Popular technology expands to support more devices, but installers should understand cabling best practices
This piece was originally published in the August 2016 issue of electroindustry.
Jeff Poulsen, Senior Electrical Engineer, Leviton
With new standards allowing for higher power levels over cabling infrastructure, Power over Ethernet (PoE) is expected to see explosive growth in the years to come. By running power and data over a single cable, PoE eliminates the need for additional wiring, reducing cost and simplifying installation. It also makes it easy to deploy devices such as IP phones, security cameras, sensors, and sales kiosks in any building location. Recent and emerging technologies allow for PoE to support even more PoE-ready devices, including the latest high-speed wireless access points, large high-end displays, and computers.
As PoE expands to support more power-hungry devices, however, cable installations can experience temperature increases that have the potential to affect performance. When cables are powered, a small percentage of the energy is dissipated within the cabling, causing the cable’s temperature to increase. Cables carrying PoE at low power levels, such as 15 or 30 watts, are less likely to see significant increases in temperature unless extreme conditions exist. It is only when higher power levels, such as 60 or 100 watts, are deployed at elevated temperatures and in large cable bundles or groupings that cable heating can become a potential problem.
It is important to keep cables at reasonable temperatures over their entire length and below their maximum operating temperature at any location to avoid compromising transmission performance, causing long-term cable degradation, and in extreme cases creating hazardous conditions.
The following factors should be considered when addressing higher temperatures in a PoE installation.
- Conductor size. Horizontal cables and patch cords with the largest conductors available (i.e., the lowest wire gage number) will improve the current flow, generating less heat than smaller conductors. A higher category rating will typically have larger conductor sizes, providing advantages where there are concerns about bundle temperature rise. For this reason, Cat 6A cabling is recommended for all new PoE installations.
- Cable type and shielding. As always, select the flame rating of cable needed for an installation, (e.g., CM, CMR, CMP, and LSZH). Also consider using a shielded cabling system or unshielded cables with a segmented metallic isolation wrap, as these cables radiate heat better and will therefore reduce the cable temperature rise.
- Cables with higher temperature rating. Cables are available with 60°, 70°, 75°, or 90° C rating, but try to keep the maximum ambient temperature at a reasonable level (a common target is 45° C maximum). For higher maximum ambient temperatures, seek professional design assistance.
- Cable management. The type of cable management and trays used can play a role in cable temperatures. For example, wire cable trays or similar cable management will allow for largely unrestricted airflow around the cables.
- Cable bundling. Always adhere to the limits for maximum bundle sizes given in applicable codes, specifications, or standards bulletins. Consider loosely grouping cables rather than bundling, or bundle cables using relatively small bundle sizes that meet the needs of the installation. Avoid cramming cables into small areas such as pass-throughs or fire stops. Provide as large an area as possible for these transitions.
Of course, cabling intended to carry PoE should be installed in accordance with all building codes applicable to the given locality, including the National Electric Code®. The Telecommunications Industry Association’s TSB-184/A and ISO/IEC TR 29125 may also provide additional installation information.
Finally, be sure to seek advice from industry experts and cabling and connectivity manufacturers.
Mr. Pouslen has more than 20 patents related to cable and connectors. He participates in several standards organizations, including TIA, ISO/IEC, IEEE, and NFPA.