my april 2015 column, “moving forward,” provided an introduction to a number of public inputs for the 2017 National Electrical Code (NEC). These inputs were submitted to Code-making panel (CMP) 3, and they addressed power over Ethernet (PoE) cables. As the new cable is to be used for power and data delivery through network cabling, these cables were to be incorporated into Article 725, which covers Class 1, 2 and 3 remote control, signaling and power-limited circuits.


Since Article 725 normally covers computer data low-voltage systems, the new cable and PoE seemed to fit within the scope of Article 725. Unfortunately, the public inputs did not provide any technical information about the use of PoE or technical information on the limited-power (LP) cables proposed for this new system, so CMP 3 resolved or rejected the public inputs.


Between the public input stage of the 2017 NEC in January 2015 and the public comment stage in November 2015, the Plastic Industry Trade Association introduced this concept into the NEC and commissioned a fact-finding report on power over local area network (LAN) or four-pair data and communications cables. A fact-finding report is based on an investigation undertaken by a testing organization to discover facts and issue a report for use by the applicant (the person or industry that commissioned the report) in seeking amendments in nationally recognized installation codes and standards.


The report issuance does not constitute an endorsement of any proposed amendment and in no way implies listing, classification or other recognition by the author or testing agency, but the report is often used to authenticate technical information on a particular product under NEC panel consideration. This report’s primary purpose was to determine the parameters that affect a PoE cable’s ability to handle current in a bundle and the effect that the conductor size and cable construction have on cable heating. In addition, the report was also intended to determine the effect of different installations on cable heating, assess the worst-case installation scenarios and conclude which conditions cause the most heat to rise in the affected cables. Finally, the report also investigated the combined effects of higher power levels (direct current voltage and current) applied to communications cables installed in accordance with the NEC. 


This particular report provided test data on the heating effects of direct current (DC) on four-pair LAN cables when these cables are installed in various sizes of bundles under simulated installation conditions. Even though Class 2 cables with DC voltage and DC current limitations of 60 volts (V) or less and 2.5 amperes (A) or less have been installed for many years without a safety issue, these new PoE cables are often in extremely large bundles, unlike most Class 2 cable installations. Combined with limited-heat dissipation for cables on the interior of the bundle, the cumulative heating effect of numerous PoE cables in these large bundles results in very high temperature on the individual conductors within the cables, according to the test data in the fact-finding report. 


The report determined that, on higher data capacity systems where all four pairs of conductors in a PoE cable are needed for data-transmission speed, power is carried on the same pairs as the data on either two- or four-pair power applications. Bundles of cables were installed in cable trays, in various types of raceways, cable-routing assemblies, and cable bundles tie-wrapped together and supported similar to installations in suspended ceiling systems.


The data from the fact-finding investigation indicates that—under various installation conditions, such as installation methods commonly used for LAN cables—currents well below the limits permitted for existing NEC Class 2 systems resulted in the overheating or melting of the cables. In some of the test installations, a DC current of only 1A per conductor resulted in measured temperatures within the bundle of 140°C in open air and at least 185°C in cable-routing assemblies and PVC conduit. A 90°C insulated conductor in a normal low-voltage cable will not be able to withstand those temperatures without severe damage to the insulation. 


After extensive testing and research, limited-power cables were developed to take advantage of a number of factors that can effectively manage cable heating, such as AWG size, cable design and appropriate material selection for high temperature. These new LP cables and a new ampacity table (725.144) provide the opportunity for manufacturers to introduce and use new cable technologies and for installers to take advantage of these cable-design innovations without compromising safety.