Proposed revisions to 2017 National Electrical Code would impact PoE deployment
As currently proposed, revisions to the 2017 NEC will negatively affect PoE deployment, potentially reducing achievable cabling distances and adding complexity, cost, and confusion. A reasonable recommendation is to defer the proposed revisions to the 2020 Code cycle.
By Christopher T. DiMinico, MC Communications, Cisco Systems
In October 2015, the code-making panels of the National Fire Protection Association (NFPA) met to consider revisions to the National Electrical Code (NEC) to take effect with the publishing of the 2017 NEC. This article provides a review of the proposed revisions to the NEC that impose new compliance requirements on communications cables used to supply power to communications equipment, considering the impact of Power over Ethernet (PoE) deployment. The article will not review topics on the enforcement of the Code.
Codes, standards and regulations have an enormous impact on the design and installation of telecommunications infrastructure. The Telecommunications Industry Association (TIA) and the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) provide related standards regarding various aspects of telecommunications infrastructure, including cabling, pathways and spaces, and labeling. BICSI supplements these standards with manuals of best practices and methods by which requirements from TIA standards are implemented. Additionally, BICSI provides training in the design, implementation, integration and project management of telecommunications and data communications technology and related infrastructure through an industry-recognized designation of achievement called the Registered Communications Distribution Designer (RCDD).
In addition to generic telecommunications standards, TIA and ISO/IEC support the Institute of Electrical and Electronics Engineers (IEEE) by developing Telecommunications Systems Bulletins (TSBs) and Technical Reports (TRs) for IEEE 802.3 applications such as 1000Base-T, 2.5/5GBase-T, 10GBase-T and remote powering (PoE). IEEE and TIA/ISO have established liaison officers, enabling tightly coupled standards reflecting inputs from across the telecommunications industry.
IEEE 802.3 is in the process of developing an amendment to PoE in the IEEE 802.3bt Task Group to increase the maximum power available at a powered device by utilizing all four pairs in a twisted-pair cable. Coordination with TIA/ISO cabling committees on power delivery continue, with the development of ISO/IEC TR 29125 “Telecommunications Cabling Requirements for Remote Powering of Terminal Equipment,” and the TSB-184-A addendum to TSB-184 “Guidelines for Supporting Power Delivery Over Balanced Twisted-Pair Cabling.”
Proposed revisions to the 2017 NEC
The second draft revision to the 2017 NEC Article 840.160 and Article 725.144 are given below under the cited Article, without modifications to the source content. The safety issues claimed in support of the revisions were expressed as concerns over operation of cable beyond their temperature ratings.
Article 840.160 Communications Circuits - Communications cables, in addition to carrying the communications circuit, shall also be permitted to carry circuits for powering communications equipment. Where the power supplied over a communications cable to communications equipment is more than 60 watts, communications cable and the powering circuits shall comply with 725.144 where communications cables are used as substitute to Class 2 and 3 cables.
Table 725.144, Ampacities of Each Conductor (in Amperes) in a 4-Pair Class 2 or Class 3 Data Cable, Based on Copper Conductors at Ambient Temperature of 30°C (86°F) with All Conductors in All Cables Carrying Current, 60°C (140°F), 75°C (167°F) and 90°C (194°F) Rated Cables Note 1: For bundle sizes over 192 cables, or for conductor sizes smaller than 26 AWG, ampacities shall be permitted to be determined by qualified personnel under engineering supervision. Note 2: Where only half of the conductors in each cable are carrying current, the values in the table shall be permitted to be increased by a factor of 1.4. Informational Note: The conductor size in data cables in widespread use are typically 22-26 AWG.
Article 725.144 Transmission of Power and Data - The requirements of 725.144(A) and (B) shall apply to Class 2 and 3 circuits that transmit power and data to a powered device. The requirements of Parts I and III of Article 725 and 300.11 shall apply to Class 2 and Class 3 circuits that transmit power and data. The conductors that carry power for the data circuits shall be copper. The current in the power circuit shall not exceed the current limitation of the connectors.
Informational Note No. 1: One example of the use of cables that transmit power and data is the connection of closed circuit TV cameras (CCTV).
Informational Note No. 2: The 8P8C connector is in widespread use with powered communications systems. These connectors are typically rated at 1.3 amperes maximum.
- The ampacity ratings in Table 725.144 shall apply at an ambient temperature of 30°C (86°F).
- For ambient temperatures above 30°C (86°F), the correction factors of 310.15(8)(2) shall apply.
Informational Note: One example of the use of Class 2 cables is a network of closed circuit TV cameras using 24 AWG 60°C rated, Type CL2R, Category 5e LAN (local area network) cables.
(B) Use of Class 2-LP or Class 3-LP Cables to Transmit Power and Data. Types CL3P-LP, CL2P-LP, CL3R-LP, CL2R-LP, CL3-LP, or CL2-LP shall be permitted to supply power to equipment at a current level up to the marked ampere limit located immediately following the suffix LP and shall be permitted to transmit data to the equipment. The Class 2-LP and Class 3-LP cables shall comply with the following: (B)(1) through (B)(3), as applicable.
Informational Note No. 1: The “(xxA)” following the suffix -LP indicates the ampacity of each conductor in a cable.
Informational Note No. 2: An example of a Limited Power (LP) cable is a cable marked Type CL2-LP(0.5A), 23 AWG. A Type CL2-LP(0.5), 23 AWG could be used in any location where a Type CL2 could be used, however the LP cable would be suitable for carrying up to 0.5 A per conductor, regardless of the number of cables in a bundle. If used in a 7 cable bundle, the same cable could carry up to 1.2 amperes per conductor.
- Cables with the “-LP” suffix shall be permitted to be installed in bundles, raceways, cable trays, communications raceways, and cable routing assemblies.
- Cables with the suffix “-LP” and a marked ampere level shall follow the substitution hierarchy of Table 725.154(A) and Figure 725.154(A) for the cable type without the suffix “-LP” and without the marked ampere level.
- System design shall be permitted by qualified persons under engineering supervision.
Impact on PoE deployment
Power over Ethernet (PoE) is specified to provide power over the same generic cabling that is used for data transmission. The success of Base-T technology and PoE is largely due to the cost-effectiveness and plug-and-play simplicity of copper structured cabling. The proposed Code revisions will create a “new” class of communication cables for operating temperatures greater than 60°C with designated ampacity limits for powering. The revisions will add confusion and complexity to specifying generic telecommunications cabling. This, combined with the uncertainty of so-called “safety issues” and Code enforcement will adversely affect the adoption and cost of PoE deployment.
Additionally, TIA and ISO/IEC cabling is functionally specified over the temperature range from -10°C to +60°C. Connectors, patch cords, crossconnect wiring, patch panels, consolidation points, etc. would require extended operating temperature specifications. Channel transmission parameters such as insertion loss and DC resistance increase as a function of temperature, requiring considerations for adjusting achievable lengths.
Code revision technical aspects
Proposed Table 725.144 is inconsistent with TIA draft TSB-184-A and based solely on a single test report without collaboration by affected standards bodies. Specifically:
- Table 725.144 limits cable bundle sizes to <19 cables except for 22 AWG (<37 cables) with 0.48 A per conductor;
- TIA TSB-184-A allows for up to 74 cables in a bundle with 0.5 A per conductor (1 A per pair).
The TIA TSB-184-A findings are a result of cooperative analysis of temperature rise versus current measurements from multiple cabling manufacturers, as well as modeling between standards organizations (IEEE/TIA/ISO/IEC) during the development of remote-powering guidelines.
In accordance with the NFPA public comment process, the IEEE 802.3 Ethernet Working Group secretary submitted the text below.
Statement of Problem and Substantiation: For the following reasons, the IEEE 802.3 Working Group believes that this change to the Code is premature and should be deferred to the 2020 Code revision cycle. This delay should permit sufficient time for all interested and affected parties to review and comment.
- Lack of LP Cable specification-At the date of the written comment submission deadline, 21 August 2015, the specifications and availability of the referenced LP cabling was not, to the best of our knowledge, publicly available. This fact alone makes it impossible to provide adequate technical review of this new type of cabling, thus effectively precluding public input.
- Insufficient Technical Justification-At the date of the written comment submission deadline, 21 August 2015, no publicly available technical data has been provided detailing the testing methodologies used as the basis for the proposed changes. There is insufficient technical justification provided for the change to require the use of LP cabling.
In addition, no technical data has been provided showing that when operating a communications circuit over a communications type cable under the established guidelines that a safety condition is created or exists.
The proposed NEC revisions will negatively impact PoE deployment, potentially reducing achievable cabling distances and adding complexity, cost, and confusion of the cabling requirements, safety, and Code enforcement. There are significant differences between the proposed NEC table revisions (Table 725.144) and those in TSB-184-A. The NEC table was developed from a single report; TSB-184-A is being developed with broad-based cabling and equipment manufacturer participation (IEEE/TIA/ISO/IEC).
Consideration of the changes should be deferred to the 2020 Code revision cycle, permitting sufficient time for all interested and affected parties to review and comment on the following.
- Review of Table 725.144 current limits and operating temperatures
- Review specifications and availability of the referenced LP cabling
- TIA TR-42.7 Task Group chartered to consider differences in TSB-184-A and proposed Table 725.144, to include test methodologies and configurations
- Review of safety criteria and Code enforcement before Code adoption.
The timeline for the 2017 NEC revision process is as follows.
- January 4: Letter ballot; principal members of CMP 3 and CMP 16 vote on proposals. A 2/3 majority is required.
- February: Technical Correlating Committee reconciles drafts.
- April: 2nd draft revisions published.
- Late April: Notice of Intent to Make a Motion (NITMAM) deadline.
- June: NFPA annual meeting.
- Later in 2016: NFPA Standards Council meets.
Chris DiMinico is president of MC Communications, a telecommunications consulting firm with clients including Cisco Systems. He also is president and chief technical officer of PHY-SI, LLC, which produces high-speed test fixtures. He plays an active role in the development of a number of telecommunications industry standards. Diminico is an active participant in and technical contributor to IEEE 802.3 and TIA. He is the IEEE liaison officer to TIA TR-42.