Electrical Conduit Sizes Explained

Understanding Conduit Sizes 

Electrical conduits are available in various sizes to suit different capacities, cable types and installation environments. The overall size affects how many cables can be installed safely while also helping maintain compliance. 

How Conduit Sizes Are Measured 

Electrical conduit sizes are typically measured by their internal or nominal diameter. In most installations, conduit sizing is listed in millimetres such as 20mm, 25mm or 32mm. 

The conduit diameter provides a clear indication of how much internal space is available for cables. Larger conduit sizes provide more room for multiple cables. They also provide less resistance when feeding cables through a conduit for a faster installation. 

Metric vs Imperial Conduit Sizes 

Conduit sizes are commonly available in both metric and imperial measurements. Metric conduit sizing is now the standard for most modern electrical installations in the UK, particularly for commercial and industrial projects.

Imperial conduit sizing is generally associated with older installations and legacy systems. While imperial sizes can still be found on refurbishment or maintenance projects, metric sizes are more widely used due to standardisation and product availability. So, electricians and contractors typically use metric conduit because it is the recognised modern system and is more convenient for everyday installations.

What are the Standard Conduit Sizes? 

Some of the most common conduit sizes used across electrical installations include:

• 16mm
• 20mm
• 25mm
• 32mm
• 42mm

Conduit Sizes and Their Applications

Different conduit sizes are suited to different installation requirements. Selecting the correct size maintains safe cable routing, simplifies installation and allows enough room for cable movement and heat dissipation.

20mm Conduit Sizes

20mm conduit is one of the most commonly used in electrical installations. It is typically used for smaller cable runs, lighting circuits and general wiring applications. 

Common uses for 20mm electrical conduits:

• Domestic electrical installations
• Lighting circuits
• Single cable runs

25mm Conduit Sizes 

25mm conduit provides additional internal capacity compared to 20mm conduit and is commonly used where more cables or larger cable diameters are required.

Typical applications include:

• Commercial electrical installations
• Larger power circuits
• Data cabling installations

32mm Conduit Sizes 

32mm conduit is generally used for larger installations where higher cable capacity is required. It is commonly found in industrial and commercial environments where larger cable bundles or power distribution systems are installed.

Common uses include:

• Industrial cable installations
• Large cable bundles
• Heavy-duty cable management systems

Other Common Sizes 

While 20mm, 25mm and 32mm conduit sizes are among the most widely used, other sizes are also available for specific applications.

Other common electrical conduit sizes include: 

• 16mm conduit is often used for small cable runs, lighting drops and compact installations.
• 42mm conduit is typically used for larger industrial installations or where substantial cable capacity is required.
• 50mm conduit and above are used for heavy industrial environments or infrastructure projects involving significant routing. 

How to Choose the Correct Conduit Size

Selecting the correct conduit size involves more than simply fitting cables into the available space. Proper sizing needs to factor in installation conditions, potential future expansion and accessibility to ensure the overall longevity of a conduit system. 

Number of Cables

The number of cables being installed has a major influence on the correct electrical conduit size to choose. Selecting a conduit that is too small can restrict internal space, making cable installation more difficult and issues that can lead to damaged cables. Conversely, choosing an excessively large conduit may be unnecessary for the application.

Cable Diameter and Type

Choosing the correct conduit size helps reduce strain on cables during installation and maintenance. Different cable types and cable diameters require different amounts of space inside a conduit. Larger armoured cables or multiple data cables will typically require larger conduit sizes than standard domestic wiring.

Installers often follow what’s referred to as the 45% conduit rule, which recommends that cables should not occupy more than around 45% of the conduit’s internal space. This allows for a smooth installation and adequate airflow around the cables. 

Future Expansion

As electrical requirements change over time, upgrading or replacing an entire conduit installation can become costly and disruptive. Selecting an electrical conduit size that allows for future expansion helps create a more adaptable system, making it easier and more cost-effective to add or replace cables later on.

Heat Dissipation

Depending on the installation environment, a conduit that is too small may restrict airflow and limit effective heat dissipation around the cables. This can increase the risk of cable damage, overheating and potential safety issues, while also affecting compliance with electrical standards. Choosing the correct conduit size helps provide sufficient space for airflow and supports safer, more reliable cable performance.

What is Conduit Overfill Capacity? 

Conduit overfill capacity refers to the maximum amount of cable that can safely fit inside a conduit. Exceeding this capacity can make cable installation difficult and may increase the risk of overheating, cable damage or reduced performance.

Overfilling a conduit can also make maintenance and future cable replacement more challenging. For this reason, installers often leave additional internal space within conduit systems to allow for easier cable pulling and future expansion.

Conduits at Lenson Select

Lenson Select offers a wide range of fixings, conduits, cable trays and cable management products to support all types of electrical installations. Whether working on a commercial, industrial or domestic project, the experienced team can help you choose the right conduit solution. Fill in our contact form, give us a call on 0121 553 6699 or email us at sales@lenson-select.co.uk.