How to Size Power Cable Duct 

By on

Some colleagues had an issue earlier in the week on sizing ducts to be cast in concrete for some power cables .  It became clear that none of us had a clear idea on an approved method.  In the end we agreed that selecting the duct based on a 40% fill factor was a good idea.  This may not be the best approach.  If anyone has other ideas, please add below.

Using fill factor:

1. Required fill factor k (0.4 in our case)

2. Cable diameter Cd,  giving cable area, a:

6cc7ed63d44f12dcabb4486f27cd91fb

3. Cable total area Ca = sum of area for all cables

4. Minimum duct diameter  Cc:

3445e1acc7a26e6ea9a105c79abf241b

To make life a little easier, I have put together a cable duct sizing  tool which will do the arithmetic.

Once the calculation is complete, choose the next largest size of ducts.  What-if scenarios can also be carried out to optimize the quantity/size of ducts.  In a real installation, it would be good practice to have multiple ducts the same size if possible and put a couple of spare ones in as well.

In addition to determining the size of cable ducts and number of cables, we also need to think about the maximum number of cables in a duct.  The more cables, the more problems there will be with pulling (friction, bunching, etc.).  It seems a good thing to limit the number of cables per duct to five or less. 

This is a post that raises questions in addition to answers.  If you have any good ideas or tips on the duct sizing, please add below.  I'll add the good stuff into the post, so that over time we end up with a great resource. 



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Random Numbers

Using laser optical pulses the random number generator utilizes the time between arrival of random photos to generate the numbers, ensuring true accuracy...

Switchboard - Forms of Internal Separation

IEC 61439 'Low-voltage switchgear and controlgear assemblies', specifies standard arrangements of switchboard (call forms of internal separation). The...

Post Editing Tips

If you at all familiar with programs like office and outlook, then adding and editing posts is pretty straightforward and intuitive.  However, there are...

Railway Electrification Voltages

This post is quick introduction and overview to different railway electrification voltages used in answer to a question sent in via email. While there...

DC Component of Asymmetrical Faults

The image (reproduced from IEC 60909) shows a typical fault in an ac system.  From the illustration it can seen that there is an initial dc component ...

IEEE Winds of Change

IEEE TV has a part series of videos on wind power and it's implication. For a really good overview to the technologies and issues around wind power, these...

IEC Document Designation

Often document control is dictated by project requirements, for example a particular organisation may have an existing numbering system. Existing company...

Lighting Design - An Introduction

From the earliest times, humans have found ways to create light. Pre-historic peoples used natural materials (moss, grass, etc.) soaked in animal fat and...

ANSI (IEEE) Protective Device Numbering

The widely used United Sates standard ANSI/IEEE C37.2 'Electrical Power System Device Function Numbers, Acronyms, and Contact Designations' deals with...

What does N+1 mean?

The term 'N+1' relates to redundancy and simply means that if you required 'N' items of equipment for something to work, you would have one additional...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note