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2018-09-19 Consulting Specifying Engineer
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.
By: Rick H. Reyburn, PE, and Barry Lasseigne Jr., NV5, Las Vegas
- 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 (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.
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.
NEC 725.143 contains requirements for the support of conductors. This section states that Class 2 and Class 3 cables shall not be strapped, taped, or attached by any means to the exterior of a conduit or other raceway as a means of support. These conductors shall be permitted to be installed as permitted in NEC 300.11(C)(2), which permits a raceway to be used as a means of support for Class 2 circuit conductors or cables if the raceway contains only the power supply conductors for the equipment being electrically controlled.
All PoE cable assemblies shall be securely fastened in place. The structural support elements shall not be immediately compromised in the event of a fire. Typically, PoE cabling found above a ceiling is permitted as long as it is supported independently of a suspended ceiling and laid inside of a properly supported raceway, cable tray, or other supporting means.
NEC Chapter 3 includes wiring methods and materials, such as conduits, wireways, cable trays, and low-voltage suspended-ceiling power distribution systems. Using "J-Hooks" is a typical method employed to support cables and it is recognized by TIA standards. However, J-Hooks are not included by the NEC in any specific article, but they are UL-listed for the application and would, therefore, be acceptable via NEC 110.3.
When determining the support methods for wiring systems in ceiling cavities, understanding the requirements of a fire-rating assembly classification is imperative to ensure proper installation methods. In general, for both fire-rated and non-fire-rated ceiling assemblies, the support of cables shall not be directly secured to the ceiling. A secure and independent support must be provided for the electrical wiring methods. However, cables are permitted to be supported by the ceiling of a fire-rated ceiling assembly if the wiring has been tested as part of the fire-rated assembly.
When specifying cables for integrated communications and power systems, certain aspects of the cable's design can limit both the applications and the performance. Things to consider when attempting to limit heat generation in a system are the conductor size, the temperature rating, the installation type, and the twisted-pair shielding.
When determining the required ampacity of the cables, the higher-numbered category cables will generally provide a lower internal resistance; for example, Cat-5e is typically more resistive than Cat-6a.
This metric is critical in preventing power losses across the communication channel. Joule heating is responsible for a majority of all power dissipation in resistive cables and can be detrimental to a communication circuit's functionality. Every incremental increase in internal resistance provides equal incremental power losses in the system.
In a situation where high power dissipation is unavoidable, it is imperative to select cables capable of performing under warmer conditions. Higher cable-temperature ratings will ensure that the cable can withstand the combined heating of internal current flow and ambient temperature. The warmer a cable gets, the higher its resistance becomes, which could eventually lead to excessive heating and cause permanent damage to the conductors. Fire, loss of transmission, and shorted circuits are all possible problems that have been encountered by neglecting temperature effects on cable performance.
Additional thermal considerations must be considered when installing many cables together in bundles. When packed tightly together, the cables in the center of a bundle are unable to dissipate heat into the external environment and can only radiate heat to their neighboring cables. Over-packing cable trays and conduits, encountering fire stops, and high ambient temperatures are all factors that can degrade conductors due to poor installation planning. In installations with large bundles of cable, it is important to refer to NEC Table 725.144 to apply temperature correction when determining the ampacity of the cables.
The composition and construction of communications and power cables also are influencing factors when trying to mitigate heat gain in cables. Various insulation types, shields, and conductor materials all play a factor in heat capacity and dissipation. As discussed, conductors with lower-resistance copper (larger copper conductors) have lower resistive losses, creating less heat. Also, cables with metallic shielding tend to conduct heat away from the conductor more efficiently than those made only with polymeric materials. Additionally, minimizing the electromagnetic effects in the cable prevents unwanted currents, thus unwanted heat, from flowing through any conductors.
Cable length and voltage drop
As with all current-carrying conductors, there is some maximum allowable distance before the resistance of the wire causes an excessive voltage drop. While not a mandatory requirement in all jurisdictions, the NEC recommends limiting voltage drop to an overall 5% from the point of service to the final outlet or device.
In general, it is recommended to limit the voltage drop to 2% for feeders and 3% for final branch circuiting. For Ethernet category cables, this distance is limited to 100 m (328 ft), with 90 m (295 ft) allowed from the patch panel and 10 m (33 ft) from the wall jack to the device. When the powered device (PD) is attached to the power-sourcing equipment (PSE), the PD sends a signal to the PSE requesting the amount of power it requires to operate.
For example, if a PoE camera requests 30 W from the PSE, the PSE will respond with a 30-W signal to control the camera. When maintained under the 100-m threshold, the voltage drop of the cable is mostly negligible and has no effect on the camera's ability to operate. At more than 300 ft, the camera's operation may be compromised; however, devices such as injectors and fiber-optic media converters can extend the maximum distance of a system by hundreds of feet.
By implementing a PoE injector, the power signal is boosted up to the originally requested wattage at the injector location. By placing an injector slightly before the maximum cable length, the signal can be transmitted another 300 ft from the injector. The injector placement is limited, however, as it must connect to a 120-V source to provide the additional power and boost the signal.
If a longer run is desired in a location without sufficient 120-V connections, then a media converter can be substituted into the system. At or near the PSE, the category cable can be connected to the media converter and the signal will be passed along via fiber-optic cables for up to 900 ft. One advantage of media converters is the use of fiber-optic cables.
For example, in a parking lot with cameras mounted at the top of light poles that are intended to be PoE capable and average more than 300 ft away from the communications cabinet, a multiport media converter is a good choice. Placing the media converted at the power-sourcing equipment, fiber optics can be run outside into the parking lot and into a second multiport media converter. From this central connection, category cables can run to each camera within the 300-ft cable radius powering as many cameras as there are ports in the media converter. However, typically, these converters will require 120 V power.
Typical applications of power over Ethernet (PoE) cabling include:
- Daylight sensing.
- LED lighting.
- Motion detection.
- Security cameras.
- Security sensors (touchpads).
- Speakers/audio equipment.
- Wi-fi routers.
Tips for things to avoid
- Avoid applying cables without supports. Follow Building Industry Consulting Service International (BICSI) and NFPA 70: National Electrical Code (NEC) requirements for proper supporting. Minimizing the sag between hooks and using wide-based supports prevent violating the bending-radius specifications of the cable
- Verify the locations you are running the cable through. If it is identified as an air-handling plenum space, as defined by the International Mechanical Code and NEC, then the proper plenum-rated cable must be used.
Consider a device that is 250 ft away from its source patch panel. Given that a 12-AWG wire is connected from the patch panel to the wall jack and a 22-AWG connects the wall jack to the load 20 ft away. The load draws 2 amps of current and the patch panel provides 60VDC. What is the voltage seen at the load, and what is the percentage voltage drop?
Rick Reyburn is the executive director of electrical engineering for NV5.
Barry Lasseigne Jr. is pursuing a bachelor's degree in electrical engineering at the University of Nevada and works part-time at NV5 drafting and learning electrical design.