works well indoors. PE gives better water resistance in wet soil. In termite- or rodent-prone areas, add a specific protective compound or use a design that includes an additional protection layer.

Do not treat the screen as an optional extra. In a 35 kV cable, the screen is part of the safety system. It carries the capacitive charging current and, in the event of a fault, a substantial fault current until the protection operates. A screen that is too small will burn through the insulation and create a second fault point.

Ampacity Under Real Installation Conditions

The nameplate current rating is usually based on ideal conditions. Real projects involve ambient temperatures above 30°C, multiple parallel cables, solar radiation, or soil with high thermal resistivity. The thermal resistivity of soil, expressed in K·m/W, is a crucial value. A dry, compacted backfill with a value around 1.0 K·m/W is a standard assumption, but sandy or dry soil can reach much higher values.

When cables are installed in ducts or as multiple parallel runs, the ampacity falls further. The derating depends on duct spacing, soil humidity, and the number of loaded circuits. For long routes, voltage drop becomes another controlling factor. A 26/35 kV cable has a specific resistance per kilometre, so the selected size must satisfy both voltage drop and short-circuit thermal stability, not just continuous current.

Typical 35 kV Cable Applications

• Substation feeders and transformer incoming lines in urban distribution networks.
• Industrial plants and mine power distribution, where the 35 kV cable links switchgear to distribution transformers.
• Renewable energy collector lines in PV and wind farm substations, where cable routes run from the inverter or transformer to the central substation.
• Aerial distribution lines, where a 35 kV aerial insulated cable is installed on poles or racks to reduce clearances and improve public safety.

In a 35 kV aerial insulated cable rated 26/35 kV, the insulation is designed for the same system voltage as the underground version. The construction is optimized for overhead installation with tighter bending radii and simplified terminations, which makes it a practical choice for crowded utility corridors.

Aerial Insulated Cable for 1kV to 35kV Overhead LinesDesigned for overhead distribution in crowded corridors, this cable offers tight bending radii and simplified terminations for 26/35kV systems. It supports various insulation materials and complies with GB standards.View Product →

Purchasing and Acceptance Criteria to Protect Your Project

Buying 35 kV cables is not the same as buying low-voltage wires. The project owner should specify the standard, the insulation thickness, the conductor class, and the screen arrangement. The supplier should provide a type test report, routine test records, and a sample of the cable. Routine tests should include partial discharge measurement, AC voltage withstand test, and insulation resistance measurement at the factory. Partial discharge limits for XLPE cables are typically expressed in picocoulombs at a specified test voltage, and the number is written into the purchase order.

On site, verify the drum length, the cable end sealing, and the serial numbers. Compare the measured conductor resistance, capacitance, and screen thickness against the approved drawings. For large projects, require a third-party witness test before dispatch.

A useful practical reference for sorting out voltage levels, cable types, and application rules is the site's high-voltage cable selection guide.

Four Common 35 kV Cable Mistakes to Avoid

1. Overlooking screen short-circuit rating. The screen current during a single-phase fault can be significant. If the copper tape or wire screen is too small, the screen burns before the protection clears the fault.
2. Using a continuous-load ampacity table from a lower voltage class for a 35 kV cable. Insulation thickness affects heat dissipation and therefore ampacity. Always use the correct voltage class table and the correct installation condition.
3. Ignoring soil thermal resistivity in the laying corridor. Dry sandy soil has a much higher thermal resistivity than moist clay, so a cable rated in air can derate by 10–20% when it is buried in the same location.
4. Accepting cable drums delivered with damaged ends that allowed moisture entry. Moisture degrades the insulation and can cause partial discharge problems after the cable is energized.

Take the time to write out the system voltage, the minimum conductor size based on load flow, the short-circuit current available at the cable terminals, and the actual laying route. Keep that one-page technical basis in front of the supplier and the project manager. It will save you from expensive rework and from a cable that looks fine on paper but fails at the first fault event.