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Author Topic: Flexible Conduit and Size  (Read 1209 times)

Aaron McQueen

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Flexible Conduit and Size
« on: June 06, 2007, 12:51:32 pm »

My church is doing much needed renovations. The sound is fairly good and we won't be changing much, but I'm trying to resolve some of the infrastructure problems at the same time of the renovations.

We are moving the sound booth from a second floor location to the main floor. The current conduit is running through the attic/crawl space to the stage, down a raceway to the stage. Moving the sound booth to the main level means either extending that conduit down the back wall with cable extensions or new cable runs, or installing a new conduit path.

During they renovations, they are moving the duct work from the floor to the ceiling. The current duct work is rusted and letting water in which makes the room a sauna when the heat is on. The old duct work is going to be filled with concrete (probably not the entire duct, just the openings). This duct makes a perfect straight path from the sound booth location to the stage.

Is there flexible conduit I could run down the ductwork to the stage? I need flexible because of the angle at which I have to enter the duct and a smooth wall would be nice.

What size conduit? I would like 16 lines of audio plus 5 returns, video cable, network cable, and room to spare.

Thanks, in advance for any suggestions.
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Tom Young

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Re: Flexible Conduit and Size
« Reply #1 on: June 06, 2007, 04:00:04 pm »

Aaron-

Yes, there is flexible conduit. It often is used to transition from pull boxes or rigid conduit into racks, etc. Unfortunately, being a consultant I never have to source this stuff so I have no resources for you. Google on it and you'll see a ton of possibilities. Maybe Brad, Ira or Ivan will chime in with exact products.

I believe that the couplings between flex and rigid conduit can be finicky. In the SynAudCon archives there exists at least one very thorough thread on this from a few years ago.

As far as size, it is common practice to spec/use conduit based on 40% fill (40% of the conduit area is filled). This practice allows for easy pulls, or rather: it helps to prevent wire damage from hard pulls. And it also allows for additional wire to be added in the future. Good practice. But if your runs are short, this may not be necessary.

Belden, among other cable manufacturers, provides a spreadhseet or wizard that allows you to calculate the conduit size needed for any number of wire/cables of a given diameter. You can sub in the specific cable you will use (even if it is not Belden) simply by finding a Belden product with same diameter.

Check their website.
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Bob Taylor

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Re: Flexible Conduit and Size
« Reply #2 on: June 06, 2007, 06:14:49 pm »

Flex conduit would not seal out the concrete and if its the aluminum stuff will corrode in no time in contact with the concrete. Also flex is expensive and hard to find in large size and length. Sealtite conduit would break the budget.
The telco industry uses what is called inner-duct. Made of PVC, flexible and intended to be pulled inside large conduit. Seen lots in the 1 inch size with a pull rope already inside. Would not corrode and pulling multiple runs would help separate the different systems. Also pulls real easy as it is slick inside for fibre optic cable.
Bob
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Vinny Worley

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Re: Flexible Conduit and Size
« Reply #3 on: June 06, 2007, 10:54:55 pm »

Any type of flexible conduit that is unsupported/unsecured over a long length will be problematic when pulling cables inside it. If you have any turns, the friction of the cables (even with liberal amounts of lube) will cause the conduit to bunch up in the direction of the pull.  Is there just a crawl space underneath then? If so, can you push PVC conduit underneath from a vent or other opening along the side, then work it into the ductwork through a hole (that will be filled when the cement goes in)? Can you entirely demo out the floor under where the new sound booth will go? Perhaps you can get better access through that kind of opening.

If you can get it in there, I'd go with at least a pair of 3" PVC conduits (2" would be ok, just not as much room for future pulls.) (When you go to add cables later, you aren't usually able to fill the conduit to the level that you can when doing a single multi-cable pull.) Figure on less than ~50% capacity utilization.

This is a tough one; without seeing it, it is hard to suggest anything else. I could probably come down and look at it to give you some different ideas; send me a PM if you want to explore that. You're probably about an hour and a half from the office; I've driven further than that for a free consult.

Regards,

Vinny
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