Vital tips about wires, cabling

Ethernet cable not only can be used to transmit data and communication, but also power for low-power-usage devices. This article discusses design considerations and relevant codes and standards.

09/19/2018


Learning objectives

  • Understand the codes, applications, and equipment required for low-voltage electrical distribution.
  • Learn about wiring and cabling as it pertains to electrical systems.
  • Understand the limitations and design considerations when using category cable.
  • Know the codes and standards that electrical engineers must identify when specifying these systems, such as NFPA 70: National Electrical Code.

Have you ever wondered how we can provide power to lighting systems via low-voltage cables? Or maybe, how do we charge phones with a universal serial bus (USB) cable? Do these cables fall under low-voltage distribution or power distribution? Are they required to be contained in a conduit system? Are there power limitations to these cables? Are there specific listings associated with these cables? Are there codes that specifically address the cables within commercial facilities?

For all questions you may have asked yourself, we will attempt to provide answers by discussing the codes, applications, and equipment that may help you apply these concepts compliantly. However, this is a very deep subject and we will not be all-inclusive.

Category cable

Category (Cat) cable is a classification of a varying number of twisted pairs of insulated conductors used for telecommunications. There are shielded and non-shielded varieties, depending on the environment and application. Most common types have unshielded twisted pairs (UTP).

Examples include Cat-1, Cat-2, Cat-3, Cat-4, Cat-5, Cat-5e, Cat-6, Cat-6a, Cat-7, and Cat-8. The construction of these cables is described in the American National Standards Institute/Telecommunications Industry Association (ANSI/TIA)-568 (2017 Revision D) standard, which is maintained and published by the Telecommunications Industry Association (TIA).

Systems using UTP of the Cat-1 and Cat-2 variety are typically not governed by these boards because they do not handle high-speed data transfers and are intended for simple voice communication. This standard does, however, recognize Cat-5, Cat-5e, Cat-6, Cat-6a, and Cat-8 variations of communications cable.

A key difference in the category rating of a cable is the number of twisted pairs in the communication channel. By increasing the number of pairs and the quality of the conductors, the cable can be used to communicate within a more complex system for transmission of information, such as networking, data transfer, cable television, and power over Ethernet (PoE) applications.

Cat-5e cable is today's most abundantly used cable for networking and data-transfer systems, containing four twisted pairs for a total of eight conductors. Cat-5e provides a bandwidth of 100 MHz and is compatible with 10Base-T, 100Base-T, 1000Base-T, audio/video (A/V) systems, and telephony applications.

Cat-6 cable is a more robust cable that exceeds many of the parameters of its Cat-5e cousin. Typically selected with a larger-gauge wire, this cable provides higher bandwidth, higher frequency, less attenuation, and less delay skew. Cat-6 cable is capable of handling up to 10 GB Ethernet for applications such as large organizations, universities, and high-speed.

Cat-8 cables are further enhanced cables, but they are not in wide use yet.

Complex applications, such as those requiring Ethernet connections, employ a Cat-5 cable or above. Ethernet cabling can be used to control various electronic elements through an internet connection based at the A/V source switch. Examples such as security cameras, LED lighting fixtures, and fire alarm equipment can all be connected through the Ethernet-connection cable.

With the adoption of the 2017 edition of NFPA 70: National Electrical Code (NEC), Part VI of Article 840 provides new guidelines for powering the electronic communication devices. This addition to the code allows power to be transmitted with the same data cables used for communications purposes by using spare twisted pairs within the cable. This PoE system uses cables with a minimum of four twisted pairs and can supply both the control circuit and the power circuit from the same connection terminals.

Power-limited circuits

NEC Article 725 categorizes power-limited circuits into three classes: Class 1, Class 2, and Class 3.The power limitation for a Class 1 circuit is 1,000 VA (volt-amperes) and not more than 30 V.

The power and voltage limitations for Class 2 and Class 3 circuits are defined in NEC Chapter 9, Tables 11(A) and 11(B). The volt-amperes allowed do not exceed 250 VA and have several other parameters based upon whether the system is ac or dc, whether overcurrent protection is required, and what the circuit voltage is (never exceeding 150 V).

Power-limited cable classifications

Class 2 and Class 3 cables have various power, location, and support requirements specific to their application. Power regulations are specified in NEC 840.160 regarding power-limited PoE systems. Any device requiring less than 60 W is typically unrestricted; however, once the 60-W threshold has been exceeded, the additional requirements of Article 725 govern the system. It is important to mention that Article 840.160 strictly avoids any discussion on power factor or harmonic content. However, because the category cables will only be carrying direct current, it is assumed that watts and volt-amperes are equal for the specified applications.

Additional classifications dependent on location apply when dealing with applications in dedicated air ducts, plenums, risers, and general spaces and must be protected independently. The listing printed on a cable's outer insulation generally is a very good indication of where the cable is permitted for use. Class 2 and Class 3 cables are denoted with a "CL2-" or a "CL3-," respectively. These cables are further divided into specific locations types for more or less restrictive areas.

Restricted areas, such as plenums and risers, have special requirements for code-allowable installations. In the NEC, a plenum is defined as a compartment or chamber to which one or more air ducts are connected and that forms a part of the air-distribution system. It is intended to support the HVAC system by allowing the movement of environmental air. This area classification is reserved for conditioned spaces, which are typically not occupied and do not house flammable or toxic materials. Due to the inherent recirculation of air through a space, the cabling requirements of a plenum are strictly limited to minimize the spread of fire and smoke into the HVAC system.

For installations in spaces rated for environmental air recirculation, cables must be tested under NFPA 262: Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces. This measures the flame's travel distance and the optical density of smoke for insulated and/or jacketed electrical wires and cables and fiber-optic cables intended for installation in plenums and similar areas.

To route cable through an air-handling plenum, the cable insulation must be rated as fire-resistant and low-smoke-producing. Cables listed to be run through plenums must be marked with the plenum designation on the cable itself. Cables listed as plenum-rated are permitted to be routed through other spaces as cable substitutions, such as risers, walls, and general-purpose locations and dwelling units; however, the use of non-plenum-rated cables cannot be substituted through a plenum. See NEC Figure 725.154(A) for additional information.

Per NEC requirements, all CL2- and CL3- cables must be clearly labeled for location-appropriate installations and are appended with a location-specific letter to indicate acceptable practices. Cables with a "-P" can be considered plenum-rated, cables with a "-R" can be considered riser-rated, cables with an "-X" can be considered safe for dwellings units, and no appended letter is considered to be a general-purpose cable. Refer to NEC Table 725.154 for the specific application requirements of the different classifications of cables for Class 2 and Class 3 installations.

A common application for these cables is a Class 2 power supply operating at 24 V with a load of less than 100 VA, typically found in many LED lighting power supplies. Some advantages of a Class 2 cable system are the inherent safety factors in installation, maintenance, and operation. The limited voltage and power available prevent the initiation of fire as well as the risk of electrical shock.

For PoE systems, the category cable is typically the chosen method of power delivery. These cables generally are supplied with between 22- and 26-AWG copper conductors in the twisted pairs. The small cross-sectional area of these wires inherently limits the current permitted to pass through the cables.

The recent advancements in LED technology work in tandem with more capable cables to allow for larger systems to use this method of power delivery. It is also notable that in PoE circuits of more than 60 W, the ampacity determination of the conductor varies significantly from that of NEC Article 310. The number of bundled cables, size of the copper conductor, temperature rating, and conductor use are still the predominant factors, but they are scaled down for maximum ampacities up to 3 amps. Details are found in NEC 725.144.


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