Carrying A Powerful Message

During the recent 2017 National Electrical Code (NEC) first revision meetings in Hilton Head, S.C., a number of public inputs were submitted to introduce a new cabling system into Article 725 and Article 760. Article 725 covers Class 1, 2 and 3 remote control, signaling and power-limited circuits, and Article 760 covers nonpower-limited and power-limited fire alarm circuits. The new cabling system being proposed for these two articles is to be called limited power (LP) cable, which is to be used for delivery of power and data through network cabling, such as power over Ethernet (PoE). I investigated the cabling system and the potential application for the system with regard to fire alarm systems, remote control, signaling and power-limited installations.

I did an Internet search for “Ethernet” and found the following definition and information (actually more than I wanted, but it was very interesting reading): Ethernet is a system of computer technologies that groups together local area networks (LANs) or metropolitan area networks (MANs).

While working for Xerox, Robert Metcalf coined the word “Ethernet” in his 1973 Ph.D. dissertation. Ethernet was commercially introduced in 1980, standardized by the Institute of Electrical and Electronics Engineers (IEEE) in 1983 as IEEE 802.3, and has been refined to support higher bit rates of data transmission and longer transmission link distances. 

Ethernet is used to interconnect appliances and other personal data devices.

The original cabling systems used for Ethernet were coaxial cables, but these connection methods were later replaced with twisted-pair and fiber optic cables because the data-transfer rates had been increased from the original 3 megabits per second to 100 gigabits per second. 

In my research, I also discovered that systems communicating over Ethernet divide a data stream into shorter pieces called frames. Each frame contains source and destination addresses as well as error-checking data so that damaged data can be detected and retransmitted. 

Now, back to the public inputs for the 2017 NEC. The LP cable being introduced is used for data transmission but must also carry a certain amount of power. Class 2 and 3 systems are limited to an amount of current and voltage, as well as an amount of power or volt-­amperes (VA), by limiting the power supply as can be found in Chapter 9 of the NEC in Table 11(A) for alternating current (AC) and Table 11(B) for direct current (DC) systems or by the listing requirements in 725.121(A). These table requirements mirror the listing requirements with a power nameplate limitation of a maximum of 100 VA after one minute of operation regardless of the load and with any overcurrent protection bypassed for both AC and DC applications. This limitation ensures the power supply will not have a prolonged output higher than 100 VA.

The maximum current (Imax) that is permitted for both AC and DC is 8 amperes (A) peak maximum output current, even during a short-circuit occurrence, at zero through 30 volts (V), but the power supply nameplate is still limited to a maximum of 5A. This difference between the nameplate ampere value and the Imax is to allow stabilization of the current for the system but to ensure a maximum peak value of not greater than 8A. These values will be critical for the application of any new installations involving PoE and any new LP cable applications using Class 2 power supplies with inherent or overcurrent protection based on the notes to tables 11(A) and 11(B).

Additional public inputs require cabling used for transmission and power delivery to be listed, and the cable must be marked with the temperature rating and the size of the individual insulated conductors in the cable. The substantiation for these new LP cables for PoE indicates that DC-powered information technology devices, lighting, building controls, cameras, security and future devices that will eventually be connected to these circuits will generate higher heat in the cables, leading to safety and performance concerns within the cable masses, especially where these cables may be bundled. Consideration must be given to increase the size to keep the insulation on Ethernet cables from long-term thermal degradation, and the fire-resistance properties of the cable materials must be taken into account.

More information about the use of Ethernet systems and special cabling requirements must be provided for the 2017 NEC comments.

About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and .

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