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NEC wiring requirements for 70v systems


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#1 sellis

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Posted 21 October 2005 - 12:03 PM

I have been trying to find out if I can combine in the same conduit my 70V PA speaker wiring from a CTs 1200 amp with other low voltage communication wires (such as Cat. 6 Ethernet cables).  I have looked through the NEC and it looks like the requirement is based on what class of power supply the voltage source is rated as.  It appears that for power supplies (which I am assuming would be the amp in this case) that are rated as Class 2 or 3, the associated circuits can be combined in the same conduit with communication circuits.  If the power supply is rated as Class-1, then the circuit cannot be combined in the same conduit with communication circuits.  Has anyone looked into this before that can tell me if I am reading the code right?  Also does anyone know what class the CTs-1200 amp is rated at when outputting at 70V?  Aside from the NEC requirements, are there general guidelines on this topic to avoid crosstalk or other problems?  Thank you.

#2 DGlass

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Posted 25 October 2005 - 09:48 AM

The UL-National Electrical Code (NEC) is published by the National Fire Protection Association (NFPA) and gives safety advisory considerations for electrical wiring. UTP cables meet different UL-NEC requirements mainly on the quality of the cable jacket. NEC Article 800 Communications Cables are manufactured to meet these different cable types. Guidelines for building pathways, such as conduit, raceways, and spaces for telecommunications wiring are covered in the EIA/TIA-569 and 568 Standards as well as EIA/TIA TSB-36 and 40.
Without getting into all the codes it has never been considered a good practice to put a low current low voltage line in the same conduit as a higher voltage higher current lines because of the induced crosstalk that can be generated from the speaker lines. I have seen it go so far as to cause feedback and oscillations, at high output levels when someone placed the signal level cable feeding a system into the same conduit as the speaker lines.   sad.gif  There could be all sorts of packet loss problems with data lines that could occur.  sad.gif There are specific guidelines for the installation of UTP cable for everything from how tight to tie the bundles, maximum length of conduit, number of 90 degree bends, to maximum pulling force.

Check out:

*The “Network Performance” section of Peak Audio’s “Network Cabling” web page at: http://www.peakaudio.com/CobraNet/Network_cabling.htm

"Cabling Installation & Maintenance" magazine is good source for training videos and reference books.
http://cim.pennwellnet.com/home/home.cfm

BiCSi publishes a “Telecommunications Distribution Methods Manual” which is the text book for their Registered Communications Distribution Designer Certification.They also have videos, books and training courses.
http://www.bicsi.org/

*http://www.generalcable.com/North_America/NA_Assets/ProductBrochures/PrmsInstallGde.pdf
* http://www.cable4pc.com/utp731.htm
* http://www.cirrus.com/en/pubs/appNote/NetworkCabling.pdf
* http://www.mohawk-cdt.com/tech/instguides-utp.html
* http://www.tiaonline.org/media/press_relea...arelease=04-139
* http://www.mohawk-cdt.com/techold/standards569.html#4

#3 sellis

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Posted 25 October 2005 - 06:07 PM

Thank you for the information.  From what I understood from your response, it is not good practice to combine in the same conduit communication cables with power wiring.  That is information that I have heard with regards to standard 120V and higher voltage AC, but did not know if the 70V DC PA wiring was considered a high enough voltage or current to be a problem (for example power over Ethernet uses a higher voltage and current but is typically not considered a problem and is run within the same cable).

I believe that separating the PA wires from comm wires is good advice, but unfortunately good advice is not always good enough for some people.  The client I am working for does not want to pay to have separate conduits unless required by an applicable code or standard.  The ANSI/TIA/EIA-569-A standard states: "Co-installation of telecommunications cable and power cable is governed by applicable electrical code for safety."  The applicable electrical code for safety in this case is the NEC and whether or not the PA cable is considered a power cable in the NEC seems to be based on what class of power source the circuit is powered by.  This brings me to my original question: When outputting at 70V, is the CTs-1200 amp considered a class 1, class 2, or class 3 power source?  In the CTs 2-Channel power amplifier user's guide it says that CTs amps require Class 2 wiring and on the photo of the amp in the guide it says "Class 2 wiring outputs" above the screw terminals (page 14).  I just want to verify that it is still rated as Class 2 even when outputting at 70 volts.  Is this correct?  Thank you for your help.

#4 DGlass

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Posted 26 October 2005 - 10:23 AM

As you have already noted the back of the amplifier already states to use "Class 2 Wiring". That is what you should use, as a minimum, if you want to meet the NEC for "safety".  However, having a safe system and an operational one are two different things. You may be able to put them in the same conduit by safety codes but should you? blink.gif
I would also try and contact the people at BICSI as they are the people that certify Network Structured Cabling installers. They may have further insight as to any actual regulations for a DDT (Don’t Do That).
Good Luck

For those of you who are following this thread one thing to note here: as a general rule Audio Systems and Network Structured Cabling fall under the “Communication” section of the NEC. I have seen in some situations were the local inspector has insisted on following the "Electrical" part of the code ignoring the "Communication" part or that the local codes require all wiring to be in conduits. Make sure you know your local codes as well. The main reason for having conduit is to protect cabling as it travels through walls and ceilings as well as to keep burning cable from spreading flames or smoke. This is what the NEC covers for the NFPA. The NEC does not take into account any cross transmissions of signal due to coupling of electrical fields that occur when lines of different signal types and/or potentials are run in tight proximity to each other over long distances. This unwanted transfer of signals from one set of wires to another is called Crosstalk. Crosstalk if severe enough can cause havoc with data transmissions lines. Yes UTP/Cat. cable uses tightly twisted pairs to reduce crosstalk but, in this case, you still have the AC potential difference of up to 70 volts. This is why I say it is not an excepted practice to install UTP/Cat. wiring in the same conduit as 70 volt speaker lines. My philosophy is it is better to engineer possible problems out from the beginning than to try and fix them later. smile.gif