Separating power and communications conduits

I am working with a school district that requires a minimum 12-inch separation of power and communications conduits, and concrete encasement of all conduits

Dec 1st, 2000
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Q: I am working with a school district that requires a minimum 12-inch separation of power and communications conduits, and concrete encasement of all conduits. This is causing us problems, as many areas we have to work in are too narrow to allow the conduits to be installed in parallel. And stacking will force us so deep as to require shoring, which we are trying to avoid.

I would like to reference a code or standard that provides for the 3-inch separation between power and communications conduits when the conduits are encased in concrete, but have only found the table in Chapter 8, page 38 of the Telecommunications Distribution Methods Manual (TDMM), 8th edition on compact disc. I do not believe that will carry the weight necessary to get the district to allow a variance in its specifications. Can you direct me to specific National Electrical Code, Telecommunications Industry Association (TIA-Arlington, VA) standard, or other standard or code that will help to validate my position?

Douglas Vigon, RCDD
Vector Resources Inc.
Los Angeles, CA

A: The text in BICSI's (Tampa, FL) TDMM can also be found in the National Electrical Safety Code (NESC), Section 320 B 2, Separation From Other Underground Installation. It states, "Separations Between Supply and Communication Conduit Systems: Conduit systems to be occupied by communication conductors shall be separated from conduit systems to be used for supply systems by 75 mm (3 in.) of concrete, 100 mm (4 in.) of masonry, or 300 mm (12 in.) of well-tamped earth."

For those not familiar with the NESC, it is to outside-plant facilities what the NEC is to facilities inside the building. Like the NEC, the NESC's purpose is the practical safeguarding of persons during the installation, operation, or maintenance of electrical supply and telecommunications lines and equipment.

The NESC contains the basic provisions that are considered necessary for safety, and is not intended as a design specification. If your client has the 12-inch separation requirement in the contract documents to address another concern, you may still be ordering shoring materials.

More on home networking

I received the following letter from Jim Nienhuis with Herman Miller (Zeeland, MI):

I just read your article in the October issue about home networking. I learned a lot about IEEE-1394 that I was not aware of before. I've been following networking alternatives as part of my research at Herman Miller and thought I'd pass along some information.

I have been particularly interested in power-line carrier capabilities, which were touched on in your article by the X-10 reference. There is actually much more to it than X-10, which is far too slow for any useful data communication. Companies like Intellon, Eniki, Inari, and Itran have systems that can provide up to 14 Mbits/sec on existing power conductors. You can find more on them at individual company sites and the HomePlug Powerline Alliance Web site.

Just to round out the list of alternatives, there's IEEE 802.11b used in wireless local area network (WLAN) equipment-most notably Apple's AirPort. Intel has a competitive system as well as others that run at 11 Mbits/sec. IEEE 802.11a will bring the speed up to 54 Mbits/sec.

In the future, there's Time Domain's Ultra Wideband Frequency low-power communication proposal at 10 Mbits/sec and a range of 150 feet. In the far-out category is Media Fusion with 2.5 Gbits/sec over the power grid for a distance of thousands of miles. (They're looking for investors, if you're interested.)

The last house I built probably has more cable than anyone will ever use, but if I needed to set up a network in an existing house, the Intellon power-line carrier capability looks interesting. I'm curious about your thoughts on the power-line carrier technologies. Do you think they have a chance?

My response:
Today, virtually any device found in the home-PCs, printers, TVs, VCRs, set-top boxes, and security systems-can be networked together through the existing power connection. Just think of the convenience of connecting any device to the network through a power outlet.

Networking devices facilitate sharing of Internet access and high-bandwidth services. The problem is speed, or lack thereof in products currently available. Power lines were created to transmit power, not to transmit data. The technology is tricky, the lines are noisy, and there's a chance of interference between adjacent houses.

All of that aside, the HomePlug Alliance is busy writing standards. Thus far, they have chosen orthogonal frequency-division multiplexing (OFDM) technology from Intellon. The alliance plans to release a complete specification based on Intellon's technology with its 14-Mbit/sec raw data rate by the end of this year. Semiconductor companies are expected to announce silicon this year, and home-networking products based on the standards will be available in early 2001.

But they are not the only standards-setting body for power-line home networking. A group within the Electronic Industries Alliance/ Consumer Electronics Association (EIA/CEA-Arlington, VA), the home-networking committee of the EIA/CEA R7.3 power-line group, will create its own standard. First we had none; now there will be at least two. If these guys don't "play well" together, power line will end up like wireless, with multiple standards, each optimized for a different task and interfering with each other.

Thanks for the investment tip, but I'll stick to my rental properties.

Donna Ballast is a communications analyst at the University of Texas at Austin and a BICSI registered communications distribution designer (RCDD). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, the University of Texas, Austin, TX 78713; tel: (512) 471-0112, e-mail:

Cabling Installation & Maintenance Editorial Mission

Designers, installers and owners of premises and campus communication systems are challenged by changing standards, products and technologies. Keeping pace with these changes requires access to current information from experts in voice, data and video infrastructure solutions. Cabling Installation & Maintenance provides analysis and interpretation of standards and technologies, presentation of design and installation techniques, and selection and use of cable and campus communications systems.

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