Underground Electrical Cable: Types, Sizing & Installation

An underground cable is an electrical cable installed below ground to carry power between electrical equipment, buildings, distribution points and other loads. The right underground electrical cable depends on factors such as system voltage, load current, cable length, installation method, voltage drop and environmental conditions.

From residential power supplies to commercial and industrial distribution systems choosing the correct cable is important for safe operation and reliable performance. This guide explains underground cable types, applications, cable sizing, voltage drop, specifications, installation, burial depth and cost considerations.

What Is an Underground Cable?

An underground cable is an insulated electrical cable designed to carry electricity below ground. Depending on its construction and approval it may be installed inside a conduit, duct or directly buried in the ground.

An underground electrical cable can contain several layers including a conductor, insulation and protective outer sheath. Some cable designs also include shielding, armour, moisture protection or other layers for specific applications.

Unlike overhead conductors underground cables are protected from direct exposure to weather but they still have to withstand conditions such as moisture, temperature changes, soil conditions and mechanical stress.

What Are Underground Cables Used For?

Underground cables are used in residential, commercial, industrial and utility electrical systems. Common applications include:

The cable construction and voltage rating should always match the electrical system and the intended installation method.

• Supplying electricity to homes and buildings
• Connecting electrical panels and distribution equipment
• Powering detached buildings and outdoor equipment
• Industrial power distribution
• Underground utility networks
• Street and site electrical installations
• Medium voltage and high voltage distribution systems

Types of Underground Cables

Underground cables can be classified according to voltage rating, insulation, construction and installation method. These classifications can overlap so a single cable may belong to more than one category.

Low Voltage Underground Cables

Low voltage underground cables are commonly used in residential, commercial and smaller electrical installations. Depending on the cable design and local requirements they may be installed in an underground raceway or where specifically permitted directly buried.

Medium Voltage Underground Cables

Medium voltage cables are commonly used in larger distribution systems, industrial facilities, substations and utility networks. They generally use specialized insulation and shielding systems and their allowable current capacity can vary depending on whether the cable is installed in a duct, conduit or directly buried in earth.

High Voltage Underground Cables

High voltage underground cables are designed for major power transmission and distribution applications. These cables require specialized insulation, shielding and protective construction. XLPE insulated designs are widely used for modern high voltage underground power systems.

Armoured Underground Cables

Armoured cables include an additional protective layer that helps provide mechanical protection. They can be useful where a cable may be exposed to physical damage or mechanical stress. However the correct armour type and cable construction depend on the installation environment and applicable requirements.

Direct Burial Cables

Direct burial cables are specifically designed and identified for installation directly in the ground without relying on a conventional underground raceway.

Not every underground electrical cable is suitable for direct burial. The cable must be approved for the intended installation and environmental conditions.

XLPE Underground Cables

XLPE stands for cross linked polyethylene, an insulation material widely used in power cables. XLPE underground cables can be manufactured with copper or aluminium conductors and may include additional shielding, armour and protective sheathing depending on the voltage and application.

It is also important to understand that these categories are not mutually exclusive. For example a cable can be XLPE insulated, armoured and suitable for direct burial at the same time.

What Cable Is Used for Underground Installation?

The correct cable for underground installation depends on both the electrical requirements and the physical installation conditions. Before selecting a cable consider:

A cable should not be selected based only on conductor size. The complete cable specification and its suitability for the installation must also be verified.

• System voltage
• Load current
• Single phase or three phase supply
• Cable length
• Conductor material
• Cable ampacity
• Voltage drop
• Direct burial or conduit installation
• Temperature and soil conditions
• Cable grouping
• Moisture exposure
• Mechanical protection
• Applicable electrical codes and manufacturer requirements

How to Choose the Right Underground Cable Size?

Underground cable sizing involves checking whether the conductor can safely carry the required current while also maintaining acceptable voltage at the load. For a more detailed explanation of the general cable sizing process see our Electrical Cable Size Guide.

1. Determine the Load Current

Firstly determine the expected design current of the circuit. The required current depends on the connected load, supply voltage, power rating, power factor and whether the circuit is single phase or three phase.

2. Check the Cable Length

Cable length is particularly important for underground installations because long runs can produce greater voltage drop. A cable that is suitable for a short run may require a larger conductor when the same load is supplied over a much longer distance.

3. Check Ampacity

Ampacity is the amount of current a cable can carry safely under specified installation conditions. Underground installation conditions can affect allowable ampacity.

Factors such as cable arrangement, temperature, soil conditions and whether the cable is directly buried or installed in a duct can influence the result.

Our Wire Size Chart provides additional information about conductor size, AWG, mm², ampacity and the factors that affect cable selection.

4. Check Voltage Drop

After checking ampacity voltage drop should also be evaluated especially on longer cable runs. Voltage drop increases with factors such as higher current, greater cable length and higher conductor resistance.

Increasing conductor size can reduce resistance and therefore reduce voltage loss. For the detailed calculation process see our How to Calculate Voltage Drop guide.

5. Verify the Installation Conditions

The final cable selection should account for the actual installation conditions including temperature, grouping, burial method, protection and applicable electrical requirements.

The selected cable should satisfy both the required current carrying capacity and the voltage drop requirement rather than relying on one value alone.

Underground Cable and Voltage Drop

Voltage drop is especially important when an underground cable has to carry power over a long distance. A simplified relationship is:

Voltage Drop = Current × Resistance

Conductor resistance depends on factors such as conductor material, cross sectional area, temperature and cable length. As cable length increases, the total resistance of the circuit generally increases which can result in greater voltage loss.

For single phase circuits the outgoing and return conductors are both part of the voltage drop path. Three phase calculations use a different relationship because of the phase arrangement. If you want to understand how cable distance affects voltage loss in more detail see our Cable Length Voltage Drop Guide.

For repeated calculations our Wire Voltage Drop Calculator can help compare cable sizes and estimate voltage drop using the relevant circuit inputs.

Underground Cable Specifications

When comparing underground cables look beyond the conductor size. Important specifications can include:

Always check the manufacturer's technical data before using a cable for a specific underground application.

How Are Underground Cables Installed?

The installation method depends on the cable construction, location, voltage and applicable requirements. A typical underground cable installation involves:

Manufacturer installation guidance recommends suitable trench preparation and protective backfill to help prevent cable damage during installation and service.

• Plan the route – Identify a suitable route while considering existing underground utilities and access requirements.
• Select the cable – Confirm voltage rating, ampacity, insulation, protection and installation suitability.
• Prepare the trench or raceway – Make sure the installation method meets the relevant requirements.
• Provide suitable protection – Use appropriate bedding, backfill, conduit or other protection where required.
• Lay or pull the cable carefully – Follow the manufacturer's limits for pulling tension and bend radius.
• Complete the terminations – Use suitable connectors, glands and termination equipment.
• Inspect and test – Check the completed installation before putting the circuit into service.

How Deep Should an Underground Cable Be Buried?

There is no single burial depth that applies to every underground electrical cable. Required cover can depend on the wiring method, location, voltage, protection and applicable electrical code. Requirements may differ between direct burial cables, cables installed in raceways, driveways, landscaped areas and other locations.

For this reason do not assume that one burial depth is suitable for every project. Check the applicable electrical requirements and the cable manufacturer's installation instructions before digging or burying the cable.

Underground Cable vs Overhead Cable

The appropriate method depends on the project design, installation environment, cost, maintenance requirements and applicable regulations.

What Affects Underground Cable Price?

The price of an underground cable can vary significantly because cables with different electrical and mechanical specifications can have very different manufacturing costs. Common cost factors include:

For example there is no single universal price for a 33kV underground cable. A quotation needs to specify the cable construction, conductor size, material, length, insulation and other project requirements before different prices can be compared accurately.

• Conductor material
• Conductor size
• Cable length
• Voltage rating
• Number of cores
• Insulation type
• Armour and shielding
• Manufacturer
• Cable accessories and terminations
• Installation requirements

Conclusion

Choosing the right underground cable requires more than selecting a conductor size. System voltage, load current, cable length, installation method, ampacity, voltage drop, environmental conditions and mechanical protection all need to be considered.

For longer underground cable runs voltage drop can become an important part of cable selection. Checking both ampacity and voltage drop along with the manufacturer's specifications and applicable electrical requirements helps ensure the cable is suitable for the intended installation.

FAQs

What is an underground cable?

An underground cable is an insulated electrical cable designed to carry electricity below ground. Depending on its construction and approval it can be installed in a raceway or directly buried in earth.

What cable is used for underground?

The correct cable depends on the system voltage, load current, cable length, installation method, environmental conditions and required protection. Only cables suitable for the intended underground installation should be used.

What is underground wire called?

It is commonly called an underground cable, underground electrical cable or for cables specifically designed for direct installation on earth, a direct burial cable.

Which cable is used for underground?

Low voltage, medium voltage and high voltage cables can all be used for underground power systems. Armoured and direct burial cables are also available for applications requiring specific mechanical protection or installation methods.

How do I choose an underground cable size?

Start with the load current, system voltage, cable length, installation method and allowable voltage drop. Then check the cable's ampacity and voltage drop performance before selecting the final conductor size.

Does cable length affect underground cable size?

Yes. Longer cable runs generally produce greater voltage drop. If the voltage loss is too high a larger conductor may be required even when the smaller cable has enough current carrying capacity.

How much does 33kV underground cable cost?

There is no single price for a 33kV underground cable. The cost depends on factors such as conductor material and size, insulation, shielding, armour, manufacturer, cable length and project specifications.

What is the difference between underground cable and direct burial cable?

Underground cable is a broad term for cables installed below ground. Direct burial cable is specifically designed and identified for installation directly on earth without relying on a conventional underground raceway.