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Author Topic: when to use what conduit?  (Read 5328 times)

Howard Gabriel

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when to use what conduit?
« on: September 22, 2008, 02:30:53 AM »

Hey there.

So I hear often that Rigid Metal conduit should be used for all audio cables that travel a significant distance but "significant" is never quantified...

So, what are the hard-and-fast rules for using different types of conduit?

What is PVC good for?
When should Rigid Metal only be used?
What sort of runs (length) determine a particular conduit type?
Should data and video be in Rigid Metal aswell or do they have different tolerances?
What about digital cable, I presume it doesn't matter on conduit type because it is not susceptable to noise?

The questions could go on... and they just might.
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Karl P(eterson)

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Re: when to use what conduit?
« Reply #1 on: September 22, 2008, 09:54:31 AM »

Underground conduit is built out of PVC and preferably pressure tested for leaks before it gets covered up.

Any conduit that is above ground (in walls, ceilings, etc) needs to be EMT or rise/plenum rated plastic conduit such as KWIK PATH. Around here that plastic is a premium over EMT so we just have every last piece of anything run out of EMT. EMT looks nicer and holds its shape far better anyways.

The reason for this is mostly a code one having to do with fire codes and local electrical inspectors. It has been this way in the areas I have worked in recent memory, but it could be different in your area. Your low voltage contractor will know what to do.

Karl P




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Brad Weber

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Re: when to use what conduit?
« Reply #2 on: September 22, 2008, 10:23:05 AM »

This is the type of general issue that is difficult to address in forum posts.  The length of the run is far from the only factor.  As Karl noted, there are code aspects as well as physical, practical and technical aspects that are also factors in when to use conduit and what to use.  For example:

a) A cable run may not be a noise problem if there is nothing else near it but would likely be an issue if there is power run parallel to it just 6" away for the whole length.

b) If it is a plenum or riser environment and you don't want to run plenum or riser cable, then code requires the cable be in conduit.

c) If the cabling is going to go through a wall or above a a ceiling that is not accessible, you probably want conduit to make installation and changes practical.

d) If you are running mic and speaker lines from the same points and they can't be physically separated, you probably want the mic lines run is their own conduit.

These are just some typical examples of factors that can apply, there are many others.

Rigid Metal conduit is nice but it is expensive and EMT is much more common.  As Karl also noted, PVC is common for underground applications.

Digital signal are still susceptible to noise, although the effect may be different.

Interestingly, there is an effort currently going on within SynAudCon to try to evaluate the specific impact of different wire and conduit types with the goal of hopefully better defining the effectiveness of different combinations and when they may be appropriate.
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Brad Weber
muse Audio Video

Howard Gabriel

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Re: when to use what conduit?
« Reply #3 on: September 23, 2008, 10:17:46 PM »

Many thanks for the replies.

Obviously I'm no installer but I can't help being curious about the things I CAN'T see... and cabling is one of those things in a new building that we don't get to see.


Replies shed a lot of light but before I put this one to bed, what would be the typical assignment of cables in different conduits:

Obviously electrical and speaker cable I presume have their own and do not run with anything else, but is audio, video and data "typically" sent in the same piece of conduit or are they "typically" seperated for other reasons (not noise but ease of locating perhaps)?

I presume fiber optic can run with anything but would that typically be in another conduit aswell (being so tiny)?
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Brad Weber

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Re: when to use what conduit?
« Reply #4 on: September 24, 2008, 08:24:41 AM »

There are once again both technical and practical issues involved.  On the technical side, it would be pretty common to run video, line level audio, mic level audio and speaker level audio as four separate runs in order to separate the different types and level of signal.  You might combine line level audio and video in one run in some cases or line level audio and mic audio for very short runs.

This is where you also get into the practical aspects of things like how large a conduit a junction box or gang box can handle, how large a conduit can fit in a slab and so on.  In many cases the limitations on conduit size may force breaking the connectivity up into multiple conduits in which case it makes sense to separate the signals.  Conversely, if dealing with existing conduit the conduit available may force you to group some signals.  Future connectivity and plain old cable pulling issues (how difficult does having mixed cable types make it to add a cable, how difficult is it to pull the cable bundle, could adding a cable damage the existing cable, etc.) may also lead you to group some signals and separate others.  And as you noted, you might even have a combination due to locations, for example you might run video and line level audio in separate conduits to a junction box at the stage at which point they then run together in a single conduit for the shorter run going to the floor boxes and wall plates.

It can even be simple things like a multi-compartment floor box that requires getting the appropriate signals into each section of the floor box and has limited knockouts.  The type of signals in each compartment and the number and size of knockouts in the box may dictate just how much signal separation is practical, it is the a judgement call is to how to separate and combine the cabling to work within those constraints.
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Brad Weber
muse Audio Video
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