Unlisted cables and the NEC
Data and communications cabling installed in a building is required to be listed per the National Electrical Code (NEC). Unlisted cables need to transition to a listed cable through the use of a splice or termination after entering a building. How soon does this transition have to take place? Can the transition from an unlisted to listed cable be extended into a building? This article provides the answers.
Per the definition in the NEC, equipment or material is listed if it is included in a list published by an organization that is recognized by the authority having jurisdiction (AHJ). The listed equipment is required to be periodically evaluated; it must meet the appropriate standard and be suitable for its purpose. (This is a paraphrase of the definition; see the “Conditions” sidebar for the whole definition.)
For data and communications cabling, listed designations can be found in Articles 725, 770, 800, 820, and 830. Each article has paragraphs defining the cable listing for the cable types unique to each article. The listing designations correspond to the cables’ performance requirements for the spread of fire and smoke, and their intended use. Cables may have other markings and designations that look official, but that does not mean that the NEC recognizes the cables for use within a building. For example, a communication riser (CMR)-listed cable is intended to be used and connected to a communications system and meets the flame spread requirements for a riser application. Buried communications cables commonly installed in outside environments have a PE-39 or PE-89 designation. This corresponds to a specification issued by the Rural Utility Service (RUS).
The RUS is a division of the United States Dept. of Agriculture and has published standards for the Rural Electrification Administration (REA). PE-39 is a filled-outside-plant cable. Although there are some cables that are rated for both outside plant in indoor installation, PE-39 is not. PE-39 cable is not intended to be routed inside a building and does not carry any of the listing designations recognized by the NEC, such as communication (CM), communication cable limited use (CMX), CMR, or communication plenum (CMP) (NEC Table 800.179 Cable Markings). A cable marked with PE-39 would need to transition to cable with a valid NEC listing designation as it enters a building.
Deciphering the code
Whereas Article 725 plays a large role for the distribution of data cabling within a building, this is not the case for data entering a building. Depending on the size and function of a building, the data signals may be brought in on fiber optic cable or on traditional copper telephone cable. Fiber optic cabling is covered under Article 770, and copper telephone cables are covered under Article 800. Small commercial facilities may also have the option of bringing outside plant data service to their buildings through the use of cable TV (CATV) cabling covered in Article 820 or a Network Broad Band Communications System under Article 830, which was added in the 1999 version of the NEC.
Rare cases in which a Class 2 data cable is routed from one building to another, such as in the data connection to a guard house, are addressed in Article 725, paragraph 725.141. Article 725 essentially refers to the requirements of Article 800 for twisted pair cables and 820 for coaxial cables entering a building.
Articles 770, 800, and 820 each have paragraphs that describe how unlisted cables shall be permitted to be installed where the length of the cable within the building, measured from the point of entrance, does not exceed 50 ft and the cable enters the building from the outside. The individual articles address their own cables’ uniqueness and further define if the cable has to be terminated in an enclosure, a protector panel, or a grounding block. The NEC was helpful in defining the unlisted cable distance the same for each article. The trades commonly refer to the unlisted cable past the entrance point as the “50-ft rule.”
Though in the simplest terms, 50 ft is the maximum distance of unlisted cable that can be routed into a building, you can actually run more than 50 ft. The key is in understanding what the NEC defines as the point of entrance. Articles 770, 800, 820, and 830 have similar definitions for the point of entrance. In those articles, point of entrance means the point within a building at which the wire or cable emerges from an external wall, from a concrete floor slab, or from a rigid metal conduit (type RMC) or an intermediate metal conduit (type IMC) connected to a grounding conductor to an electrode.
For cables defined in 770, 800, and 820, if the unlisted cable that enters the building is immediately exposed, you need to start counting 50 ft of cable from the wall or concrete slab. You can provide protection to the cable using inner-duct cable tray or electrical metallic tubing (EMT) raceway, but the 50-ft limit starts where the cable exits the wall of concrete slab.
To extend into a building beyond 50 ft, the conduit entering the wall or concrete floor slab needs to be extended with IMC or RMC conduit. This allows the entrance point to move from the wall or concrete slab. The 50-ft limit starts when the cable exits the IMC or RMC conduit. Extending the entrance point with IMC or RMC is a useful provision in applications when it is not practical to have the entrance facility on a ground floor or adjacent to the exterior of the building.
Other items to note
In the case where unlisted and nonconductive fiber optic cable is entering a building, NEC paragraph 770.48(B) allows the entrance facility to be extended with EMT and rigid PVC conduit. The optical fiber and cable jacketing material is inherently nonconductive. Conductive materials such as an armored wrap, steel strength members, or a tracer wire can be added, which would result in the cable being listed as a conductive cable. The designer needs to be mindful of article 300.22 (C), which does not allow rigid PVC conduit to be used to extend the entrance point in the above space of a ceiling that is used to transport air.
For the unshielded twisted pair cables of Article 800, the code requires primary protection circuits for cables entering a building to be located “in, on, or immediately adjacent to the structure or building served and as close as practical to the point of entrance.” The definition of “as close as practical” is subjective and may be a good topic for discussion with a local code official. When primary protection devices are installed inside the building, transitioning to a listed cable can occur at the protector block. If the protector is located outside the building, the unlisted cable can be routed into the building following the provisions noted above. Although if the pathway from the outside protector block to the building has some protection from the outdoor environment, it is recommended that the transition to a listed cable occur at the protector block.
Article 830 does not address the entrance or unlisted cables in the same way as the other articles discussed. Article 830 uses the 50-ft rule for network-powered broadband communications low-power (BLX)-listed cables where a primary protector or a network interface unit with an integral protector is located at the point of entrance. Limited use network-powered broadband communications low-power cable (BLU) has to be routed in RMC or IMC for the length it is routed in a building.
The markings on the cable jacket are important in designating a cable’s intended purpose and whether the NEC would allow it to be routed in a building. It’s the responsibility of the owner, designer, or contractor to understand the listing designations in the NEC and apply them to correct building environment. Manufacturers have developed several types of fiber optic cables, including outdoor, water-blocked fiber optic cables, which are also listed by the NEC to be routed inside a building.
If necessary, the entrance point can be extended into a building beyond where the conduit stubs into the exterior building wall or concrete slab. It may be more convenient to extend the entrance point due to telecom operations or the nature of the building layout. However, there is a significant cost in routing cable in rigid or intermediate metallic conduit to extend the entrance point. RMC can be almost two times the cost of EMT or add approximately $54/linear ft to a cable run in lieu of a listed cable that may be run exposed outside of conduit.
For other articles in the series, check out the following articles at www.csemag.com .
February 2010, “Unlocking the code” is an article about how to decipher the NEC for easier application on design and construction projects.
March 2010, “Bonding conduit sleeves” is an article that discusses where and how the NEC requires bonding and grounding for conduit sleeves.
TABLE 1: REFERENCE TABLE
|Article 770||Article 800||Article 820|
|Fiber optic cable||Communications (telephone)||CATV|
|770.2: Point of entrance definition||800.2: Point of entrance definition||820.2: Point of entrance definition|
|770.48: Unlisted cables entering a building||820.48: Unlisted cables entering a building||800.48: Unlisted cables entering a building|
|770.154: Applications of listed fiber optic cables||800.154: Applications of listed communication wires and cables and communication raceways||820.154: Applications of listed CATV cables and CATV raceways|
|770.179: (Fiber optic) cable markings||Table 800.179: (Communications wires and cables—) cable markings||Table 820.179: Coaxial cable markings|
|OFNP, OFCP, OFNR, OFCR, OFNG, OFCG, OFN, OFC||CMP, CMR, CMG, CM, CMX, CMUC||CATVP, CATVR, CATV, CATVX|