Reusing Conduit 8

[Reesh14]

I know that there are calculations to determine the minimum conduit size required for conductors, but is there also a maximum size that can be used? I am replacing a handful of 208V,3 phase rooftop units. The new units have significantly less load than the existing. Therefore lower rated circuit breakers and wire sizes will be used for the new units. I would like to reuse the existing conduits if possible. In a few cases the existing conduit size meets the size of the required load, but in most cases, the existing conduit is 1 to 2 inches larger than the minimum conduit size required for the load. Length of the run varies between 50-175 feet. Can over sized conduits be used? or is there some issue with insulation or something else?

example: existing rooftop unit: 350 mcm wire size, 2-1/2" conduit, and fed by 300A circuit breaker. The new rooftop unit has an MCA of only 34A and a recommended 35A MOCP device. Thus #8 wire will be installed and only 3/4" conduit is needed. Can the 2-1/2 inch conduit be reused?

Thank you.

 

[EEJaime]

You can absolutely use over sized conduit, there is no code or installation restriction to doing so. Installation is a breeze and you would save money on demolition and new conduit material and installation.

 

[SteveWag]

Yes, often times oversized conduits are used.

Steve Wagner

 

[Laplacian]

There is one possible issue with using slightly oversized conduits where conductors jam together when pulled in. Look up "jam ratio" for more information.

http://www.polywater.com/jamming.html

 

[safy2004]

Several years ago I faced the same problem, Sure you can use oversized conduit without any troubles, its the right way if you have those conduits in hands, and there is no need to buy more!

 

[waross]

You don't have to change the conductors either if they are in good condition. You can pigtail smaller conductors on each end of the existing conductors for ease of connection to the new breakers and equipment.
Depending on your code, you may have to add a code sized grounding conductor if one is not present.
For the most econoical installation;
1> I would reuse the existing conductors if the grounding is up to code.
2> If an equipment grounding must be added, I would remove the existing conductors and install the new conductors with the grounding conductor. Reclaim and sell the old copper to offset the cost of the installation.
respectfully

 

[itsmoked]

With big conduit you can even use the vacuum cleaner wire pulling method.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[apowerengr]

You may need conduit reducers if the knockout area is not large enough to accommodate the oversized conduits, otherwise it sounds like a great opportunity.

 

[powerjunx]

Reesh14, preferred size conduits were presented on the table for minimum size as prescribe by the code. This, as we know, prescription is just minimum, if you oversized it then the code never restrict to use it. While using it beyond minimum sized, certainly you just creating your own burden.






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