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



Restricted Earth Fault Protection

The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of...

Medium Voltage Switchgear Room Design Guide

Many medium voltage (MV) indoor switchgear rooms  exist worldwide. The complexity of these rooms varies considerably depending on location, function and...

Calculating Cable Fault Ratings

When selecting a cable, the performance of the cable under fault conditions is an important consideration. It is important that calculations be carried...

Meeting room of the future

The IET site has a video of a visit showing of a high tech meeting room developed at Napier University in Edinburgh. It a good demonstration of innovative...

HTML Symbol Entities

HTML supports a variety of entity symbols which can be entered using either numbers or an entity name.  The number or name is preceded by the ‘&’ sign...

Tip – Latitude and Longitude on Large Scale Plans

If you are working on a large plan, get the real coordinates [latitude, longitude] for two or more points and add them to the drawing. That way you can...

Magicians of Engineering

The other day I was reading 'Night of the New Magicians' by Mary Pope Osborn with my son.  The story is about a young boy and girl who travel back in time...

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...

International System of Units (SI System)

The International System of Units (abbreviated SI) is the world's most widely used system of units.  The system consists of a set of units and prefixes...

Maxwell's Equations - Introduction

Maxwell's Equations are a set of fundamental relationships, which govern how electric and magnetic fields interact. The equations explain how these fields...

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