# 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:

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

4. Minimum duct diameter  Cc:

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.

More interesting Notes:

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

Differential protection, the good old days

This morning I was explaining how differential protection works to a junior engineer. To give him something to read I opened up the NPAG (Network Protection...

Post Authorship

In 2011, with the introduction of it’s Panda search ranking algorithms, Google introduced tools for determining the original author of posts.  The intention...

Are We Losing Professional Integrity

I have been thinking recently that there appears to be less professional integrity around than when I first started my career in electrical engineering...

Low Voltage Switchroom Design Guide

Low voltage (LV) switchrooms are common across all industries and one of the more common spatial requirements which need to be designed into a project...

Periodic Electrical Installation Inspection – What to Inspect?

This is the second post in a series of two on periodic electrical inspections. In the first post, I discussed how often inspections should be carried out...

Introduction to Traction Substations

Following on from my post on railway electrification voltages, I thought an introduction to traction substations would be a good idea. Traction substations...

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...

GE's Shingijutsu Factory

GE's latest thinking on product manufacturing is he Shingijutsu philosophy or Lean production system. They have started applying this at the Louisville...

Lighting - Lamps

Lamps are the essential part of any luminaire. These are the light generating components. Since the advent of electrical lighting in the middle of the...

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

## 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