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



Surface Treatment – Ladders, Trays and Baskets

Steel ladders, trays and baskets form the backbone of cable containment systems. Often these items need some form of surface treatment to prevent corrosion...

Voltage Levels – Confused?

I was having a conversation the other day about voltage levels.  While everyone was in agreement that low voltage was 1000 V and less, there was more confusion...

Understanding electric motor insulation & temperature

Anyone specifying or using electric motors should have a basic understanding how the insulation is related to temperature. Three classes of insulation...

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...

Understanding Motor Duty Rating

One of the comments on my Motor Starting Series was asking for something on duty cycles. Here it is. As a purchaser of a motor, you have responsibility...

Periodic Electrical Installation Inspection – How Often?

How often installations are inspected is up to the owner of the installation, provided such durations do not exceed any regulatory maximums in force. ...

Smarter Electrical Distribution

The other day I came across an article in Technology Review on the development of a smart transformer. A professor at North Carolina State University is...

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 ...

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...

Photovoltaic (PV) - Utility Power Grid Interface

Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird. During periods when the PV system generates energy...

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