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What is the right flexible conduit size to use?

Choosing the right flexible conduit size for a project involves several key steps. First, determine the type and number of cables that will be used. Next, calculate the total cross-sectional area of these cables. Finally, ensure that the chosen flexible conduit size can accommodate this total area while complying with relevant wiring regulations. Take a look below at a few suggested steps to help you.

Steps to Determine the Right Flexible Conduit Size:

1. Identify the Cables:

  • Determine the type and size (e.g., mm²) of the cables.

  • Count the number of cables.

electrical wir and cables.JPG

2. Calculate Total Conduit Fill:

  • Flexible conduits have specific fill capacities depending on their size. These are usually specified in percentage fill, for example the fill should not exceed 40% of the conduit’s internal area with the wires installed.

  • Refer to conduit fill tables from the NEC or relevant electrical code to find the maximum allowable fill for each conduit size.

3. Calculate Total Conduit Fill:

  • Flexible conduits have specific fill capacities depending on their size. These are usually specified in percentage fill, for example the fill should not exceed 40% of the conduit’s internal area with the wires installed.

  • Refer to conduit fill tables from the NEC or relevant electrical code to find the maximum allowable fill for each conduit size.

As a general rule of thumb, ensure that there is an extra 40% space in the conduit when the wires are installed. 

Example Calculation:

 

  • Assume:

       You have 3 cables that have a cross-sectional area of 1.5 mm².

 

  • Calculate Total Area:

       Total fill = 3 cables x 1.5 mm² = 4.5 mm².

 

  • Check 40% Fill:

       You need a conduit where 40% of its internal area is at least 4.5 mm².

 

  • Determine Conduit Fill Capacity:

       For example, a 20 mm flexible conduit might have a maximum fill capacity of approximately 58 mm²             at 40% fill.

 

  • Choose Conduit Size:

       Since 4.5 mm² (total fill) is much less than 58 mm² (capacity of a 20mm conduit at 40% fill), a 20mm               flexible conduit would be appropriate. so it’s a good fit.

Tips:

  • Safety Margin: Consider leaving some margin below the maximum fill capacity for easier installation and future additions

  • Consult BS 7671: Always refer to the latest edition of BS 7671 for specific requirements and guidelines.

  • If you need any guidance or advice, please feel free to get in touch with us. We are here to assist you and provide the support you need to ensure your project is successful.

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