35kV XLPE Power Cable: What Buyers Need to Confirm Before Ordering

“35kV XLPE cable, 3 × 120mm².”

At first glance, this looks like a fairly complete cable inquiry. It gives the voltage class, insulation material, core number and conductor size.

For a cable manufacturer, however, there is still not enough information to define the product.

What does “35kV” mean under the project standard? Is the required cable 21/35(40.5)kV or 26/35(40.5)kV? Is the conductor copper or aluminum? What metallic screen is required? Will the cable be armoured? Is the route underground, in a cable trench, or exposed to possible water ingress?

At this voltage level, a medium voltage power cable cannot be specified by conductor size alone. The electrical field control system, metallic screen, water protection and installation conditions all become part of the cable design.

“35kV Cable” Is Not a Complete Voltage Specification

One of the first points to confirm is the rated voltage notation.

Power cables are commonly specified as:

U₀/U (Um)

where:

· U₀ is the rated power-frequency voltage between conductor and earth or metallic screen

· U is the rated power-frequency voltage between conductors

· Um is the highest system voltage for which the equipment is designed

Under the Chinese GB/T 12706.3-2020 system, 35kV-class cables are covered up to Um = 40.5kV.

Typical voltage designations include:

· 21/35(40.5)kV

· 26/35(40.5)kV

These are not interchangeable labels. The required rating depends on the system and its earthing conditions.

This is why a professional reply to:

“We need 35kV cable.”

should usually begin with another question:

What U₀/U(Um) rating and applicable standard does the project require?

Do Not Automatically Apply IEC 60502-2 to Every “35kV” Cable

This point deserves particular attention in international enquiries.

IEC 60502-2 covers extruded-insulation power cables from 6kV up to 30kV, with Um up to 36kV, for fixed installations such as distribution networks and industrial systems.

That means a cable specified as:

26/35(40.5)kV

should not simply be described as “IEC 60502-2 compliant.”

The voltage range is different.

For projects using the Chinese 35kV, Um 40.5kV system, GB/T 12706.3-2020 is the relevant national cable standard.

For other international projects, the buyer should provide the project standard rather than asking the manufacturer to infer it from the words “35kV cable.”

Why XLPE Is Common at This Voltage Level

XLPE, or cross-linked polyethylene, is widely used as the insulation material in modern medium voltage cable systems.

Its role is more significant here than simply providing a heat-resistant layer around the conductor.

At medium voltage, the insulation system must work together with semiconductive screens to control the electric field around the conductor. A typical XLPE cable therefore includes more layers than a low-voltage power cable.

A simplified construction looks like this:

Conductor → Conductor Screen → XLPE Insulation → Insulation Screen → Metallic Screen → Bedding / Inner Sheath → Armour, if required → Outer Sheath

So when a buyer asks for an XLPE power cable, “XLPE” defines only one important part of the construction. It does not tell the manufacturer what metallic screen, armour, sheath or water-blocking system is required.

The Two Semiconductive Screens Are Not Optional Details

One of the most obvious structural differences between a 0.6/1kV cable and a 35kV-class cable is the electric field control system.

Conductor Screen

The semiconductive conductor screen sits between the conductor and the XLPE insulation.

Its purpose is to provide a smooth electrical interface around the stranded conductor and reduce local electric-field concentration caused by conductor surface irregularities.

Insulation Screen

A second semiconductive layer is applied over the XLPE insulation.

Together with the metallic screen outside it, this helps establish controlled electrical stress across the insulation system.

These layers are an integral part of medium-voltage cable design. A 35kV cable is therefore not simply a low-voltage cable with thicker XLPE insulation.

Metallic Screen: What Should the Buyer Specify?

Outside the insulation screen, a medium voltage power cable normally includes a metallic screen.

Depending on the design and project specification, this may use:

· copper wires

· copper tape

· a combination of copper wires and tape

· another approved metallic screening construction

The metallic screen serves important electrical functions, including providing a path associated with earth-fault current and maintaining the required screened cable system.

For procurement, it is not enough to write:

“With copper screen.”

The manufacturer may also need to know the required screen cross-section or project fault-current requirements.

This is particularly important when the project specification has already calculated the required metallic screen size.

Single-Core or Three-Core?

Both single-core and three-core designs are used in medium-voltage systems.

The correct choice depends on the project design rather than on a general rule.

Single-Core Cable

A single-core arrangement may be selected for:

· high-current circuits

· larger conductor sizes

· installations where each phase is routed separately

· projects with specific laying and thermal requirements

If armour is required on a single-core AC cable, armour material also needs attention because magnetic steel armour can create additional losses.

Three-Core Cable

Three-core construction places the three phase conductors within one overall cable.

It can simplify certain installation arrangements, although cable diameter, weight, bending and installation handling may become more significant as conductor size increases.

So a request such as:

3 × 120mm²

should be clarified.

Does it mean one 3C × 120mm² cable, or three separate 1C × 120mm² cables?

Those are different products.

Underground Installation Raises Another Question: Water

For underground medium-voltage cable routes, water should be discussed before production rather than after installation.

Two separate risks should be considered.

Radial Water Ingress

Can moisture penetrate through the outer cable layers from the surrounding environment?

The answer depends partly on the sheath and overall cable construction.

Longitudinal Water Penetration

If water enters at a damaged point or exposed end, can it migrate along the cable?

Where this is a project concern, a water-blocking structure may be specified.

Possible designs may use water-blocking tapes, yarns or other construction features, depending on the cable design.

For an underground project, therefore:

“35kV underground cable”

is still not a complete specification.

The supplier should know whether the route includes:

· wet soil

· cable ducts

· water accumulation

· high groundwater

· cable trenches

· requirements for longitudinal water blocking

Armour Should Follow the Installation Condition

Not every 35kV cable needs armour.

Armour may be required where the installation presents mechanical damage risks, but the appropriate structure depends on whether the cable is single-core or three-core, the laying method and the project specification.

The inquiry should therefore state whether the cable is intended for:

· direct burial

· underground duct

· cable trench

· cable tray

· indoor substation

· outdoor fixed installation

The phrase:

Armoured medium voltage cable

is useful as a starting point, but the actual armour structure should still be confirmed during technical review.

Copper or Aluminum Conductor?

Both copper and aluminum conductors are used in medium voltage power cable systems.

Copper offers higher conductivity and allows a smaller conductor cross-section for comparable electrical performance.

Aluminum is lighter and can provide a cost advantage, particularly for large conductor sizes and long project routes.

But changing from copper to aluminum is not a simple material substitution.

The design needs to account for:

· conductor resistance

· ampacity

· voltage drop

· short-circuit thermal performance

· conductor size

· termination compatibility

For this reason, a manufacturer should not replace a specified copper conductor with aluminum at the same nominal cross-section without a proper engineering review.

What Should a Complete 35kV Cable Inquiry Include?

A useful inquiry should contain more than voltage and conductor size.

This information gives the manufacturer a much clearer basis for technical confirmation and quotation.

A Better Way to Write the Specification

For a project using the Chinese 35kV standard system, an inquiry might begin with:

26/35(40.5)kV Cu/XLPE medium voltage power cable, 3C × 120mm², metallic screened, armoured, PE outer sheath, compliant with GB/T 12706.3-2020.

That description is already much more useful than:

35kV 3 × 120 cable.

But even the longer version may still require confirmation of:

· metallic screen size

· exact armour construction

· water-blocking requirement

· conductor class

· flame performance

· installation conditions

· required cable length per drum

A technical specification should therefore be treated as a controlled engineering document, not simply as a product name.

What Buyers Should Check Before Mass Production

Before a 35kV XLPE power cable enters mass production, the supplier and buyer should be working from the same confirmed specification.

That document should align with:

· the quotation

· technical datasheet

· applicable standard

· cable drawing

· conductor size

· insulation thickness

· screen construction

· sheath material

· cable marking

· testing requirements

For medium-voltage cable, changes that look small on a purchase order can affect the cable structure significantly.

Confirming the technical version first is much easier than resolving discrepancies after the cable has been produced.

Final Thoughts

“35kV cable” tells a manufacturer the general voltage class. It does not define the cable.

A properly specified medium voltage cable also needs an exact U₀/U(Um) rating, conductor material, XLPE insulation system, metallic screen, core arrangement, armour and sheath requirements, installation conditions and applicable standard.

For projects using a 35kV system with Um = 40.5kV, buyers should also pay close attention to the standard being requested. IEC 60502-2 covers cables only up to Um = 36kV, while GB/T 12706.3-2020 covers the Chinese 35kV, Um 40.5kV cable class.

For a quotation from HONGTONG CABLE, providing the system voltage, conductor size, core configuration, metallic screen requirement, installation environment and project standard at the beginning will make the technical confirmation—and the quotation that follows—far more accurate.