Unlocking the Code: Requirements for A/V System Cabling

Audio/visual systems must be installed to meet the cabling requirements of the NEC. The audio/visual (A/V) system in an office or school is often installed long after the base-build phase of construction of a new building is completed. The A/V system may not even be included in the tenant improvement phase of a project.

09/03/2010


The audio/visual (A/V) system in an office or school is often installed long after the base-build phase of construction of a new building is completed. The A/V system may not even be included in the tenant improvement phase of a project. Smaller systems may be installed by the building facility or IT department after a building has been occupied. Often the only building design provisions to accommodate A/V cabling will be an empty conduit to a floor box or a poke-through in the floor to avoid cables being routed on the floor to reach a podium and conference room tables.

Due to the complexity of larger A/V systems, they are more likely to be installed by a contractor specializing in this type of work. Regardless whether an A/V system is being installed as part of new construction, included in a retrofit, or being installed as part of an upgrade of an existing system, it still needs to meet the cabling requirements of the National Electric Code (NEC).

Key Issues

The cabling circuits for an A/V system are considered NEC Class 2 or 3 power limited signaling circuits. This is the same section of the code that applies to network cabling. For A/V systems that use IP (Internet Protocol) networks for communications, the A/V signals are data traffic signals.

However, there is still a lot of A/V cabling distribution that doesn’t look anything like the cabling for a common data network. For a network structured cabling system, there would be large quantities of unshielded twisted pair (UTP) Category 5e, 6, or 6a cabling centrally routed back to a communication room. An A/V system consists of a variety of different cable types with the majority of cabling interconnecting within the surrounding area of the room receiving the A/V equipment. In a modern data network almost all of the signals are Ethernet/IP based. In an A/V system there may be only one Ethernet connection with the remaining cables supporting audio, component video, broadband video, HDMI, RS232 serial, USB, and DVI signals.

 

Figure 1. A block diagram of a simple A/V system. In this simplified system there is no audio component, no serial control to the projector, and no automated control for the projection screen. Source: CH2M HILL.

Figure 1 shows a block diagram of a simple A/V system. In this simplified system there is no audio component, no serial control to the projector, and no automated control for the projection screen. Figure 2 represents the same system implemented in a conference room. In this example, the only concern is the display video cable. The cable from the laptop to the projector transitions from the tabletop to a floor box, in a conduit up the wall cavity, through a ceiling space, and then through the ceiling tile to the projector. There may or may not be an outlet for the video connection on the table.

Figure 2. This represents the same system as Figure 1, implemented in a conference room. Source: CH2M HILL.

The cable shown needs to be properly rated for the environment it is routed in. In the design of this system, here are some questions you should ask yourself:

  • Is the cable rated to be routed exposed in the ceiling space?
  • Is the ceiling space being used as a return air plenum, thereby requiring the cable to be plenum rated?
  • The conduit stub is coming up the wall and the projector is in the middle of the room. How is the cable supported?
  • There may be only one floor box for power to the table devices, communications cable, and A/V cabling. Is there proper separation between the different circuits?

Figure 3 shows a block diagram for a more complicated system. There are a variety of different signals and different types of cables required depending on the signals, but the same questions apply as did for the system in Figure 2.

Figure 3. This block diagram shows a more complicated system. Different signals and different types of cables are required depending on the signals, but the same questions apply as did for the system in Figure 2. Source: CH2M HILL.

Deciphering the Code

Since A/V cabling is covered under the same section of the code as network data cabling, the same rules apply in the application of cable routing, supports, and the cable selection type for the environment for which it is installed. The NEC allows for the Class 2 and 3 circuits to be routed exposed in the ceiling and wall spaces as long as the cable is properly rated. If ceiling space is not used to transport air, a CL2 or CL3 rated cable meets the requirements. If the ceiling space is used as an “other space used to transport air” as defined in NEC paragraph 300.22(C), then the cable needs to be plenum rated and have CL2P or CL3P rating. Since the code allows communications rated cables (CM, CMR, CMP) to be substituted for Class 2 and 3 cables, you may likely see these designations on the cable jacket. The exposed cabling in the ceiling space can be supported the same as data cabling using J-hooks, in conduit, and in cable tray. The cable must not be supported from ceiling support wires, or lie on the ceiling tiles, on the recessed lighting, or on other utilities.

In the floor box or conduit to the floor box, the data (Ethernet) cable does not have to be separated from the A/V cable per the code because they are both Class 2 or 3 circuits. Due to the size and quantity of the data and A/V cables, the designer may choose to use separate or multiple conduits to the floor box. Whereas it is rare for the data cable to be terminated prior to being pulled through the conduit, often an installer may choose to install a video cable with connectors installed at the factory. This takes considerably more conduit space to install the cable. Once the cables are in the floor box, the data and A/V cables can share the same portion of the box. The power routed to the floor box is required by the code to be in a separate conduit and in a separated compartment within the floor box.

Although we have generically identified A/V cabling as being a Class 2 or 3 circuits, audio cables are covered under their own section of the code. Audio systems are addressed under Article 640, Audio Signal Processing, Amplification, and Reproduction Equipment. For the routing of audio cable in plenum spaces and in cable tray, Article 640 references the same paragraphs in Chapter 3 as Article 725. Refer to Table 1 for more detail. For the audio cable on the output side of the amplifier (the cable to the speakers), Article 640 paragraph 640.9(C) refers you to Article 725 for Class 1, 2, and 3 wiring.

 

Table 1. Code requirements reference table. Source: CH2M HILL.

In designing a system, the designer must verify with the audio equipment manufacturer that the output of the amplifier is Class 2 or 3 wiring compliant to allow it to be routed with the video, control, network, and communications cabling. This information may be located in the amplifier installation manual or printed on the back of the amplifier. With careful product selection, a large A/V system can be specified using audio amplifiers requiring only Class 2 wiring. For example, Extron manufactures its XPA 200x series of 70 V rack-mounted amplifiers that are rated at 200 W per channel requiring Class 2 wiring. Bogen manufactures its large M class audio power amplifiers rated at 70 V, 4 ohms at 1200 W, and the back panel clearly states that Class 2 wiring is required.

Article 725 does have one restriction in routing Class 2 audio cables with other Class 2 wiring. Section 725.139 addresses the conditions when conductors of different circuits are in the same cable, enclosure, and raceway. Paragraph 725.139(F) states, “Audio system circuits described in 640.9(C), and installed using Class 2 or Class 3 wiring methods in compliance with 725.133 and 725.154, shall not be permitted to be installed in the same cable or raceway with Class 2 or Class 3 conductors or cables.” Although audio cables on the output side of a Class 2 amplifier can follow all of the 725 rules for routing, cable types, and installation as other Class 2 circuits, they cannot be in the same cable or raceway. This would imply that an A/V cabinet with video, control, and audio outputs would need a separate conduit for the audio cable. There appears to be no restrictions in the code allowing the Class 2 audio circuits to be routed exposed or in the same cable tray with other Class 2 circuits.

Other Items to Note

All the different A/V signals require their own types of cable. With the pervasiveness of data network UTP cabling, there has been an increasing amount of technology to allow A/V signals to use network UTP cabling. A few years ago, companies started selling adapters to allow the interface with UTP. Now we are seeing A/V equipment being shipped with 8-position, 8-conductor UTP jacks for more than just the data connection. Using UTP cable has the advantage of being easy to buy, widely understood, and supported with products in the industry, and there is a large workforce of technicians trained in its installation. However, many A/V signals are not IP based and there are technical issues to get the variety of A/V signals to work on UTP. We will likely see a trend of more A/V component manufactures developing products to use UTP network cable for A/V signals. For now, the cabling installed is specific to the signals being sent. Whether using cabling specific to an A/V signal or UTP data cable and adapters, the code requirements for the cable type and installation are covered in NEC Article 725.

Keep in mind that the code is not concerned with the performance of the A/V system. The A/V system designer still needs to consider the separation of cables or signals from a performance perspective. Signals of different voltage levels should be separated to avoid signal cross talk. Microphone level (mV), line level (2 V and less), and speaker level (25 to 100 V) signals should all be separated. Even data signal pulses from Ethernet circuits can induce noise onto audio cabling circuits. Power circuits should not be routed parallel in close proximity to A/V signal cable and should only cross each other at 90 degrees. The code may allow Class 2 circuits to share the same cable tray, but the standard practice for the separation of signal types has to be considered for the system to perform properly.

Fire stopping can easily be overlooked by an installer placing A/V cabling after the initial construction of the building has been completed. Where the base-build contractor would have an architectural set of drawings showing the fire-rated walls, an installer for an A/V system may not think to ask to see the architectural drawings, or the drawings may no longer be available. This does not relieve the installer of the A/V system from following the NEC or building code requirements for maintaining the integrity of a rated wall or system when routing the cables for the A/V system.

The power cords for items such as projectors, switchers, video converters, DVD players, and such are shipped with a typical appliance-type of power cord. This type of power cord is UL rated for its intended use, which is to connect the appliance to a power receptacle. These types of power cords are not rated or suitable for routing through walls, through a floor box, or into a ceiling cavity. Planning is required to make sure there is a power receptacle near where the A/V equipment will be installed.

Conclusion

It is not only the contractor’s responsibility to meet the code requirements when installing an A/V system. As a building owner or tenant of a building, you have the responsibility to install the cabling and system components to meet the building code requirements. The codes are in place to protect people and the property. This responsibility does not end with the contractor after the initial installation.

The most common code issues seen in small installations are cables that are not rated for the environment, improper separation between power and signal cables, and the improper support of cables in the overhead ceiling space. The markings on the cable jacket are important in designating its intended purpose and whether the NEC would allow it to be routed in a building. It’s the responsibility of the owner, designer, contractor, and installer to understand the listing designations in the NEC and apply them to correct building environment.

 

 

Kuhlman has 22 years of experience in the design and construction of telecommunications infrastructure. He is a member of Consulting-Specifying Engineer's Editorial Advisory Board.

 

NEC 2008 root articles:

Video and Signal Control Cabling: Article 725 for Class 2 and 3 circuits

Audio Systems: Article 640. Article 640 refers to Article 725 for Class 1, 2, and 3 cabling to speakers.



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